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Evaluation of the Thermal Environment

Chapter 8

EVALUATION OF THE THERMAL 

ENVIRONMENT

WILLIAM R. SANTEE, PhD*; and WILLIAM T. MATTHEW, BS†

INTRODUCTION

THE ENVIRONMENT

Definition of Environment

Definition of Thermal Environment

Insensible Heat Exchange

Other Factors

Temperature of Bodies of Water 

Meteorological Parameters

MEASUREMENTS

Complexity of the Environment

Meteorological Scale

Measurement Objectives

Parameter Measurements

Stations and Instrument Packages

LINKING METEOROLOGY TO MODELS AND USERS

Operative Temperature

The New Effective Temperature 

Humid Operative Temperature

SUMMARY

ATTACHMENT: INSTRUMENTS AND METHODS FOR MEASURING 

PARAMETERS OF THE THERMAL ENVIRONMENT

*Research Physical Scientist, Biophysics & Biomedical Modeling Division, US Army Research Institute of Environmental Medicine, Building 42, Kansas 

Street (Office MCMR-EMB), Natick, Massachusetts 01760-5007

†Biophysicist (Ret), Biophysics & Biomedical Modeling Division, US Army Research Institute of Environmental Medicine, Building 42, Kansas Street, 

Natick, Massachusetts 01760-5007; currently, PO Box 25, Woodville, Massachusetts 01784
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

INTRODUCTION

heatstroke) reaches a level that significantly impacts 

operations or captures media attention. In addition, 

knowledge of the basic factors involved in thermal 

stress, particularly meteorology, is not part of classical 

medical education. 

Within the US military, the weather mission is 

not always a priority for the Army, because the Air 

Force is assigned primary responsibility for weather. 

Thus, one cannot fault the medical staff for not being meteorologists. However, for medical personnel 

operating in the field, having a basic understanding 

of how weather impacts soldiers can be critical. 

The purpose of this chapter is to present the basic 

information regarding the collection of weather 

data—relating the data to the actual science of heat 

exchange—and to review methods that allow the 

interpretation of weather data into practical applications for the management and prevention of 

morbidity associated with the thermal elements of 

the environment. 

From General George Washington’s successful 

winter attack in Trenton, New Jersey, on December 26, 

1776 to present operations in the heat of Iraq and the 

cold and altitude of the Afghanistan mountains, the 

environment—as represented primarily by meteorological extremes—has played a role in military operations. Although, on occasion, a catastrophic weather 

event or a major storm can impact a military operation 

directly, the majority of medical problems related to 

the thermal state of soldiers result from their exposure 

to multiple physical and environmental stress factors 

that, in combination, might produce chronic or acute 

physiological strain. 

Perhaps because exposure to the physical environment is a persistent but not dramatic factor relative to 

other tactical and medical concerns, the prevention of 

environmental injuries related to the weather is often 

a secondary concern. The prevention of thermal environmental injuries is often not a command priority 

until a specific ailment (eg, frostbite, hypothermia, or 

THE ENVIRONMENT

Definition of Environment

At the most basic level, environment is defined as 

the surroundings of an object, organism, population, 

or system. By that definition, the environment can be 

virtually the universe. A more practical and limited 

definition is necessary, as any reference to the environment is only meaningful in the context of a specific focus. For military medicine, the obvious focus is military 

personnel. To further reduce the scope of the environment to manageable levels, the description should be 

limited to factors that may have a specific effect on the 

individual or population. Thus, it is meaningful to refer 

to the social, psychological, or physical environment 

rather than the totality of the environment. 

Definition of Thermal Environment

The primary effect of the environment on military 

personnel is that of ambient conditions on heat exchange. At a given point in time and space, an individual who occupies that point experiences a specific 

set of physical conditions. Those conditions that determine the potential for heat exchange between the individual and the surrounding environment define the 

thermal environment. Thus, the thermal environment 

is defined as those aspects of the physical surroundings of the individual or population that directly affect 

the potential for heat exchange between the subject or 

population and its environment. In terms of environmental stress, the thermal environment represents the 

primary stress potential of the physical environment. 

The actual strain experienced is dependent on the 

thermal environment, certain physical characteristics 

of the individual (eg, posture, insulation, or clothing), 

and human physiology (hydration, energy reserves, 

metabolic rate, and activity level).

Energy Balance Equation

The energy balance for an individual can be written as:

(1) •M ± •E ± •R ± •C ± •K ± •Wk ± •S = 0 [W].

The variables are metabolism (M), evaporative heat 

loss (E), radiative heat exchange (R), convection (C), 

conductance (K), work (Wk), and storage (S). Significant 

changes in S, as reflected by changes in body temperature, are indications of thermal strain. E, R, C, and K

are the environmental pathways for heat transfer. 

These pathways are not measured directly, however. 

Instead, measurements are made of the meteorological 

parameters that determine the potential for transfer 

along each pathway.

The whole-body equation is expressed in terms 

of the rate of change in an energy flux (units in W or 

J • s–1), but almost all heat exchanges occur through 
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Evaluation of the Thermal Environment

the body surface. It is therefore often preferable to 

express heat exchange parameters in relation to the 

skin surface area involved in heat exchange as density 

fluxes (W • m–2). It is also more meaningful to reorder 

the equation to indicate that the final balance might 

result in a change in the storage term, S. 

(2) S = M – (±Wk) ± E ± R ± C ± K [W • m–2]

(3) ∆S = 1,000 • cb • m •∆Tb ÷ (∆t • AD) [W • m–2].

The value of expressing the energy balance in terms 

of S is that any change in storage (∆S), during time 

interval ∆t, will result in a change in body temperature 

(∆Tb). The practical significance is that any change in 

measurable body temperature indicates a shift in the 

energy balance of the body. Hence, monitoring body 

temperature will provide a direct indication of the thermal state of the body. The specific heat of body tissue 

(cb) is 3.5 kJ • (kg • K)

–1.1

 Body mass (m) in kilograms 

and height (ht) in meters can be used to estimate the 

Dubois (AD) body surface area.2

(4) AD = 0.202 m0.425 ht0.725 [m2].

AD is used to approximate the body surface area involved in convective, radiative, and evaporative heat 

exchange, but not conductance.

Biophysical/Environmental Variables

Four general environmental parameters determine 

the potential for heat exchange between an object and 

the environment. The first three parameters—(1) temperature, (2) wind speed, and (3) radiation—determine 

dry or sensible heat exchange potential. The fourth parameter, water vapor pressure (humidity), determines 

the potential for insensible heat exchange that involves 

a phase change from liquid to vapor.

Conductance. Conductance is heat exchange between a surface and a solid. It is dependent on the 

temperature difference (∆T) between the body surface 

and the other solid surface, the thermal conductivity 

of the material (kc), and the distance (l) through which 

the heat is conducted.

(5) K = (kc/l) • ∆T [W • m–2].

To compute actual heat loss, the area of surface contact 

(Ac) must also be known. For a standing person, the 

contact area for conductance is through the soles of 

the feet or footwear. Unless the surface temperature 

is unusually high or low and the soles are poorly insulated, conductance is usually negligible. Heat loss 

from even a very small surface area can be important. 

For an astronaut handling cold-soaked tools on an extravehicular activity, an individual repairing/refueling 

a vehicle in extreme cold with inadequate handwear, 

or an injured soldier or rifleman lying/sleeping on frozen ground, conductance can be a significant concern. 

A combination of radiant heating and conductance 

quickly teaches vehicle crewmen to protect themselves 

from the surface of armored vehicles exposed to direct 

sunlight in the desert.

Convection. Convection is heat transfer through a 

fluid medium, normally air or water. A hot blast of air 

or a cool breeze is a sensory interpretation of convective heat exchange. The general equation for convection is driven by the difference between mean skin or 

surface temperature (–Ts) and air temperature (Ta).

(6) C = hc • (–Ts – Ta) [W • m–2].

The convective heat transfer coefficient hc is dependent on the density, viscosity, and velocity of the 

fluid. There are two types of convection: (1) free and 

(2) forced. Free convection involves still or very lowvelocity fluids, whereas forced convection dominates 

at higher fluid velocities. Free convection would occur 

if a heated rock were placed in still water. By warming the surrounding water, a pattern of fluid densities 

and temperatures would form around the rock. If the 

object had been heated at a constant rate rather than 

cooled, that pattern would stabilize. Forced convection 

is equivalent to placing the same rock into a flowing 

stream. The patterns of temperature and density that 

form in the stream flow are driven in part by the mechanical interaction of the rock and the flowing water, 

whereas in still water the primary factor is a small buoyancy flow from warming of the surrounding water. 

The practical significance of free versus forced convection is that the convective heat transfer coefficient 

hc is calculated with slightly different parameters for 

each type of convection. Free convection dominates 

in still fluids, and forced convection dominates above 

0.3 m • s–1.

3

 In the narrow zone between those two 

thresholds, both free and forced convection must be 

considered.4 When forced convection dominates, hc for 

a standing individual is roughly proportional to the 

square root of the air velocity (u). As a consequence, 

a velocity change of 0.5 m • s–1 has a relatively more 

significant effect on hc at low wind speeds (below 2.5 

m • s–1) than at higher wind speeds.

To calculate hc for forced convection in air without 

the additional effects of the individual parameters of 

motion or clothing requires a series of calculations4,5:

Thermal conductivity of air, 

(7) kf = 2.41 • 10–2 + 7.8 • 10–5 • Ta [W • m–2 •  ̊C–1].
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

Kinematic viscosity of air,

(8) ν = 1.33 • 10–5 + 9.0 • 10–8 • Ta [m2 • s–1].

Reynolds number,

(9) Re = u • L/ν [N.D.]

and the Nusselt number,

(10) Nu = cPraReb [N.D.]

(11) Nu = c′′Reb [N.D.]

(12) hc = Nu • kf/L [W • m–2 • K–1].

The characteristic dimension (L) for a standing cylinder is the diameter (m), and approximate values for 

c′′ and b are 0.33 and 0.55.6 The Prandlt number (Pr) is 

assumed to be a constant (0.72) for air temperatures in 

the 10° to 50°C range, as is the shape factor (a) for a cylinder.6

 The actual rate of heat transfer is not determined 

wholly by environmental conditions. The convective 

heat transfer coefficient (hc) is also determined, in part, 

by the shape or posture of the individual, as well as 

by the velocity and other properties of the fluid. The 

majority of physiological research assumes a standing 

or walking subject. The shape factor4,7 for a patient on a 

litter on the ground is not the same as it is for a standing 

person. The shape factor would not only differ with 

posture, but also there would be a parallel equation 

for free convection at lower air velocities, and clothing 

and motion would also alter convective heat transfer. 

Because of the greater density of water, the rate of 

heat loss in water is much higher than it is in air. In 

water, temperature has an effect that is 77 times greater 

for a nude person at rest in still water than the same 

temperature difference in air.8–10 Both activity and water flow will increase the rate of heat loss. A relatively 

small difference in the temperature gradient between 

a body and the surrounding water has a much more 

significant effect on an individual’s thermal balance. 

The danger of water immersion was reinforced by the 

deaths of four Ranger students following a streamcrossing exercise at Florida’s Eglin Air Force Base. The 

estimated air and water temperatures were 17° to 18°C 

and 12° to 15°C, respectively.11 

It is difficult to separate the environmental potential 

for convective heat loss from the effects of clothing and 

activity. In a fluid, a series of layers of homogeneous 

temperature and velocity form about the body surface; 

this is collectively referred to as the boundary layer. 

By modifying the rate of heat transfer between the 

surface and the environment, this layer acts as insulation between the body surface and the environment. 

The thickness and stability of the boundary layer is 

dependent on the velocity and turbulence of the fluid. 

Boundary layers are more stable when the fluid flow is 

streamlined (laminar) rather than turbulent. Extreme 

turbulence can disrupt the formation or stability of a 

boundary layer, increasing heat loss.

Convection is also modified by body motion (the 

pendulum effect), the movement of air within clothing 

and in and out of clothing through openings (pumping 

or ventilation), and clothing insulation. Another clothing interaction occurs when wind penetrates clothing, 

thereby reducing the effective insulation. Such effects 

fall outside the scope of this chapter, but some understanding of how these dynamic, physical factors affect 

convective heat exchange is useful.

The pendulum effect12 results from the motion of 

extremities. The effect of motion has been studied by 

mounting naphthalene discs on both human subjects6

and mannequins that simulate walking motion.13Empirical equations for calculating hc during different 

activities14 can be used to compensate for the effect 

of motion on convection. Another approach is to calculate an effective air velocity (ve

) that assumes that 

the effect of the observed air velocity is enhanced by 

body motion and that body motion is proportional to 

whole-body metabolic rate ( •M).15

(13) ve = va + 0.004 ( •M – 105) [m • s–1]

Measured air velocity (u or va) and effective air 

velocity (ve) are measured in meters/second. Corrections for only the pendulum effect are adequate for 

the nude state. The effect of clothing insulation can 

be estimated by calculating a thermal efficiency factor 

(Fcl) from the intrinsic insulation of clothing (Icl) and 

boundary layer (Ia), as proposed by Burton in Gagge 

and Nishi.14 The Fcl value for a nude individual is 1 and 

always less than 1 when clothing is worn. Some investigators feel that a pumping coefficient that adjusts for 

the pendulum effects and air exchange from both air 

movement within the clothing and ventilation should 

be incorporated into the equations. The combination 

of clothing and motion has been studied in human 

subjects16,17 and walking mannequins.18,19 Tests were 

often conducted with variable wind speeds to determine the contribution of wind penetration. There have 

been several approaches to quantifying this pumping 

effect. One approach is to treat the effects of motion 

as a convective effect by increasing the heat transfer 

convection. The other approach is to express the effect 

as a reduction in clothing insulation, which is possible 

because clothing insulation is partially a resistance to 

convective heat exchange. 

Convection is an important heat exchange pathway 

in the cold. The windchill index20 predicts cold toler-
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Evaluation of the Thermal Environment

ance and injury on the basis of convection heat loss. 

In the heat, evaporation is usually more important in 

maintaining homeostasis, but convection is an important pathway for heat exchange in some circumstances. 

When air temperature is greater than skin temperature, 

the normal direction of convective heat exchange is reversed, the body will gain heat from the environment, 

and a cooling breeze becomes a warming breeze. Another situation is the relative importance of evaporative 

versus convective cooling. Normally, evaporative heat 

loss is the most effective means of transferring heat to 

the environment; but, in conditions of extremely high 

humidity or when impermeable clothing is worn (eg, 

the Suit, Contamination Avoidance, Liquid Protective [SCALP] or Toxicological Agent Protective [TAP] 

chemical ensembles), evaporative transfer is greatly 

reduced or blocked and the primary source of cooling 

is convective heat loss.

Radiation. Radiation is often equated with direct 

sunlight and solar load. Radiation from the sun interacts 

with the atmosphere before reaching the body surface. 

The total or net radiant term in the energy balance 

equation is the sum of six different heat transfer components. Five of these components are incoming from 

environmental sources. Solar load is a combination of 

three of those components: (1) direct, (2) reflected, and 

(3) scattered solar radiation. The other three components follow: (4) incoming ground thermal radiation, 

(5) diffuse or sky thermal radiation, and (6) outgoing 

thermal radiation emitted from the body surfaces 

(Exhibit 8-1).

Natural nonionizing radiation from the ultraviolet 

(UV) to thermal or near infrared (IR) spectrum (200 to 

8 • 104 nm) is an important component in determining 

an individual’s thermal balance. The wavelength of 

nonionizing radiation is dependent on the temperature 

of the radiating surfaces (Wien’s law). The sun—which 

is a large, extremely high-temperature source (6,000 

K)—is the principal source of natural radiation. In 

an outdoor environment, the ultimate source of most 

radiation is the sun.

Radiation is normally categorized as short-wave (solar) radiation and long-wave (IR or thermal) radiation. 

UV radiation in the 200- to 400-nm range is included in 

the discussion of the short-wave radiation component. 

Although it is important at the cellular level in terms of 

cell damage and vitamin D production (see Chapter 9), 

UV contributes little to the thermal load. Visible light 

(400–750 nm) is detectable by the human eye; the colors 

seen are actually the light that is reflected by surfaces. 

Much of the long-wave radiation is absorbed by water 

in the atmosphere and reemitted rather than transmitted directly through the atmosphere as direct beam 

radiation. Radiation—particularly in the visible spectrum (400–750 nm)—that is transmitted from the sun 

through the Earth’s atmosphere to the surface without 

being absorbed and reemitted is classified as solar radiation. Solar radiation includes some near-IR radiation. 

Solar radiation is divided into direct, diffuse, and 

reflected solar radiation. The amount of direct solar 

radiation that reaches the body surface (Figure 8-1) 

is dependent on the intensity of the direct rays and 

a combination of the body surface area, posture, and 

position of the body relative to the elevation (solar 

angle) and direction (azimuth) of the sun.21,22 Diffuse 

solar radiation consists of short-wave radiation that is 

scattered or reflected downward as it passes through 

the clouds and atmosphere. In contrast to direct solar 

radiation, diffuse solar radiation is assumed to be an 

isotropic source, so the intensity of diffuse radiation is 

more or less equal on all exposed surfaces regardless of 

the angle of incidence. On overcast days, all radiation 

from the sky is diffuse.

Reflected solar radiation is solar radiation (direct 

and diffuse) that is reflected back from the surface. 

The intensity of the reflected radiation depends on the 

intensity of the incoming radiation and the reflectivity or albedo of the surface. Ice, snow, smooth rock, 

and sand have high albedos. Normally, the ground 

reflects incoming radiation equally in all directions; 

therefore, like diffuse radiation, reflected radiation 

is isotropic. Topography can also affect reflection. In 

alpine areas, the walls of a south-facing cirque can act 

like a parabolic mirror, capturing and intensifying the 

incoming solar radiation so that the cirque functions 

like a low-temperature solar oven23 (Figure 8-2). At 

higher elevations, a combination of the thinner atmosphere, the high albedo of smooth rock, ice or snow, 

and terrain features might significantly increase the 

total amount of incoming solar radiation (including 

UV radiation) and, thus in addition to warming, an 

increased potential for sunburn (Exhibit 8-1).

The remaining radiation components are in the 

thermal or IR spectral range (800–808 • 104 nm). According to the Stefan-Boltzmann equation, an object 

emits long-wave radiation (Rb):

(14) Rb = σ • TK

4 [W • m–2].

The Stefan-Boltzmann constant (σ) is 5.67 • 

10–8 W • m–2 • K–4, and TK is the surface temperature in degrees Kelvin (–

Ts + 273). The atmosphere 

absorbs some of the radiation that reaches the 

outer atmosphere. Ozone absorbs UV radiation, 

and water molecules absorb thermal (IR) energy. 

Energy that is absorbed into the atmosphere warms 

the upper atmosphere. Some of this energy can 

be remitted at a different wavelength, depending 
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

EXHIBIT 8-1

SOLAR LOAD

Importance of Solar Load

The importance of radiation and, more specifically, the solar load (

•

Qs) to the overall level of environmental thermal 

stress is the subject of some debate. The soldier in the field knows that solar radiation is important. Conversely, environmental physiologists have tended to treat •

Qs as a relatively minor contributor to the overall energy balance of 

humans. The methods for estimating radiant load are often based on relatively crude instruments, such as black globe 

thermometers; but, as is often the case, direct measurements of radiation are completely absent. Until recently, popular 

media weather reports offered the public only generalities regarding solar radiation (eg, sunny, partially cloudy, or 

overcast). Indices for ultraviolet exposure and the potential hazards of sunburn have appeared in local weather forecasts during the summer months and year-round in the subtropics (see Chapter 9, Exhibit 9-6). Military meteorological 

databases do not generally record information regarding solar radiation unless black globe temperatures have been 

incorporated into wet bulb globe temperature values. There is an apparent discrepancy between the perceptions of 

field soldiers and their scientific support regarding solar radiation.

As Lotens and Pieters(1) have indicated, the real importance of •

Qs occurs when the overall level of thermal stress is high. 

A radiant gain of 50 W is relatively small; however, if not counterbalanced by an increase in evaporative or convective 

heat loss, this small imbalance will represent the continual accumulation or storage of energy within the body tissues 

and a slow, gradual increase in body temperature. When overall stress levels hover at a critical threshold, a small heat 

gain can be the factor that upsets the balance.

A simplified calculation for a whole-body heat storage would be: 

(1) ∆S’ = cp • m • ∆Tb [kJ].

The specific heat of body tissue (cp) is approximately 3.49 kJ • kg–1 • °K–1. We can assume that, at a rectal temperature 

(Tre) of approximately 39.5°C, 50% of a military population would exhibit hyperthermic symptoms. Assuming that 

a neutral initial Tre is 37°C, an increase in Tre to 39.5°C approximates an increase in body temperature (∆Tb) of 2.5°C. 

For a 70-kg individual, if •Qs = 50 W, constant exposure to that level of solar load would result in hyperthermia in approximately 4 h.

(2) ∆S’ = 3.49 • 70 • 2.5 [kJ]

 = 610 kJ

= 610 • 103 J.

(3) •Qs = 50 [W = J • s–1].

(4) t = 610 • 103 ÷ 50 [s]

= 1.22 • 104 ÷ 3,600 [h]

 = 3.4 h.

For •Qs = 200 W, the estimated time to hyperthermia would be one-fourth, or approximately 51 min. In actuality, most 

people would not “cook-off” in 51 min. Instead, thermoregulatory mechanisms, particularly the sweat mechanism, 

would be strained to compensate for an additional 200 W.

Breckenridge and Pratt(2) demonstrated how the solar load was offset by an increase in evaporation by using the increase 

in evaporative loss to estimate difference in solar load on soldiers wearing different colors and shades of uniforms. 

They estimated the solar load for soldiers in a black uniform to be 169 W (145 kg-cal • h–1) and for soldiers in a white 

uniform to be 107 W (92 kg-cal • h–1)—a difference of 62 W. Estimates for green and khaki uniforms made the following year were 131 W and 107 W, respectively, which is a smaller difference of 24 W.

The question posed by Breckenridge and Pratt(2) and other empirical studies(3,4) that found •Qs for clothed mannequins 

ranging from 11 to 166 W is whether or not the solar contribution to energy balance is significant. The general consensus 

of environmental physiologists and modelers has been that the importance of •Qs relative to other thermoregulatory 

factors is small and that the additional effort required to obtain a more accurate estimate of •Qs is not justified. The 

corollary to this conclusion is that, if there is only a limited need to quantify •Qs accurately, there is less need to be 

(Exhibit 8-1 continues)
211

Evaluation of the Thermal Environment

concerned about any difference in between uniforms. The preceding calculations, which demonstrate the potential 

impact of even a 50 W •Qs , suggest that solar radiation can be an important factor in thermal balance, but this does not 

directly address the issue of whether or not a more accurate estimate of •Qs is a priority.

Clothing, Color, and •Qs

It would seem intuitively obvious to anyone standing in direct sunlight that exposure to solar radiation is a significant 

factor; yet, when energy balance equations are computed, the relative contribution of radiation to the overall energy 

balance can seem relatively small. Converting those observations to a complete understanding of the full impact of 

exposure to radiation under field conditions is difficult. Sunburn is a clear hazard, and the degree or severity of the 

burn can be directly related to the cumulative exposure to total solar radiation, but is not necessarily a reliable indicator 

of •Qs because sunburn impacts bare skin, whereas •Qs can be mitigated by clothing. Conversely, the change attributed 

to moving from full sunlight to shade is quite real, but some of the effect can be from thermal or long-wave radiation 

being reemitted by the ground or sun-heated walls.

One source of some confusion regarding solar radiation is an apparent paradox; •Qs is smaller for clothed individuals 

than nude individuals. The intuitive response is that, because clothed individuals are warmer than near-nude individuals in the same environment, •

Qs should be greater when clothed. When you go to the beach, would you put on 

clothing to reduce your •Qs? Actually, the answer is yes. Just as you would wear clothing to reduce your ultraviolet 

exposure (and sunburn), clothing will reduce the absorption of solar energy in the longer wavelengths. The apparent 

paradox happens because, whereas clothing reduces solar gain, it also reduces heat loss by convection and evaporation. Because radiation is usually a smaller factor in the overall heat balance, the net result is that the clothed state is 

warmer than the nude state. But, if you want a greater •Qs, take off your clothing.

In respect to solar load ( •Qs ), the body is essentially a solar collector. The total amount of energy absorbed by the collector is dependent on the intensity of the solar radiation (I), the surface absorptivity (α) and transmission (τ), and the 

exposed surface area (A). For bare skin, absorptivity (α) varies with the degree of skin pigmentation. The approximate 

range of α is 0.4 to 0.9, but there is considerable variability between people and source wavelength.(5–8)

Clothing modifies •Qs in two ways: (1) it modifies all energy exchange pathways by changing the areas for energy 

exchange and (2) it modifies the rate of heat transfer. It slightly increases the effective absorption surface area (Ae) and 

modifies the amount of radiation that is absorbed. Clothing increases the outer surface just as increasing the diameter 

of any cylinder would increase its surface area. A clothing area factor (f

cl) is used to adjust Ae for clothing.

The second factor is clothing insulation. Clothing insulation refers to the resistance of clothing to heat transfer by 

convection and radiation. In practice, clothing insulation decreases the rate of heat transfer by convection, radiation, 

and conductance. Radiation that is transmitted directly through clothing mass retains its full wavelength distribution, 

whereas radiation that is absorbed at the clothing surface can heat the clothing surface or increase the temperature 

of the clothing layer. Some absorbed radiation can be re-reflected multiple times within the clothing material and 

eventually reach the body surface without any change in wavelength. This reflected solar component is technically 

transmitted rather than absorbed. Solar radiation that is absorbed by the clothing can be stored in the clothing layer, 

thereby raising clothing temperatures, or it can be lost by emission, conductance, or convection to the body or the 

environment. Breckenridge and Goldman(3,4)used an insulation efficiency factor (U), derived from clothing insulation 

values, to indicate the portion of absorbed solar radiation that was eventually transferred to the body.

Data sources: (1) Lotens WA, Pieters AMJ. Transfer of radiative heat through clothing ensembles. Ergonomics. 1995;38:1132–1155. (2) 

Breckenridge JR, Pratt RL. Effects of Clothing Color on Solar Heat Load. Natick, Mass: Environmental Protection Branch, Quartermaster 

Research and Engineering Center; 1961. Technical Report EP-155. (3) Breckenridge JR, Goldman RF. Solar heat load in man. J Appl 

Physiol. 1971;31:659–721. (4) Breckenridge JR, Goldman RF. Human solar heat load. ASHRAE Trans. 1972;78:110–119. (5) Hardy JD. 

Physiological effect of high intensity infrared heating. ASHRAE J. 1962;4:36–42. (6) Roller WL, Goldman RF. Prediction of solar heat 

load on man. J Appl Physiol. 1968;24:717–721. (7) Fanger PO. Thermal Comfort. New York, NY: McGraw-Hill; 1970. (8) McIntyre DA. 

Indoor Climate. Essex, England: Applied Science Publishers Ltd; 1980. 

Exhibit 8-1 continued

on the temperature of the emitting molecules in accordance with the Stefan-Boltzmann equation. The source 

of this sky thermal radiation is the atmosphere; thus, it 

is another diffuse, isotropic source. In a similar manner, a portion of the radiation striking the earth might 

be absorbed, thereby warming the ground. Ground 

thermal radiation can be calculated in the same manner as a function of surface temperature. Incoming 

thermal radiation is isotropic and absorbed over the 

same surface as diffuse and reflected solar radiation.
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

Whether or not radiation is absorbed, reflected, or 

transmitted depends on the physical properties of the 

surface. The absorptivity (α) of a surface determines 

the absorption of radiant solar energy in the visible 

spectrum, whereas emissivity (ε) determines the absorption and emission of thermal radiation. Because 

α refers to the visible range, color and surface texture 

are also factors. In the IR range, texture is the primary 

clue to (ε) properties.

Under most circumstances, the meteorological information required to separate incoming radiation into 

the incoming solar and IR components is not available. 

Instead, values for the effective radiant field (Re) and 

mean radiant temperature (Tr) are estimated. The assumption behind these two terms is that the radiant 

load is distributed equally over the body. The effective 

radiant field (Re) is the total radiant energy exchange in 

an imaginary isothermal black enclosure at Tr if body 

surface temperature (–Ts) equals Ta.

14 hr is the radiant 

heat transfer coefficient.

(15) Re = hr • (–Tr – Ta) [W • m–2].

As discussed in the instrumentation section, Tr can 

be measured with a relatively simple instrument: the 

black globe thermometer. The globe thermometer essentially integrates radiation into a single net value. 

In contrast, attempting to sum the radiation term (R) 

from six or more separate components is much more Direct Solar Load (W)

300

250

200

150

100

50

0

Calculated

16 Aug

17 Aug

Time

6 8 10 12 14

Fig. 8-1. Solar load ( •Qs) for direct solar radiation for a standing 

figure without clothing (α = 0.68). Because of the interaction 

of solar intensity and solar angle, the maximum •Qs does not 

coincide with the maximum solar intensity, but does at the 

optimum combination of surface area exposure and solar 

intensity. Discrete values were based on measurements of 

incoming solar radiation at Fort Bliss, Texas. Calculated values 

were derived using input values from a solar radiation model.

complex. The concept of Tr is widely accepted by 

physiologists, but might present some problems in 

outdoor environments where the solar load is a more 

significant factor in the thermal environment. The 

radiative heat transfer coefficient (hr) may be derived 

from the Stefan-Boltzmann law:

(16) hr = 4 • (Ar/AD) • σ • ε • [(–Ts + –Tr)/2 + 273.2]3 

[nude, W • m–2 • K–1]

(17) hr = 4 • (Ar/AD) • σ • ε • f

cl • [(–

Ts + –Tr)/2 + 

273.2]3 [clothed, W • m–2 • K–1].

The ratio of radiated surface area (Ar) to AD is approximately 0.71 for a seated or standing individual.24,25

The surface temperature (–Ts) may be the mean skin temperature (–

Tsk) for the nude condition or the mean clothing surface temperature (–

Tcl) for a clothed individual. 

The Stefan-Boltzmann constant (σ) and emissivity (ε) 

were introduced earlier. An additional term, the ratio 

of the outer clothed surface area to the skin surface 

area (f

cl), is introduced into the equation to adjust for an 

increase in surface area if clothing is worn.14 In parallel with the cooling efficiency factor (Fcl), the clothing 

area factor (f

cl) is 1 without clothing. If a calculated or 

direct measurement value is available for Rg, (–Tr) can 

be approximated by adding a fixed value to –Ta using an 

equation developed by Matthew and colleagues.26 As 

long-wave radiation varies with –Tgrd and –Tsky temperatures, the equation is adjusted for different values of –

Ta. A 

rougher approximation based on assumptions regarding the range of Rg

 values is no sun ([–Tr] = –Ta), overcast or

cloudy ([–Tr] = –Ta + 20), partially cloudy ([–Tr] = –Ta + 35), and 

clear sky ([–Tr] = –Ta + 50).26

The relationship between the net radiation heat 

exchange (R) is derived from Re, Tsk, and hr by the 

equation14: 

(18) R = Re + hr (Ta – –Tsk) [nude, W • m–2].

If clothing is worn, the cooling efficiency factor (Fcl) 

is added to the equation:

(19) R = Fcl Re+ hrFcl(Ta – –Tsk) [clothed, W • m–2].

One interesting point is that wearing clothing 

significantly reduces the solar load22,27 by insulating 

the body from external radiation. Relative to heat 

transfer by evaporation, the solar load would seem 

to be unimportant. However, as noted by Lotens and 

Pieters,28 when the body’s thermal state is at a critical 

level, even a relatively small change in the solar load 

can be an important factor. In adjusting the original 

Givoni-Goldman heat strain model, Shapiro et al29 and 

Moran et al30 essentially make that point.
213

Evaluation of the Thermal Environment

Insensible Heat Exchange

The evaporation of sweat is potentially the most effective physiological mechanism for transferring heat 

from the body to the environment. When a favorable 

water vapor pressure gradient exists, evaporative 

heat loss is the dominant mechanism for maintaining thermal balance. Above an air temperature of 

35°C, evaporation is the only effective physiological 

mechanism for heat loss.27 During military build-ups, 

when personnel are abruptly introduced into thermally stressful environments, an important objective 

is the physiological adjustment of thermoregulatory 

mechanisms. The acclimatization process, which is 

also important during basic training of new personnel 

in warmer climates, is characterized by a more rapid 

onset of sweating. An acclimatized individual activates 

thermoregulatory sweating at a lower threshold or 

set point, which in turn reduces heat storage (S) and 

minimizes the increase in Tb. 

Adolph27 cited a maximum verifiable sweat rate 

of 1.7 l • h–1, but indicated that the actual maximum 

sweat rate was 3.5 l • h–1. Clark and colleagues3 cite 

maximum and sustained sweat production levels of 

2.1 l • h–1 and 0.9 l • h–1, respectively. The latent heat 

of vaporization for sweat is 2.43 kJ • g–1, which would 

result in a maximum cooling potential of 1,215 W and 

608 W, respectively. 

The driving force for evaporative heat exchange 

is the water vapor pressure gradient from the skin to 

the environment. 

(20) E = ω • he • (Ps,sk – Pw) [W • m–2].

Skin-wettedness (ω) is the ratio of the skin area 

completely covered by sweat to total skin surface area. 

Values for ω range from 0.06 (no active sweat) to 1.0.1

Many thermal models assume that, for an acclimatized 

individual, skin-wettedness is 100% (ω = 1.0). Saturated water vapor pressure (Ps

) can be obtained from 

a table or calculated for air temperature range from 

0°C to 60°C using Antoine’s equation14: 

(21) Ps= exp(16.6536 – [4030.183/(t – 235)]) [kPa].

Pa can be derived, for a given Ta, from the saturated 

vapor pressure (Ps) and ambient relative humidity. 

For example, at 25°C and 70% relative humidity, Ps

is 3.0 kPa and Pa is 70% of that value, or 2.1 kPa. If 

the convective heat transfer coefficient (hc) is known 

or calculated from wind speed and air temperature, 

the evaporative heat transfer coefficient (he) is usually 

Fig. 8-2. Comparison of (a) solar collector to (b) alpine 

cirques, depressions that can act as natural solar collectors. 

These alpine cirques are several retreating, unnamed, small 

valley glaciers and several recently deglacierized cirques and 

ridges north of the terminus of Tonsina Glacier, north central 

Chugach Mountains, Alaska. Much of the ice disappeared 

during the last few decades of the 20th century.

Illustration: Reproduced with permission from AMETEK, 

Inc (Radnor, Pa). 

Photograph: Courtesy of Bruce F. Molnia, US Geological 

Survey. Available at: http://www.usgs.gov/images/topical/glaciers/dsc00053.jpg. 

a b
214

Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

derived using the Lewis relationship (LR).31 At sea 

level, LR is the ratio between evaporative (he) and 

convective (hc) heat transfer coefficients (LR = he/hc).

(22) he= LR • hc [W • m–2 • kPa–1].

For air, LR is a constant value (16.5 K • kPa–1). 

Evaporative heat transfer is analogous to convective 

heat transfer, except that the driving force is the difference in water vapor pressure rather than a temperature 

gradient, thus the concept of a drying or desiccating 

wind. LR is called an analogy because the theoretical 

basis for the relationship is not sound. However, the 

only practical alternative to computing he using hc or 

environmental parameters and the LR is to derive the 

coefficient from experimental data. 

Although in unusual circumstances moisture can 

condense on body surfaces, evaporative heat exchange 

is primarily a pathway for net heat loss from the body 

surface to the environment. When clothing is worn, 

water evaporated from the skin surface can condense 

in the clothing, a form of heat regain, as the heat of 

vaporization is released in the clothing layer. However, regain in clothing represents a disruption of 

evaporative heat loss from the body rather than a net 

gain of heat from the environment, and it is possible 

for some of the regain to be lost by either radiation, 

convection, and reevaporation from the clothing to 

the environment. 

Considerable effort has been expended on the preparation of psychrometric charts that demonstrate the 

thermodynamic relationship between air temperature 

and humidity parameters, such as wet bulb or dewpoint temperature to relative humidity or water vapor 

pressure32 (Figure 8-3). Typically, the horizontal axis is 

air temperature, the top curve represents a saturation 

temperature (dew-point or wet bulb temperature at 

saturation), and the vertical axis is an expression of 

the moisture content of the air (water vapor pressure 

or density, relative humidity, or multiple scales). These 

Fig. 8-3. Basic psychrometric chart (barometric pressure 1,000 mb [750 mm Hg]) illustrating the relationship between temperature and different measurements of humidity. If temperature is known, most humidity measurements can be converted 

into other formats. WB: wet bulb.

Illustration: Adapted with permission from Ellis FP, Smith FE, Walters JD. Measurement of environmental warmth in SI 

units. Br J Ind Med. 1972;29:361–377. 

0.044

0.042

0.040

0.038

0.036

0.034

0.032

0.030

0.028

0.026

0.024

0.022

0.020

0.018

0.016

0.014

0.012

0.010

0.008

0.006

0.004

0.002

0.000

Water Content (kg/kg of dry air)

Dry Bulb Temperature (°C)

Vapor Pressure (mm Hg)

48

45

42

39

36

33

30

27

24

21

18

15

12

9

6

3

0

Wet Bulb Temperature (°C)

Vapor Pressure (mb)

64

60

56

52

48

44

40

36

32

28

24

20

16

12

8

5 10 15 20 25 30 35 40 45 50 55 60 65

90%

80%

70%

60%

50%

40%

30%

20%

10%

100%

WB 40
215

Evaluation of the Thermal Environment

charts provide a valuable illustration of the thermodynamic relationship between air temperature and the 

capacity to retain water vapor. More elaborate psychrometric charts can be used to illustrate graphically the 

heat balance equation.33 Some models used to predict 

heat strain are constructed as modified psychrometric 

charts or nomograms.34,35From a practical perspective, 

attempting to use a page-sized chart to obtain more 

than an estimate of psychrometric relationships is 

sometimes difficult.

Other Factors

Common vs Special Environmental Factors

The four basic pathways for heat exchange and 

measurement of environmental parameters determine 

the potential thermal stress in a specific environment. 

These basic relationships exist regardless of the branch 

of service, task, or specific environment. What can vary 

between environments is the relative importance of 

each pathway. The same basic meteorological parameters still define that potential for energy exchange; but, 

in some environments, the importance or mix of factors 

varies. There cannot be, for example, any significant 

evaporative heat loss to the environment unless there 

is a favorable water vapor pressure gradient. If the 

air is saturated at 100% relative humidity for a given 

Ta, the ambient pressure (Pa) is equal to the saturated 

pressure (Ps), and if Ta ≈ –Tsk, the pressure gradient (ΔP) 

is greatly reduced, and there will be little or no evaporative heat loss even if the skin surface is completely 

wetted (ω = 1.0). However, if –Tsk > Ta, the vapor pressure 

at the skin surface will be greater than the ambient Pa, 

regardless of the relative humidity, and evaporative 

heat loss can occur. Base security personnel (regardless of their branch of service), who perform similar 

duties, are usually exposed to similar environments. 

Base security personnel normally operate in “built 

environments,” where hardtop road surfaces and 

buildings are common features of the environment. 

However, to provide security at more remote sites 

on a base, a military security unit can work more 

like a combat patrol, either from vehicles or on the 

ground. What can vary between environments are 

special conditions (eg, proximity to airstrips, large 

hangars, or other maintenance buildings) that are 

neither outdoor nor normal building interiors. Even 

more varied are the vehicles, ships, and aircraft operated by even one service branch. Ships, for example, 

are basically metal islands that shift orientation and 

position. Except on minesweepers, the metal on ships 

increases the potential for conduction and reflected 

or emitted radiation.

Pressure

At low elevations, atmospheric pressure is measured 

primarily to correct other meteorological values relative 

to standard temperature and pressure conditions. At 

higher elevations, reduced pressure has an important 

effect on oxygen saturation curves. Consequently, atmospheric pressure is a more critical environmental parameter in alpine environments. In terms of heat exchange, 

as atmospheric pressure decreases, air density and convection heat loss decline. The reverse occurs with evaporative heat transfer: as pressure decreases, the potential 

for evaporative heat transfer increases. Adjustments for 

the effect of reduced pressure on convective and evaporative heat transfer coefficients are usually presented as 

exponential equations,14 although Chang and Gonzalez36 have suggested a linear relationship. Barometric 

pressure is also used to predict changes in weather 

from movement of high- and low-pressure air masses.

Precipitation

Rain. There is no doubt that standing in cold shower 

water will cause rapid heat loss. Conversely, to rewarm 

an individual, a hot shower is nearly as effective as 

immersion. However, under normal circumstances, 

military personnel caught in the rain are clothed, 

and the duration of the exposure is usually relatively 

short. The thermal consequences of exposure to rain 

thus depend on the prevailing air temperature and 

the extent to which insulation has been reduced by 

water saturation. The probability of an adverse effect 

increases with the amount and frequency of rainfall, 

but quantifying the amount of rain has limited value 

in predicting any direct effect.

Snow. Although direct contact of exposed skin to 

snow results in rapid heat loss through the melting process (the skin gives up 79 calories of heat for each gram 

of snow melted), most of the skin area is covered. The 

more important indirect effect of snow is the progressive wetting of clothing and the consequent reduction in 

insulation qualities. The Extended Cold Weather Clothing System incorporates a breathable, water-resistant 

membrane in the outer shell garments to minimize 

that problem. In extreme cold, snow is less likely to 

melt on clothing surfaces, so there is less need for water 

repellency. Deep snow and frozen ground increase the 

potential for cold injury. Snow varies in moisture content 

and structure, and those characteristics may change over 

time in response to wind and temperature. The potential 

danger presented by snow depends on the interaction 

between air temperature and snow. When melting snow 

soaks into clothing, cold injuries can occur even when 

air temperatures are well above 0°C.
216

Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

Temperature of Bodies of Water

Although water is not a standard weather data 

element, in training or operational settings, tactical 

movement can require partial or near complete immersion in littoral ocean water, lakes, ponds, swamps, 

marshes, streams, and rivers. Factors that influence 

the thermal effect of water immersion are duration, 

depth of immersion (eg, feet, knees, and torso), water 

temperature, and flow rate. For water or any fluid, 

temperature differences are the driving force for 

convective transfer. Water temperatures can be stratified; thus, it might be valuable initially to measure a 

vertical profile of water temperatures over the range 

of exposure depths. 

A particularly difficult topic to assess is partial 

immersion. The 1996 deaths of Rangers in training 

were probably attributable to hypothermia in the aftermath of partial immersion. Marine and SEAL (SEa, 

Air, and Land) units frequently operate in the coastal 

environment where partial and periodic immersions 

are normal requirements of their duties. Flooding and 

stream crossings can make immersion a factor in desert 

operations as well.

Meteorological Parameters

Meteorological measurements that define the 

thermal environment are (a) temperature, including 

ground and air temperatures; (b) wind speed and direction; (c) humidity; and (d) the separate components of 

radiation, including solar (direct, diffuse, and reflected) 

and IR (sky and ground) parameters. Other relevant influences may include other surface temperatures, such 

as tools, fuel pump handles, armored vehicle surfaces, 

and asphalt. An awareness of diurnal and seasonal 

cycle variations is also important. Some cycles, such as 

solar radiation, are obvious. Other less apparent cycles 

include (a) the different lags in response time between 

ground and air temperature, (b) the skewed distribution of air temperature because of a combination of 

sunlight and ground warming, (c) temperature-related 

variation in relative humidity, and (d) less predictable 

diurnal variations in wind speed and direction.

MEASUREMENTS 

There are many ways to measure the four basic 

heat transfer parameters. Factors that determine how 

measurements are made include the following: (a) complexity of the environment, (b) meteorological scale, 

(c) measurement objectives, (d) individual parameter 

measurements, (e) other meteorological parameters, 

and (f) stations and instrument packages.

Complexity of the Environment 

Local conditions can significantly alter each of the 

four basic heat transfer parameters. It is important to 

measure parameters at the operational site. In addition, 

measurements should be appropriate to the tasks or 

activities. Normally, air temperature is measured near 

the center of mass or head level. However, in some 

environments, especially deserts, the vertical temperature profile can be extreme. The air temperature 

at head level may be in the 40s (Centigrade), whereas 

at ground level it can be in excess of 55°C. The thermal 

stress experienced by a prone rifleman will be underpredicted on the basis of a measurement taken at a 

standard height. If defensive earthworks are used, even 

subsurface temperatures can be important. The presence of overhead vegetation, cloud cover, large bodies 

of water, or rock might affect the daily temperature 

distribution pattern.

Wind is channeled by topographic features. Surface 

roughness or vegetation affects airflow patterns in a 

vertical profile analogous to the temperature profile. 

There are also daily patterns of air movement associated with large bodies of water and upslope and 

downslope air drainages. Cold air drainage can be of 

particular importance in alpine regions.

The site exposure relative to the position of the 

sun is important when solar radiation is considered. 

A field position on the military crest of a south-facing 

slope will receive significantly more solar radiation 

than a similar position with a northern exposure. 

Certain surfaces (eg, sand or snow) have high surface 

reflectance or albedo, which affects local radiation 

levels. Sunburn and eyestrain can also result, but 

those effects are not thermal effects in terms of heat 

exchange. Cloud cover significantly alters the characteristics of radiation, as will various types of overhead 

cover (Figure 8-4). Air moisture content also displays 

a vertical profile and is affected by movements of 

air masses, local precipitation or surface water, and 

vegetation.

In planning operations from a remote site, it is 

useful to recognize that climatic data can be mean 

values that were measured at sites favorable to human habitation. An average relative humidity of 50% 

does not guarantee that periods of extreme humidity 

cannot occur. Humidity will vary with the movement 

of different air masses through an area or even with 

changes in wind direction. The diurnal cycle of air 

temperature is mirrored by the variation in relative 
217

Evaluation of the Thermal Environment

humidity. When air temperatures are typically lowest 

just before sunrise, relative humidity is often at the 

daily high (Figure 8-5). Monthly mean air temperatures might obscure an extreme range in both daily 

temperatures and between days. Rain on cold days can 

also have a short-term effect on heat exchange, even 

with waterproof clothing; but, it is generally treated as 

insignificant short-term phenomena when analyzing 

the thermal environment.

Site Heterogeneity (Landscape)

In an attempt to integrate ecology with other cultural and economic factors, a new discipline called 

“landscape” emerged in the 1980s. One of the useful 

elements of landscape was the basic premise that the 

environment was a physical mosaic of patches, rather 

than a homogeneous state, and that target populations moved within that mosaic.37 This concept of 

heterogeneity or patchiness succinctly delineates the 

basic problem in applying laboratory models to the 

field environment. The real world is often not a good 

laboratory because the environment and interactions 

within the environment are too complex. Landscape 

methods were developed to apply models to a patchy 

environment. A relatively simple method is to identify 

and measure characteristic regions or zones within the 

total environment and then do a time-space study to 

determine when and how long each zone is occupied. 

The International Organization for Standardization, in 

Standard 7933,38 suggests such a weighting procedure 

to accommodate workers who move between thermal 

environments.39 Each zone or microenvironment might 

display cyclical behavior, such as diurnal or seasonal 

variability. Site-specific features include the following: 

• north-south facing slopes, 

• leeward-windward sides of an island, 

• vegetation, 

• road surfaces, 

• hanging troughs or cirques (mountains),

• depressions or hollows

• elevated terrain,

• land-sea breezes, 

• upslope and downslope winds, and 

• lower air temperatures associated with bogs 

and other wetlands.

Urban Environments

The focus on thermal environments tends to be 

either large-scale, remote field sites or indoor urban 

locations. One of the most complex environments 

for military operations is an urban area, either damaged or intact. All the thermal elements of a natural 

environment are present, but the urban topography 

is complex. A notable feature is wind channeling, 

Fig. 8-4. Effect of cloud cover on direct and diffuse solar 

radiation measured at the US Marine Corps Mountain Warfare Training Center, California, on September 9, 1994, as 

demonstrated by comparison of clear and cloudy portions 

of plot.

Direct Solar

Diffuse Solar

Cloudy

Solar Radiation

1,200

1,000

800

600

400

200

0

0 2 4 6 8 10 12 14 16 18 20 22

Time

Fig. 8-5. Demonstration of the relationship between measurements of air temperature (Ta

) and relative humidity (RH). 

When air temperatures reach the daily maximum because 

of increased moisture capacity or saturation level, relative 

humidity is minimal. The situation is reversed at the morning 

minimum temperatures. Measurements were taken at the US 

Marine Corps Mountain Warfare Training Center, California, 

on September 4–8, 1994.

Air Temperature (°C)

Relative Humidity (%)

Time (hours)

RH

Ta

35

30

25

20

15

10

5

0

60

50

40

30

20

10

0

0 12 0 12 0 12 0 12 0 12 0
218

Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

because winds are compressed and funneled between 

buildings as they would be in a natural canyon. 

Another canyon feature is the difference between 

north- and south-facing solar exposures. Relative to a 

position on earth, the sun travels an east-to-west pattern, reaching its zenith in the northern hemisphere, 

directly south of a site. Areas with moderately tall 

buildings on streets running north-south often have 

a definite shift throughout the day between shady 

and sunny areas of the street. Initially, direct solar 

radiation will strike buildings on the west side of 

the street, leaving the east side in the shade; but, as 

the sun rises and moves from east to south, the entire 

street is illuminated by the high, southern sun. As the 

sun sets to the west, the west side of the street is cast 

into the shadows, and the east side now receives most 

of the available direct solar radiation. A combination 

of tall buildings and a low winter sun can result in 

very little solar radiation or light reaching the streets. 

On a smaller scale, trees along a street can cause a 

pattern of alternating shade and sun-warmed patches; 

variations in surface texture can also contribute to the 

complexity of the urban environment. Asphalt heats 

rapidly because of solar radiation, whereas a smooth 

concrete surface can reflect radiation and heat less 

rapidly. A grassy surface will be cooler, and shrubs 

might break up strong direct winds and create more 

turbulence. 

Interior Environments

The distinctions between outdoor or field environments and those inside buildings can vary in the relative importance of different environmental parameters, 

but the same four parameters still determine the thermal environment. Building interiors cover an almost 

infinite variety, ranging from modern high-rise offices 

to unheated garages and hangars. In a closed, climatecontrolled building, thermal conditions are often referred to by room temperature, despite the contribution 

of the other parameters, because all four parameters 

are essentially constant at a given temperature setting.

A building with climate control (eg, heating, ventilation, and air conditioning) is generally not an extreme 

environment. Inside most buildings, the intensity of 

activities and the associated metabolic heat production 

is often in a narrow, relatively low range. Individuals 

can be uncomfortable, and this might adversely affect 

performance or morale, but environmental conditions 

are less likely to have an acute effect on health. Occupants of an office building have several options to 

adjust to an uncomfortable climate, including altering 

heat flow, ventilation, and air-conditioning, or adjusting clothing when humidity is low enough to allow 

some evaporative loss.

A modern building without climate control—as can 

happen as a result of a massive power failure, catastrophic weather events, or warfare—is quite another 

situation. The heat balance of an unheated building is 

similar to that of the human body without any metabolic heat production and greatly reduced evaporation. 

The lag between external conditions and a significant 

change in interior temperatures is sometimes attributed to thermal inertia. Changes in temperature are 

slow because of the large mass of the building and the 

rate of heat transfer at the building surface. 

When the mass of the building reaches a low, even 

temperature with the mass of the walls and building 

contents at a uniform temperature, the building may 

be termed “cold-soaked.” Confounding factors include 

external heat sources (eg, solar radiation) or poor insulation provided by a large glass expanse. A temperature 

difference in the opposite direction will obviously 

reverse the direction of the net heat exchange, and 

the building will become “heat soaked.” However, the 

basic principles are the same. 

Buildings are difficult environments to assess because a building is an ensemble of separate microscale 

environments, often with asymmetrical elements. The 

combination of doorways, windows, climate control, 

and furniture may be unique. Often, interior spaces 

must be quantified on a smaller scale (ie, each desk and 

occupant). The same basic rules apply, but asymmetry 

and smaller scale variability make it a more difficult 

process. Windows and point heat sources (eg, radiators) produce an uneven exposure to radiant sources. 

McIntyre25 has emphasized that asymmetry can contribute to a sense of discomfort. Major radiant sources 

(eg, ship engines) also contribute to asymmetry. Electronic office equipment can be another heat source. To 

evaluate an indoor thermal environment, an IR scan 

can be useful in pinpointing asymmetry of radiation 

sources, including sources and sinks in the windows 

and walls. The building interior is spatially complex, so 

drafts can be detected best with a handheld electronic 

anemometer, which is sensitive to low air movement 

and can be easily used at different heights and small 

spaces (eg, underneath desks).

A factor that is not directly related to the thermal 

environment is clothing. The amount of clothing worn 

and its distribution on the body can vary significantly, 

but building climate control systems are based on the 

presumption of a certain level of clothing insulation. 

It can be unfashionable or it can violate dress codes to 

match the building thermal environment. For example, 

military personnel must often conform to specific 

guidance regarding clothing. It is difficult to conform 

to a common clothing standard in a building with a 
219

Evaluation of the Thermal Environment

variable set of building microclimates. Of course, activity level, age, and body mass are also factors that can 

dictate differences in dress. 

Nonurban Environments

Environments with gentle topography and open, 

low vegetation (eg, plains, farmland, open marshes, 

and open deserts) offer relatively simple landscapes, 

whereas the topography and orientation of mountains and canyons clearly parallel the complexity of 

the urban environment. Just as navigation by Global 

Positioning Satellites is sometimes blocked in mountains, canyons, or a city, wind and solar radiation are 

blocked or channeled by broken topography. Airflow 

can follow distinct daily upslope and downslope 

patterns in the mountains as cold, denser air flows 

down and warmer air rises from the valley and basin 

floors. Winds can also be channeled through canyons 

or ravines so that velocity and direction vary with 

topography. In areas with large bodies of water, the 

daily variation in onshore and offshore winds is also 

a factor.

On a finer scale, broken terrain or vegetation creates 

air turbulence as the smoothly flowing upper air contacts boulder fields or patches of tall and short vegetation. There are often distinct differences in vegetation 

on canyons and slopes that are orientated north and 

south, because the variation in solar radiation affects 

temperature and soil moisture. In high mountain 

ranges, the leeward side of mountains can receive 

significantly less moisture than the windward side. 

These phenomena are particularly well known in the 

western United States, where the drier eastern side of 

the ranges is said to be in a “rain shadow.” Heavy or 

patchy vegetation also creates a complex landscape. 

Extreme cases include the infamous triple canopy 

forests of Southeast Asia.

Military Interior Environments

The interiors of armored vehicles, helicopters, and 

different fixed-wing aircraft (eg, fighters, high-altitude 

bombers, tankers, cargo planes, and other forms of 

air transport) are all unique environments. Human 

factors, including the potential for thermal stress, are 

a design element for each of these mobile environments. The actual or resultant environment can be a 

product of external meteorology, the parameters and 

physical properties of the vehicle (eg, mass, material 

properties, and color), internal heat sources (engines), 

climate control, and the metabolic heat production of 

the crews. In considering just the thermal environment 

of an armored vehicle, the vehicle’s interior environment could be modified by (a) varying the amount 

of exposure to solar radiation from full exposure to 

full shade, (b) either opening or completely buttoning 

down hatches, (c) turning the engine on or off, and (d) 

noting the presence or absence of crew members. Each 

of those conditions can vary, depending on the tactical 

situation, concerns over acoustic or thermal image, and 

other such situations.

Meteorological Scale

The area of operation for a military unit varies with 

the unit’s level of organization. A progression from 

platoon to corps or army level encompasses a larger 

and larger area. In terms of meteorology, the scale of 

concern expands in a similar manner. Macroscale or 

synoptic meteorology considers the general distribution of air masses and pressure over a large area (eg, 

an entire theater of operation). The general weather 

maps of the United States and Europe showing air 

masses, pressure isobars, and other data are macroscale 

or synoptic weather maps. The primary focus of synoptic weather is areas of high or low pressure, general 

wind patterns, and the movement of large air masses 

or fronts. The clearest images of synoptic patterns are 

obtained via satellites or other remote sensing systems. 

The time scale for synoptic weather might be in days 

or weeks. 

General weather patterns reflect the trends in longer 

term forecasts, but synoptic patterns often cannot adequately forecast the weather that will be experienced 

by an individual or unit operating within a relatively 

smaller area. Synoptic weather indicates the general 

trends or conditions or potential for weather events (eg, 

the percentage of probability of local precipitation), but 

it does not provide adequate or sufficient information 

for local or immediate conditions.

Mesoscale meteorology concerns local weather patterns on a time scale of minutes to days. As a weather 

front moves through an area, the distribution of local 

winds—the dominant mesoscale feature—determines 

the local weather events that affect individuals and 

small units. The local weather radar, often presented 

with television weather reports, is concerned with 

mesoscale weather.

The dividing line between mesoscale and microscale weather is not entirely clear. The weather 

that individuals experience directly is microscale 

weather. Some meteorologists consider local variations in wind patterns (eg, upslope or onshore winds 

caused by local topographic or geographic differences) to be microscale events. Other meteorologists, 

perhaps depending on the area or scale of the effect, 

can see the same phenomena as a mesoscale event. 
220

Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

EXHIBIT 8-2 

OPERATION OVERLORD

The decision to proceed with the Normandy invasion on June 6, 1944 illustrates the interaction of climatic data and 

weather forecasting. During World War II, from the Russo-Finnish War (1939–1940) to the Battle of the Bulge in the 

Ardennes (1945), weather was both a strategic and personal factor that separated victory and defeat, although it had 

more to do with the strategic concerns than individual physiology.

The information in the following account is based on official military histories by Harrison(1) and Pogue.(2)Unless otherwise indicated, both sources present the same information in approximately the same detail. The two sources focus on 

the requisite conditions for the invasion and the events leading to the final decision to commit forces on June 6. There is 

no direct indication that the physiological effects of water or air temperatures were a factor in selecting D-Day. There is 

some indication of concern that fall and winter conditions might impend later operations. Harrison(1) stated in reference 

to delaying the invasion until June 19 that “ . . . the longer the postponement, the shorter the period of good campaign 

weather on the Continent.” However, a more important concern was the difficulty in maintaining secrecy and morale if the 

invasion was delayed. It should be noted that operational weather concerns generally focus more on mobility and logistical problems than on soldier discomfort or environmental exposure. There is little indication that the effect of weather on 

individual soldiers was of paramount concern in planning major operations during World War II. Unlike early European 

and American armies, the armies of modern Europe did not curtail operations and enter winter quarters or camps.

Nonetheless, the selection and progression of the decision to proceed with the Normandy invasion indicate the importance of both long- and short-term meteorology on military planning. The general pattern of climatic trends (eg, 

rainfall, air temperatures) allows the prediction of a set of meteorological limits or bounds for military operations. 

The initial planning was for a spring or summer attack. Invasion preparations were to be completed by Y-Day, and a 

tentative date was set after May 1. This was changed to June 1 to accommodate an enlarged operation.(2)

The actual invasion date, D-Day, was to be the earliest possible date after Y-Day. The critical elements were determined 

to be a combination of daylight and high tides, with a secondary concern for sufficient moonlight to support airborne 

operations. Daylight was required for naval gunnery observations. The actual invasion would occur within 3 h of high 

tide, but at least 30 min of daylight was required before the first invasion wave. The projected windows for D-Day 

could be projected from moon phases and tidal information obtained from nautical almanacs. The earliest window 

was June 5–7. The next window would begin on June 19.

Climatic information can give an approximation of the anticipated air and water temperatures, and almanacs definitely 

predict the timing of tides and moon phases. However, no table could accurately predict wind or cloud conditions. 

Pogue(2) states that, “In the last days of May, the Supreme Commander began to watch the weather forecasts very 

closely. He got in the habit of talking over the reports with the Chief Meteorological Officer . . . so that he understood 

fully the value of the reports and the basis on which they were made.”

The forecast on May 29 was favorable for the following week. D-Day was set for June 5, 1944.(2) The two sources defer 

regarding forecast information through June 3. Weather information remained favorable (the morning of May 3),(1)

with Pogue(2) stating that forecasts were “less hopeful” on May 2, but Harrison1 stating that Eisenhower described 

it as “favorable” in a telegram to Washington on May 3. However, by the evening of June 3, inclement and unstable 

weather was forecast. In addition, because of instability, weather forecasts could not be considered reliable for more 

than 24 h.(1) Despite the unfavorable forecast, some naval elements were initially given orders to proceed.

At 0430 h on June 4, the forecast indicated low, overcast conditions for June 5, which would limit vital air support. 

The operation was delayed for 24 h until June 6. A new forecast, presented at 2130, indicated a new weather front and 

a break in the weather. Conditions would be adequate for overnight bombing and naval gunnery on the morning of 

June 6. Wind speeds would decrease later in the morning. The final decision was made at 2145 on June 4,(1) a little 

more than 24 h before operations. Another weather briefing, at midnight—which would have allowed a delay of the 

operation, but would not allow sufficient time to reschedule the operation for the following day—provided no new 

information.(1) At 0330 on June 5, weather was bad, but the forecast made at 0200 on June 5 indicated a 36-h break, but 

could not forecast further.(2)

(Exhibit 8-2 continues)
221

Evaluation of the Thermal Environment

The historical record indicates that the forecast was accurate, and the invasion proceeded as planned. When the next 

invasion window (June 19) arrived, a storm effectively closed Omaha Beach until June 23 and severely damaged 

the artificial harbors that were crucial during the early stages of the invasion. A later June landing would have been 

unsuccessful. Had the Allied forces not invaded on June 6, the course of World War II might have been significantly 

different. Although it is of limited value to speculate on alternative outcomes, without a successful June invasion, the 

German military might have been able to concentrate successfully their efforts against the Russian forces, or the Russians might have defeated the German forces in the absence of any Western allies.

The decisions regarding the timing of the Normandy invasion illustrate a progression from broad climatic considerations 

to a narrower time frame with increasing forecast accuracy. The initial selection of Y-Day was concerned mostly with 

military readiness (eg, when was the earliest possible date for a summer invasion?). It is also important to note that 

the final decision was a human one, with the responsibility ultimately resting on the Commander’s shoulders.

Data sources: (1) Harrison GA. United States Army in World War II: The European Theater of Operations: Cross-channel Attack. Washington, DC: Office of Military History, Department of the Army; 1951. (2) Pogue FC. The Supreme Command. Washington, DC: Office of 

Military History, Department of the Army; 1954.

Exhibit 8-2 continued

Onshore and offshore winds are mesoscale events 

for a large island or a long coastline, but they might 

be a microscale factor for a moderate-sized inland 

lake. A single point or local ground-based weather 

station or observer collects only microscale weather 

data. A time scale suggested for microscale weather40

of minutes to hours indicates the immediate nature 

of microscale events.

The different scales are significant in terms of 

weather forecasts and local thermal environments. 

It is important to recognize that, as the operational 

area of a military unit expands, the scope of relevant 

weather data also expands from local observations 

and measurements to an ability to predict the range 

of weather conditions and thermal environments that 

can be experienced over the entire area of operation. 

The relevance of the local data to mesoscale or synoptic weather depends on the homogeneity of the local 

environment and how representative the site is of the 

larger region.

Measurement Objectives

The collection of meteorological data is dependent 

on the objectives of the user. Long-range or strategic 

planning requires different data than planning for a 

mission with a specific window for execution. As the 

execution time nears, more and more accurate forecasts 

are both required and possible. When the mission begins, real-time, ground-level weather data are added 

to the requirements for forecasts. Specific operations or 

tasks often have different meteorological requirements. 

Helicopter pilots generally operate from ground level 

to 600 m above surface features and can range over a 

large area, whereas a rifleman occupying a defensive 

bunker has a much more limited area. Differences 

that are of no significance to the former can be very 

important to the latter.

Although not directly related to thermal environment, certainly the drama over General Eisenhower’s 

planning for the Normandy invasion captures the 

importance of short-term weather forecasts. Long-term 

data allowed selection of a target zone (time and place), 

and short-term forecasts determined the decision to 

commit; however, only real-time data determined 

what conditions existed when the troops landed on 

the beaches (Exhibit 8-2). 

Whether planning for another Normandy invasion 

or a peacekeeping mission in the Caribbean, environmental data must be considered in three stages. The 

first stage is long-term planning during the predeployment phase. Climatic data provide the means to conduct a predeployment reconnaissance of the potential 

thermal environments. During an ongoing, long-term 

operation, the same climatic data provide medical 

support staff with the basis for planning logistics 

(clothing, water, and special needs), medical treatment 

(patient load, special medicine or care, staff preparation, transport, or evacuation), and normal seasonal 

changes that could be anticipated from climatic data. 

The accuracy of a forecast increases as time approaches 

real time. A forecast made for the year can predict only 

general seasonal trends, but the accuracy of the forecast 

increases as the time frame compresses from month 

to week to day to hour and as the uncertainties of the 

preconditions are reduced. 

Sources for long-term strategic or predeployment 

planning consist primarily of climatic data from military 
222

Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

and civilian sources, including almanacs, records from 

US military bases, foreign military sources, military 

histories, and government and private meteorological 

services. Limitations of published climatic databases are 

readily apparent. When US forces were committed to 

Somalia, the climatic and meteorological data available 

to most planners were limited to major cities. In Somalia 

and many other countries, the major urban areas are 

located along the coast. Little information is available on 

interior zones. If data consist only of average monthly 

temperature and precipitation, it is difficult to determine 

any meaningful information regarding daily extremes, 

wind speeds, and seasonal transitions that do not conform to the month-by-month calendar divisions. Other 

data that do not lend themselves to monthly averages 

include the distribution of precipitation and values for 

humidity and humidity variability. Day length and estimates of solar load4,41,42 can be calculated from latitude 

or almanacs. Even with the preceding guidance, the task 

of relating solar phenomena to local terrestrial sites is 

difficult. Computer programs are available for calculating solar time, angle, day length, and other factors.43

An approximation of conditions can be derived from 

knowledge of regional or climatic information. Climatic 

classifications (physical geography or ecology) provide 

a general framework for understanding the seasonal 

patterns. Better references include more detailed information (eg, monthly average and absolute extremes 

for air temperature, relative humidity at morning and 

midday, and days per month with precipitation).

Long-Term Climatology for Predeployment 

Reconnaissance and Planning

It is useful to know what weather information is 

available to the US military. Within the US military, the 

primary responsibility for meteorological data collection lies with the US Air Force and its meteorological 

teams. The Air Force Combat Climatological Center 

(AFCCC) in Asheville, North Carolina, can provide 

tailored climatological summaries of relevant environmental data for specified regions worldwide. Information on submitting requests for climatological data 

products can be obtained from the following address: 

AFCCC/DOO, 151 Patton Avenue, Room 120, Asheville, NC 28801-5002. Using the following Web site, 

https://notus2.afccc.af.mil (US Air Force Staff Weather 

Officers [SWOs]) will also provide climatological summaries for their areas of responsibility to interested 

military users. During ground-based operations, the 

US Corps of Engineers is an important secondary 

source for information, because they have knowledge 

of the significant effect of inclement weather on flooding, road passability, traffic, and construction.

Real-Time or Near Real-Time Data and Forecasts

SWOs and their meteorology teams have the responsibility to provide real-time weather data and 

forecasts to military users. These SWOs are attached 

to Army units and operate the Army’s Integrated 

Meteorological System, which is located at separate 

brigade, division, corps, and echelons above corps 

headquarters deployed outside the continental United 

States, and at some continental United States locations.

The Integrated Meteorological System44 provides a 

broad range of weather information products (including current data and forecasts of local air temperature, 

humidity, wind speed, and solar radiation), as well as 

recent weather satellite imagery.

The Headquarters Air Force Weather Agency at 

Offutt Air Force Base, Nebraska, maintains the capability to provide current satellite and other weather 

information around the world. This information can 

be accessed at the following Web site (where it is necessary to register for a password): https://weather.

afwa.af.mil/. 

Parameter Measurements

Primary Environmental Parameters and Instruments

There are a variety of measurements that can be 

made to assess the potential impact of the thermal environment on military personnel and their missions. It 

is important to be aware of the types and limitations of 

environmental measurements. In addition to providing 

basic knowledge to assist in any direct effort to collect 

local weather data, knowledge of weather instruments 

and measurements will enable an individual to make 

an informed request for better information from other 

agencies and sources. Users need to know what information to collect and how to use it. 

In principle, measuring air temperature, wind 

speed, solar radiation, and humidity (the four basic 

parameters that define the thermal environment) 

is not difficult to do. However, when more careful 

consideration is given to instrument type, number 

of sensors, location, sensor placement and operation, 

data acquisition, and processing, the process becomes 

more complex. It is necessary to determine whether or 

not there is adequate justification of costs in time and 

other resources to collect meteorological data. In other 

words, can you get it elsewhere?

The decision to collect weather data directly is dependent, to a large degree, on whether or not available 

data are adequate or relevant to local needs. The local 

site conditions and circumstances of reported thermal 

injuries might be important. Under tactical conditions, 
223

Evaluation of the Thermal Environment

ground-based military personnel are often restricted in 

their choice of environment. An infantry member can 

be assigned a defensive position and told to defend in 

place until relieved. Environmental parameters that 

affect thermal balance can be local, and substantial differences in exposure to sun and wind can be a matter 

of only a few meters. It is difficult to appreciate how 

little local weather data are available, even for retrospective analyses. The windchill index20 only requires 

Ta and wind speed, yet Whayne and DeBakey45 were 

unable to evaluate the significance of windchill on the 

incidence of cold injuries in World War II from the lack 

of proximate weather data.

Individuals deployed in military operational settings typically do not have direct access to weather 

information. Meteorological data and weather effects 

are considered intelligence issues, and information 

is provided to those organizational levels that have 

the ability and capability to use the information. Certainly, in planning an operation, all personnel should 

be provided with basic information regarding the 

anticipated weather conditions. However, when one 

considers an individual member of the infantry assigned to hold an exact position, the utility of weather 

information may be reduced, because as a member of 

a military group, his or her individual options can be 

extremely restricted. It would, for example, be possible to provide thermometers on zipper pulls or even 

environmental sensor suites to the individual. The 

question, then, is what can the individual do with the 

information? Low-ranking personnel without leadership authority might be unable to do much but complain. In contrast, an independent, small-unit leader 

(whether a noncommissioned officer supervising a 

platoon loading supplies in a warehouse, a leader 

in a Special Operations Forces ground unit, or any 

tactical leader with authority to make independent 

decisions) can use information on the thermal environment to plan work–rest cycles, to rotate personnel 

warming shelters, or to incorporate thermal factors 

into the selection of a position. When civilians select 

a campsite or site for a solar-heated home, environmental conditions—especially exposure to sun and 

wind—are part of that decision. If anything, those 

factors should be more important to military personnel in temporary shelters.

It is unrealistic to expect that a medical unit would 

attempt to operate a complete weather station. The 

most basic rule for weather data collection is to keep it 

simple. In a tactical operation, even a simple weather 

kit or sensor suite can be difficult to manage. Data 

collection can be limited to a pocket thermometer 

hung from a tree branch. Simple, proximate data that 

are available to an informed, responsible individual 

become far more valuable than an elaborate system 

that is inaccessible to the individual. It is also useful 

to understand what information can be obtained with 

minimal effort. A field medical officer can tell from a 

minimum-maximum thermometer if the possibility of 

frostbite exists, whereas the wet bulb globe temperature (WBGT) index,46 or more sophisticated models, 

requires three or four environmental parameters to 

predict probability of heat injuries. 

The primary meteorological measurements are air 

temperature, humidity, wind speed, and incoming 

radiation. Other measurements that can be of value 

include atmospheric pressure, ground temperature, 

and precipitation. A variety of instruments and sensors 

for monitoring meteorological conditions are available, 

from simple analogue pocket instruments to sophisticated electronic sensors. The attachment describes the 

various options for monitoring the thermal environment, including types and limits of environmental 

measurements. In addition to any direct effort to collect 

data, knowledge of weather instruments and measurement will develop one’s ability to make an informed 

request for better information.

Stations and Instrument Packages

As noted, the US Air Force has the primary responsibility for providing weather support (AR 115-10, 

2010). In the future, expanded US Air Force and Army 

weather capabilities, currently under development, 

may be able to provide detailed local surface weather. 

However, at present, in areas where the thermal environment has the potential to cause significant injury, 

the systematic collection of local weather data may be 

appropriate. Units should utilize existing Army and 

US Air Force weather assets and expertise whenever 

possible rather than establish independent weather capabilities; but, it is useful to have a working knowledge 

of weather equipment and data collection. 

For continuous operations at a specific site, a fixed, 

permanent weather station can supplement data from 

other sources. Such meteorological data are more 

often unavailable or insufficient when an operation is mobile. The most effective way to establish a 

meteorological collection system is to purchase commercial weather instruments as a complete station. 

This station should include the basic measurements 

mentioned previously—air temperature, humidity, 

and wind speed—and, if practical, radiation should 

also be monitored.

The heart of a commercial station is an electronic 

data acquisition (DA) system. Instrument signals 

are received, translated into appropriate units, and 

displayed or stored for later recovery or transmission. 
224

Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

The basic weather station used during our field studies consists of a battery-powered DA system. The 

sensors include one temperature and humidity sensor inside a shielded mount, a ground temperature 

sensor, an anemometer (wind gauge), and a wind 

direction sensor. A pyranometer is used to measure 

incoming global radiation. On some stations, a 15-

cm black globe thermometer was also included. The 

instruments are mounted on a 2-m steel tripod with 

an enclosure for the DA system and storage modules. 

If a lighter tripod is used, the entire station can fit into 

two large suitcases.

In that system, a portable computer is used to program and download data. Other stations incorporate 

programming, display, and output elements into the 

DA system. For studies focused on meteorology, the 

basic station can be expanded by adding a barometer, 

more radiation sensors (including a shaded pyranometer and two net radiometers), a second anemometer 

at another height, and additional air and surface thermometers. This array is supplemented by a separate 

WBGT sensor suite. On complex terrain, additional 

stations are deployed. 

Simpler home-weather stations that measure the 

basic air temperature, humidity, and wind parameters 

are available. The basic DA system displays data and 

has limited storage and data processing capabilities. 

The basic system of one home station weighs 1.6 kg 

and can be linked to a personal computer. Limitations 

of these far more economical units include accuracy, 

Fig. 8-7. Handheld Kestrel 4000 unit. 

Photograph: Reproduced with permission from NielsenKellerman (Boothwyn, Pa). 

Fig. 8-6. WeatherPod miniature weather station. The unit 

provides environmental measurements through communications to a base unit, with Global Positioning Satellite 

(GPS) locations. The light (1 kg) unit senses and transmits 

air temperature, relative humidity, barometric pressure, and 

GPS position. In addition, it incorporates a solar sensor that 

detects low light (twilight) and approximate solar intensity. 

Wind speed and direction are measured using a miniaturized sonic anemometer. To avoid drawing attention, the unit 

has no moving parts. Versions with radio or satellite-linked 

wireless communications systems are available. 

Photograph: Reproduced from the US Army Research Institute of Environmental Medicine (Natick, Mass).
225

Evaluation of the Thermal Environment

no radiation or ground temperature measurements, 

the inability to expand instrumentation, less than 

ideal mounts for the wind indicator, and alternating 

current power requirements for some systems. The 

most important concern is an inability to calibrate or 

recalibrate the instruments. Periodic comparisons to 

measurements made with handheld instruments can 

establish the level of confidence that can be assigned to 

the system. Despite the limitations, these systems are 

an attractive alternative to no weather data at all. At 

present, there is a commercially available battery- or 

solar-powered, miniature weather station that weighs 

less than 1 kg; has a height of approximately 15 cm; 

and measures and transmits air temperature, humidity, 

solar radiation, wind speed and direction, and rainfall 

(Figure 8-6).47

The alternative to a full-scale weather station is 

handheld instrumentation. The basic measurements 

have not changed, but the instruments might be less 

sophisticated. Individual instruments (eg, pocket thermometers) can be purchased. Simple kits—such as a 

fire weather instrument kit (Case Belt Weather Kit FSN 

8465-00-521-3057F)—have a compact psychrometer 

(wet bulb and dry bulb temperatures), a simple wind 

gauge, a compass, and other supplies in a belt carrier. 

The kit was designed for use by ground-based forest 

LINKING METEOROLOGY TO MODELS AND USERS 

The use of thermal models allows planners to utilize 

scarce resources better and to preserve the fighting power on the battlefield, both of which can be seen as force 

multipliers. Chapter 2 of this volume describes in depth 

the basic principles of thermal modeling, and presents 

both a historic perspective and a detailed description 

of the suite of thermal models developed by the US 

Army Research Institute of Environmental Medicine 

(USARIEM). This section complements Chapter 2 by 

explaining how weather data and the biophysics of heat 

exchange are incorporated into models. The examples 

that follow are based on work done over a 50-year period by Gagge and colleagues at the J. B. Pierce Foundation (New Haven, CT). The basic equations describing 

the fundamentals of heat exchange developed at the 

J. B. Pierce Foundation have been adapted into many 

models, including some USARIEM thermal models. 

Operative Temperature

The original concept of operative temperature (To) is 

based on dry heat exchange between the body surface 

and the environment.14,50,51 The premise is to calculate 

a single value, expressed as a temperature, that would 

account for heat exchange by convection and radiation 

(C + R), if all temperatures (surfaces and air) were at 

the same operative temperature. The general equation, 

assuming no work (Wk) or storage (S), is:

(23) M – E = (hc + hr) (

–

Ts – To) [W • m–2].

Equation (23) is equal to the dry heat exchange, or R + C. 

Several equivalent equations for calculating To exist,5

but the basic equation is:

(24) To = (hcTa + –Tr hr)/(hc + hr) [ ̊C].

The American Society of Heating, Refrigerating, and 

Air-Conditioning Engineers (ASHRAE)42 describes To

as the average of –Tr and Ta, weighted by their respective 

heat transfer coefficients. To derive the heat transfer 

coefficients, wind speed (V) and –Tr, or an equivalent 

radiation term, must be known. To is the temperature 

of a uniform environment, such as an enclosed test 

chamber with the same wall and air temperatures (Ta

= –Tr), where the dry or sensible heat exchange is the 

same as the actual environment. One problem with To is 

that hc is dependent on wind speed. To use To as a basis 

firefighters to provide temperature, humidity, and 

wind data for planning operations and directing aircraft. Similar compact kits exist to measure the WBGT 

parameters. These kits feature a miniature black globe 

thermometer.

Some WBGT sensor kits are mounted as a single 

unit and use a battery-powered system to collect data 

and calculate the WBGT index. More recently, batterypowered environmental sensor systems to measure 

air temperature, dew-point or relative humidity, wind 

speed, and black globe temperature have been developed. A pocket-sized electronic unit—with the ability 

to make the required measurements, calculate derived 

parameters (eg, –Tr ) and run a sophisticated heat strain 

model—has been developed and tested.48 Sensor suite 

units can be as small as a cigarette pack (9.5 × 6.0 ×

3.0 cm) in a folded position and weigh 155 g, with a 

9-V battery.49 Several companies now offer handheld, 

battery-powered, miniature stations (Figure 8-7) that 

measure air temperature, humidity, wind speed, and 

barometric pressure, and can incorporate a Global 

Positioning Satellite unit or an electronic compass, 

and thus measure wind direction, but not solar or rain 

parameters. One such unit was observed hanging from 

a tree outside a medical clinic, and forward weather 

observers have been using these units.
226

Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

for comparison between environments, the variation 

from wind speed is accommodated by calculating the 

equivalent To for still air.52 The relationship between To

and standard operative temperature (Tso) is: 

(25) Tso= (h/h’)To + (1 – h/h’) –Ts [ ̊C]

and

(26) h • (–Ts – To) = h’ • (–Ts – Tso) [W • m–2].

The coefficient h is the combined heat transfer coefficient for convection and radiation (hc + hr

), and h′ is 

the same combined term, except that hc is calculated 

for the standardized still air velocity.50

The New Effective Temperature 

The new effective temperature53 (ET*) has been 

adopted by ASHRAE.42 ET* is the temperature of a 

uniform environment, at 50% relative humidity, where 

the sensible and insensible heat transfer from the skin 

are the same as in the actual environment. ET* uses 

the concepts of operative temperature (To), mean skinwettedness (–

ω), i

m, and the Lewis relationship (LR)31 to 

calculate a temperature at 50% relative humidity, rather 

than 100% RH, which is used for Tsoh and results in an 

equivalent heat loss from the skin.

(27) ET* = To + –ω • i

m • LR • (Pa

 – 0.5Ps,ET*) [ ̊C]

(28) i

m = Rt

/(Re,t LR) [N.D.]

Ps,ET* is the saturated water vapor pressure at ET*. 

LR is the ratio between evaporative (he) and convective (hc

) heat transfer coefficients (LR ≈ he/hc). If the 

convective value (hc) is known or calculated from 

wind speed and air temperature, then he can be estimated. Values used to calculate the water vapor 

permeability index (i

m) are the total dry resistance (Rt 

(m2 • K)/W) and total evaporative resistance (Re,t(m2

• kPa)/W) from the skin to the environment. Values 

for i

m index range from 0 to 1.0, with 0 representing 

an impermeable condition and 1.0 bare skin. Skinwettedness (ω) ranges from 0.06 (no sweat) to 1.0. As 

indicated in the 2005 ASHRAE Handbook,

42 calculation 

of ET* is relatively complex and dependent on both 

clothing and metabolic rate. Clothing, activity level, 

wind speed, 

–

Tsk, and mean skin wettedness ( 

–

w) values are the same in both environments. In Figure 8-8, 

ET* is plotted as a psychrometric relationship. The 

plot also incorporates data from Hardy54 that relates 

ET* to records of US Army heat-related fatalities in 

World War II. 

Standard ET*50,55 is a modification analogous to the 

principle of standard operative temperature. ET* is 

adjusted to standard or constant reference conditions 

(sedentary subject in 0.6 clo, air velocity of 0.2–0.5 m 

• s–1, and 50% relative humidity) to allow better comparisons between environments.

Humid Operative Temperature

In a manner similar to ET*, humid operative 

temperature (Toh) is the temperature at 100% relative 

humidity that would result in the same heat loss from 

the skin for a given combination of activity, clothing, 

wind speed, –Tsk , and ω. 

(29) Toh= To+ –ω • i

m • LR • (Pa

 – Ps,oh) [ ̊C].

The thermodynamic relationship is the same as ET* 

except for the reference vapor pressure, Ps,oh, which 

is the saturated water vapor pressure at Toh. Standard 

humid operative temperature (Tsoh) incorporates the 

still air condition for Tso with the concept of Toh, thereby 

accounting for both sensible and insensible effects.

Fig. 8-8. Nomogram for the “new,” effective temperature 

(ET*), including data points for US Army heat deaths 

provided by Hardy. ashrae: American Society of Heating, 

Refrigerating, and Air-Conditioning Engineers, Inc; clo: unit 

of clothing insulation, 1 clo = 0.155 m2 K/W; MET: unit of 

metabolism, 1 MET = 58.15 W/m2. RH: relative humidity.

Illustration: Adapted with permission from ASHRAE. Copyright 2005 © American Society of Heating, Refrigerating, 

and Air-Conditioning Engineers, Inc (www.ashrae.org). Reprinted with permission from the 2005 ASHRAE Handbook—

Fundamentals. (This text may not be copied nor distributed in 

either paper or digital form without ASHRAE’s permission.)

Data source: Hardy JD. Thermal comfort and health. ASHRAE 

J. 1971;13:43.

Dry Bulb Temperature, °C

Dry Bulb Temperature, °F

Dew-Point Temperature, °C

Ambient Vapor Pressure, kps

20 30 40 50

68 86 104 122

10

15

20

25

30

34

1

2

3

4

5

zone of

body heating Army heat

deaths

(1942-1944)

35 ET heat

stroke danger

line (proposed)

1 MET, 0.6 clo

Wind < 0.15 m/s

100% RH

41.5 ET* w = 1

40 ET* w = 0.85

intolerable

80% RH

50% RH

20% RH

very uncomfortable

uncomfortable 35 ET*

w = 0.45

30 ET* slight discomfort

w = 0.25

comfort zon

ashrae 55-1981

e

20 ET*

zone

of

body

cooling
227

Evaluation of the Thermal Environment

SUMMARY 

REFERENCES

1. Nishi Y. Measurement of thermal balance in man. In: Clark JA, Cena K, Wrocawska P, eds. Bioengineering, Thermal 

Physiology, and Comfort. New York, NY: Elsevier; 1981: 29–39.

2. DuBois D, DuBois EF. A formula to approximate surface area if height and weight be known. Arch Intern Med.

1916;17:863–871.

3. Clark JA, McArthur AJ, Monteith JL, Wheldon AE. The physics of the microclimate. In: Clark JA, Cena K, Wrocawska 

P, eds. Bioengineering, Thermal Physiology, and Comfort. New York, NY: Elsevier; 1981: 13–27.

4. Campbell GS, Norman JM. An Introduction to Environmental Biophysics. 2nd ed. New York, NY: Springer-Verlag; 1998.

5. Santee WR, Gonzalez RR. Characteristics of the thermal environment. In: Pandolf KB, Sawka MN, Gonzalez RR, eds. 

Human Performance Physiology and Environmental Medicine at Terrestrial Extremes. Indianapolis, Ind: Benchmark Press; 

1988: 1–43.

6. Nishi Y, Gagge AP. Direct evaluation of convective heat transfer coefficient by naphthalene sublimation. J Appl Physiol.

1970;29:830–838.

7. Mitchell JW. Heat transfer from spheres and other animal forms. Biophysics. 1976;16:561–569.

8. Witherspoon JM, Goldman RF, Breckenridge JR. Heat transfer coefficients of humans in cold water. J Physiol.

1971;63:459–462.

9. Nadel ER, Holmér I, Bergh U, Åstrand P-O, Stolwijk JAJ. Energy exchanges of swimming man. J Appl Physiol. 

1974;36:465–471.

10. Gonzalez RR. Biophysics of heat transfer and clothing considerations. In: Pandolf KB, Sawka MN, Gonzalez RR, eds. 

Human Performance Physiology and Environmental Medicine at Terrestrial Extremes. Indianapolis, Ind: Benchmark Press; 

1988: 45–95.

11. Wittmers LE, Savage MV. Exhibit 17-1: Ranger training incident report. In: Bellamy RF, ed. Medical Aspects of Harsh 

Environments. Vol 1. In: Bellamy RF, ed. Textbook of Military Medicine. Washington, DC: Department of the Army, Office 

of The Surgeon General, Borden Institute; 2001: 534–535. Chap 17.

12. Clark RP, Mullan BJ, Pugh LGCE, Toy N. Proceedings: heat losses from the moving limbs in running—the ‘pendulum’ 

effect. J Physiol. 1974;240:8P–9P.

To optimize performance during military operations in extreme environments, the military must understand how the environment impacts the soldier. 

Although many aspects of the physical environment 

can affect military populations in extreme environments, the factors that influence the thermal state of 

the soldier—temperature(s), radiation, wind, and 

humidity—have a pervasive potential to degrade 

military performance significantly. Thus, the rationale 

for the military to study how environmental factors 

influence the thermal state of the human is force 

conservation by early intervention or prevention of 

nonbattle injuries, especially among those individuals tasked with difficult missions in the most extreme 

environments imaginable. 

The more these environments and the limits of military capability are understood, the more protective 

equipment can be improved to prevent environmental 

casualties, as well as improved medical treatment for 

individuals who suffer from environmental injuries. 

This enhances the ability of the military to operate in 

extreme surroundings and contributes significantly 

to the vast body of civilian literature concerning environmental extremes. Likewise, collaboration with 

civilian scientists offers an opportunity for both military and civilians to understand better not only the 

limits of human endurance, but also the ability of the 

human body to respond and adapt to environmental 

stress, and to develop strategies and equipment for 

minimizing the impact of the thermal environment 

on human performance, especially during military 

operations.
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13. Chang SKW, Arens E, Gonzalez RR. Determination of the effect of walking on the forced convection heat transfer 

coefficient using an articulated manikin. ASHRAE Trans. 1988;94:71–82.

14. Gagge AP, Nishi Y. Heat exchange between human skin surface and thermal environment. In: Lee DH, ed. Handbook 

of Physiology: Section 9. Reaction to Environmental Agents. Rockville, Md: American Physiological Society; 1977: 69–92. 

15. Breckenridge JR. Effects of body motion on convection and evaporative heat exchanges through various designs of 

clothing. In: Hollies NRS, Goldman RF, eds. Clothing Comfort: Interaction of Thermal, Ventilation, Construction and Assessment Factors. Ann Arbor, Mich: Ann Arbor Science; 1977: 153–166.

16. Lotens WA. The actual insulation of multilayer clothing. Scand J Work Environ Health. 1989;15(suppl):66–75.

17. Havenith G, Heus R, Lotens WA. Resultant clothing insulation: a function of body movement, posture, wind, clothing 

fit and ensemble thickness. Ergonomics. 1990;33:67–84.

18. Holmér I, Gavhed DCE, Grahn S, Nilsson HO. Effects of wind and body movements on clothing insulation—measurements with a moveable thermal mannikin. In: Lotens WA, Havenith G, eds. Proceedings of the Fifth International 

Conference on Environmental Ergonomics. Maastritcht, The Netherlands: TNO Institute for Human Factors; November 

2–6, 1992: 66–67.

19. McCullough EA, Hong S. A database for determining the effect of walking on clothing insulation. In: Lotens WA, 

Havenith G, eds. Proceedings of the Fifth International Conference on Environmental Ergonomics. Maastritcht, The Netherlands: TNO Institute for Human Factors; November 2–6, 1992: 68–69.

20. Siple PA, Passel CF. Measurements of dry atmospheric cooling in subfreezing temperatures. Proc Am Phil Soc. 

1945;89:177–199.

21. Underwood CR, Ward EJ. The solar radiation area of man. Ergonomics. 1966;9:155–168.

22. Breckenridge JR, Goldman RF. Solar heat load in man. J Appl Physiol. 1971;31:659–721.

23. AMETEK, Inc. Power Systems Group. Solar Energy Handbook: Theory and Applications. Radnor, Pa: Chilton Book Company; 1979.

24. Fanger PO. Thermal Comfort. New York, NY: McGraw-Hill; 1970.

25. McIntyre DA. Indoor Climate. Essex, England: Applied Science Publishers; 1980.

26. Matthew WT, Santee WR, Berglund LG. Solar Load Inputs for Thermal Strain Models and the Solar Radiation Sensitive 

Components of the WBGT Index. Natick, Mass: US Army Research Institute of Environmental Medicine; 2001. Technical 

Report T01-13.

27. Adolph EF. Physiology of Man in the Desert. New York, NY: Interscience Publisher (John Wiley & Sons); 1947.

28. Lotens WA, Pieters AMJ. Transfer of radiative heat through clothing ensembles. Ergonomics. 1995;38:1132–1155.

29. Shapiro Y, Moran D, Epstein Y, Stroschein L, Pandolf KB. Validation and adjustment of the mathematical prediction 

model for human sweat rate responses to outdoor environmental conditions. Ergonomics. 1995;38:981–986.

30. Moran D, Shapiro Y, Epstein Y, Burstein R, Stroschein L, Pandolf KB. Validation and adjustment of the mathematical prediction model for human rectal temperature responses to outdoor environmental conditions. Ergonomics. 

1995;38:1011–1018.

31. Lewis WK. The evaporation of a liquid into gas. ASME Trans. 1922;4:325–335.

32. Ellis FP, Smith FE, Walters JD. Measurement of environmental warmth in SI units. Br J Ind Med. 1972;29:361–377.

33. Chambers AB. A psychrometric chart for physiological research. J Appl Physiol. 1970;29:406–412.
229

Evaluation of the Thermal Environment

34. McArdle B, Dunham W, Holling HE, et al. The Prediction of the Physiological Effects of Warm and Hot Environments: The 

P4SR Index. London, England: Medical Research Council; 1947. RNP Report 47/391.

35. Leithead CS, Lind AR. Heat Stress and Heat Disorders. Philadelphia, Pa: FA Davis Company; 1964.

36. Chang SKW, Gonzalez RR. Clothing insulation prediction in hypobaric environments. In: McBriarty JP, Henry NW, 

eds. Performance of Protective Clothing. Vol 4. Philadelphia, Pa: American Society for Testing and Materials Standards; 

1992: 604–615.

37. Urban DL, O’Neill RV, Shugart HH Jr. Landscape ecology: a hierarchical perspective can help scientists understand 

spatial patterns. BioScience. 1987;37:119–127.

38. International Organization for Standardization. Hot Environments—Analytical Determination and Interpretation of Thermal 

Stress Using Calculation of Required Sweat Rate. Geneva, Switzerland: International Organization for Standardization; 

1989. ISO 7933.

39. Parsons KC. Human Thermal Environments: The Effects of Hot, Moderate and Cold Environments on Human Health, Comfort 

and Performance. Washington, DC: Taylor and Francis; 1993.

40. Lutgens FK, Tarbuck EJ. The Atmosphere: An Introduction to Meteorology. Englewood Cliffs, NJ: Prentice-Hall; 1979. 

41. Roller WL, Goldman RF. Estimation of solar radiation environment. Int J Biometeorol. 1967;11:329–336.

42. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). 2005 ASHRAE Handbook: 

Fundamentals. Atlanta, Ga: ASHRAE; 2005.

43. Smith SM. Almanac software programs to compute rise and sunset times for the sun and moon. Ecol Soc Am Bull.

1994;75:260–264.

44. Brown RC, Harris JE. The United States Army’s Proof of Concept Integrated Meteorological System (IMETS). In: Proceedings of the Tenth Annual EOSAEL/TWI Conference. Las Cruces, NM: New Mexico State University; 1990: 228–235.

45. Whayne TF, DeBakey ME. Cold Injury, Ground Type. Washington, DC: Department of the Army, Office of The Surgeon 

General; 1958.

46. Yaglou CP, Minard D. Control of heat casualties at military training centers. AMA Arch Ind Health. 1957;16:302–316.

47. Lavine R. Special Operations weather technicians lead the way in OEF. Observer [serial online]. 2002;Sep–Oct: 3 pp.

48. Matthew WT, Stroschein LA, Blanchard LA, Gonzalez J. Technical Testing of a Prototype Heat Stress Monitor: Software 

Verification and Laboratory Evaluations of Sensor Performance. Natick, Mass: US Army Research Institute for Environmental 

Medicine; 1993. Technical Report T9-93.

49. Talley RL, Becker TJ. Environmental Health Monitor: Advanced Development of Temperature Sensor Suite. East Amherst, 

NY: Veritay Technology, Inc; 1996.

50. Gonzalez RR, Nishi Y, Gagge AP. Experimental evaluation of standard effective temperature: a new biometeorological 

index of man’s thermal discomfort. Int J Biometeorol. 1974;18:1–15.

51. Winslow CEA, Herrington LP, Gagge AP. Physiological reactions of the human body to varying environmental temperatures. Am J Physiol. 1937;120:1–22.

52. Gagge AP. Standard operative temperature. A generalized temperature scale applicable to direct and partitional calorimetry. Am J Physiol. 1940;131:93–103.

53. Gagge AP, Stolwijk JAJ, Nishi Y. An effective temperature scale based on a simple model of human physiological 

regulatory response. ASHRAE Trans. 1971;77:247–262.
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54. Hardy JD. Thermal comfort and health. ASHRAE J. 1971;13:43.

55. Gagge AP, Nishi Y, Gonzalez RR. Standard effective temperature: a single temperature index of temperature sensation 

and thermal discomfort. In: Thermal Comfort and Moderate Heat Stress: Proceedings of the COB Commission W45 (Human 

Requirements) Symposium. London, England: Building Research Establishment; 1972: 229–244.
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Evaluation of the Thermal Environment

ATTACHMENT: INSTRUMENTS AND METHODS FOR MEASURING PARAMETERS 

OF THE THERMAL ENVIRONMENT

Air Temperature

The most obvious parameter of the thermal environment is air temperature. It is the factor that provides the 

drive and direction for convective heat exchange. The most basic thermal environmental monitoring consists of 

systematically reading a thermometer mounted at a uniform height in a shaded, representative location. A single 

measurement of air temperature may not be adequate. Air temperature at a given point and time can display a 

substantial vertical gradient, depending on ground or floor temperature and local air movement. In addition, 

on complex terrain, there might be diurnal upslope and downslope flows or accumulation of denser, colder air 

in depressions, and so forth. Choice of the number and location of air temperature sensors is also dependent 

on the activities of the personnel. 

Fig. 8-A1. Comparison of air-to-ground temperatures measured at two heights to ground temperature at the US Marine 

Corps Mountain Warfare Training Center, California, on 

January 23, 1988. PST: Pacific Standard Time.

0 2 4 6 8 10 12 14

15

10

5

0

-5

-10

-15

Time (PST)

16 18 20 22 24

0.5 M

2.0 M

Ground

Temperature (°C)

If a multichannel data acquisition station is used with electronic sensors, it is relatively easy to collect additional air temperatures at different heights. Air temperature and other environmental parameters can be quite 

different for a patient lying on a ground litter and for the standing staff. When possible, the best solution is to 

monitor air temperature from 0.5 m—from ground level to the maximum occupied height, usually 2 m (Attachment Figure 8-A1). 

Air temperature sensors should be shaded from 

direct sunlight and mounted at a known height. On 

weather stations, temperature and humidity sensors 

are placed inside a ventilated enclosure that provides 

protection from sunlight, wind, precipitation, or 

physical damage. The standard height for US civilian 

ground stations is 1.25 to 2 m (4.1–6.6 ft), but weather 

data collection is highly dependent on the needs of the 

primary collector. The standard height for wind measurements is 10 m (30 ft). A uniform or standard height 

might not be entirely appropriate for individuals with 

differing tasks or work environments. Aviators have 

different needs than mechanized ground personnel, 

and theirs—in turn—are distinctly different from those 

of nonmechanized light infantry. The 10-m height for 

wind speed measurements reduces uniformity problems related to ground-level turbulence, but it might 

be a more appropriate measurement for aircraft than 

ground personnel. The wind speed at 2 m is probably 

more appropriate for ground personnel. Wet bulb globe 

temperature (WBGT), which was developed for ground 

personnel, is measured at 1.2 m (4 ft), which is slightly 

above the center of mass for a standing person.1

Surface Temperature 

Ground temperature is an important factor in situations in which people are in close contact with the ground, 

especially where the ground is frozen or, as in a desert, where it acts as a significant heat sink or source. In a few 

situations (eg, when handling metal surfaces that are cold soaked or exposed to extreme heat and solar radiation), 

there might be some benefit to measuring other surface temperatures. All hot surfaces are also a significant source 

of radiant energy that may be estimated using the Stefan-Boltzmann equation from the surface temperature. 

Attachment Figure 8-A2 illustrates the offset between air and ground temperatures at two different locations.

The temperature of anything that comes into prolonged contact with the body surface should be quantified 

and recorded, although it might not be necessary to monitor most surface temperatures continuously. The 

temperature of working surfaces or tools can also be important. During extravehicular walks, astronauts have 

experienced problems with cold tools. Fuel pump handles in cold weather and even a scientist’s clipboard dur-
232

Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

ing cold chamber studies can cause significant heat 

loss. The temperature of liquids is also important, 

whether encountered during military stream crossings, 

accidental immersions, or accidental spills. Liquids 

(eg, gasoline or alcohol) can remain liquid well below 

the freezing point of water. If alcohol stored outdoors 

at –29°C is spilled on the hands, the combination of 

extreme cold, the higher convective heat transfer coefficient for liquid, and possibly evaporative heat loss 

as the alcohol vaporizes can cause local cold injury.

Temperature Measurement Instruments

Thermometers and other meteorological instruments can be placed into two general categories: (1) 

relatively simple to use, often handheld instruments 

based on mechanical or thermal expansion principles; 

and (2) electronic devices, which are better suited for 

downloading into data acquisition systems for continuFig. 8-A2. Demonstration of time lag between ground and 

air temperatures because of differences in heat transfer and 

mass in a meadow in the vicinity of the Tuolumne Meadows, 

Yosemite National Park, September 8–10, 1994.

ous, real-time monitoring. The operating principle for mercury and alcohol thermometers is based on the uniform 

thermal expansion properties of the liquids, which also have a favorable range between critical temperatures for 

freezing and boiling points. Bimetal thermometers are based on the differential thermal expansion of the two 

metals. Bimetal thermometers are often used for temperatures above normal human tolerances and are more 

rugged and shock-resistant than liquid thermometers.

Thermocouples and thermistors are solid-state electronic devices that use thermoelectric or electrophysical 

properties of certain materials to measure temperature. A thermocouple sensor consists of a junction of two 

dissimilar metals that produces a small voltage, or electromotive force, proportional to the temperature at the 

junction. This phenomenon is known as the “Seebeck effect.” Thermocouple measurement circuits require a 

second reference junction, at a known temperature, in series with the sensor junction. Thus, the voltage measurement is the net difference in electromotive force between these two junctions, and it is this voltage that is 

multiplied by the appropriate Seebeck coefficient to produce the temperature reading. Thermistors and resistance 

temperature devices (RTDs) measure temperature as electrical resistance. Thermistors use a bead of semiconducting material as the sensor. RTD sensors can be fine coils of wire or very thin films of metals (eg, platinum 

on ceramic substrates). In both cases, calibration tables, curves, or equations for specific thermistors and RTDs 

are used to convert the resistance values to temperature units. This is usually done online in measuring systems 

with dedicated microprocessors and readout displays. Thermocouples are smaller, less expensive, and have a 

wider temperature range than thermistors. However, thermistors are more accurate within their temperature 

range and are easier to incorporate into general sensor systems because they do not require a reference juncture. 

Three useful types of simple instruments are the following: (1) pocket thermometer, (2) minimum-maximum 

thermometer, and (3) soil (or ground) thermometer. Pocket thermometers are short, glass alcohol instruments 

that are enclosed inside a protective metal or plastic shaft. In use, the pocket thermometer can be hung by an 

eyehole on the shaft cap. It is a lightweight, compact instrument that provides a safe method for transporting a 

reasonably accurate thermometer. Although it can be carried in a pocket and removed for quick readings of air 

or water temperature, the best use is to hang it in a shaded location for periodic reference.

A simple minimum-maximum thermometer consists of a pair of liquid thermometers that are modified to 

mark the highest or lowest reading until they are reset. The marking mechanism can be a plug at the end of the 

liquid column or a constriction in the column, similar to an oral temperature thermometer. A minimum-maximum 

thermometer can provide the immediate temperature, and it can also track the range of temperatures during the 

measurement period. It is particularly useful in determining whether the air temperature drops below freezing 

overnight. 

A ground or soil thermometer looks like a small, all-metal meat thermometer with a shaft that is pushed into 

the ground. The bimetal thermometer uses a dial display scaled to the appropriate range of soil temperatures. 

Often, bimetal expansion does not provide as accurate a measurement of temperature and, in some cases, the 

pointed metal shaft of a soil thermometer can present a hazard during transport.

25

20

15

10

5

0

-5

Ground

Air (1.5 m)

Temperature (°C)

12 18 24 6 12 18 24 6 12 18 24 6 12
233

Evaluation of the Thermal Environment

A special set of thermometers is used to obtain the WBGT index. WBGT is the weighted sum of three temperatures: (1) a shaded air temperature, (2) a natural wet bulb temperature, and (3) a black globe temperature. 

Ta is conventional. The natural wet bulb is a thermometer with a wetted wick or cover exposed to the normal 

air movement (natural convection) described in the later section on Measurement of Humidity. The black globe 

thermometer is described in the following section on radiation measurements. 

Radiation Measurement Instruments

Instruments for measuring radiation can be split into two categories: (1) thermometers and (2) direct radiation sensors. The simplest and most common instrument for measuring radiation is the Vernon black globe 

thermometer.2 A black globe thermometer is part of the WBGT instrument set. The standard instrument is a 15-

cm (or 6-inch) hollow copper sphere with a flat black outer surface and a thermometer inserted into the center. 

The flat black surface has high emissivity (ε) and absorptivity (α). The solar angle is unimportant, because a 

sphere always presents the same cross-section area perpendicular to solar beams. When air temperature and 

wind speed are known, the globe temperature, Tg, is used to compute the mean radiant temperature (–Tr), which 

represents the effect of radiation that would be emitted in a chamber with all walls heated to that temperature.3

The theoretical basis for –Tr is the Stefan-Boltzmann law that all bodies emit radiation in proportion to the fourth 

power of surface temperature in degrees Kelvin (°K). –Tr goes in the reverse direction to determine the theoretical 

temperature of surrounding surfaces that would produce that level of isotropic radiation. 

(A1) –Tr = [Tg

4

 + 2.5 • 108 • V0.6 • (Tg – Ta)]1/4.

One objection to the black globe thermometer as a radiation instrument is that it measures the combined effects of radiation, air temperature, and wind. Another problem is that many radiation sources are not isotropic. 

Fig. 8-A3. A pyranometer—a radiometer for measuring 

global solar radiation. On this instrument, the thermopile 

sensor is protected from convective heat loss by double-glass 

hemispheres. Hukseflux model LP02.

Photograph: Courtesy of US Army Research Institute of 

Environmental Medicine (Natick, Mass).

In particular, solar angle has a significant effect on 

the amount of direct solar radiation striking the body 

surface. Other objections include the globe color and 

the slow response time of the 15-cm globe thermometer. A variety of modified globe thermometers exist, 

including scaled-down spheres, flesh-colored surfaces 

that more closely simulate absorption by human skin,3

an ellipsoid comfort meter,4 and globes protected by 

transparent wind shields.5 The modified globes are attempts to correct objections to the original globe while 

retaining the simplicity of operation and the concept of 

–

Tr. Matthew and colleagues6 have developed a method 

for approximating black globe temperature and –Tr using a single radiation instrument as an alternative to 

the bulkier globe thermometer. 

The alternative to –Tr is to measure the separate 

incoming radiation components. This is the only way 

to obtain an accurate estimate of the amount of solar 

radiation, or solar load, reaching the body surface. 

Unfortunately, the investment in instruments and effort required to obtain separate values for the different 

radiation components is extensive.7 With very few 

exceptions, weather stations make only one radiation 

measurement. Consequently, only a few investigators8–11 have obtained enough of the data required to 

calculate accurately the actual solar load. Although 

a variety of different names are used to differentiate 

radiometers by function, most instruments follow the 

same basic design. A generic radiometer consists of a 

flat thermopile or photoelectric sensor disc covered 
234

Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

with a half dome of transparent material to protect the sensor surface from damage and convective heat loss (Attachment Figure 8-A3). Usually, the instrument mount incorporates levels so that the sensor can be mounted in 

a fixed position relative to the horizon. By using different orientations of the sensors, mounts that limit or block 

radiation from different sources and domes that filter out different segments of the electromagnetic spectrum, 

visible infrared, and sometimes ultraviolet, radiation can be separated into direct, diffuse, or sky, and reflected 

or ground components. To obtain continuous radiation data for the basic solar and infrared components requires 

a minimum of four or, preferably, five radiometers, plus at least one special instrument mount.12 The majority 

of weather stations have limited or no radiation instrumentation.

Pyranometers, for example, are radiometers used to measure solar radiation. The sensor is covered by a glass 

dome that limits the spectral range to the near-ultraviolet, visible, and near-infrared spectra (300–3,000 nm). A 

level pyranometer exposed to the sky measures global radiation, which is a combination of direct and diffuse 

solar radiation. The previously noted relationship developed by Matthew and colleagues6 to estimate –Tr uses 

global radiation as the alternative to measuring black globe temperature. To separate direct and diffuse solar 

radiation, a separate value for diffuse or direct solar radiation must be obtained. A pyrheliometer is aimed directly at the sun to measure direct solar radiation. Diffuse radiation is measured with a pyranometer just like 

global radiation, except that a shadow-band mount or a disc is used to block direct radiation. A shadow band 

consists of a band of metal positioned at the correct (solar) angle to block the direct sun rays as the sun travels 

above the horizon from east to west. Lambert’s cosine law and the solar angle are used to compute the intensity 

of the direct solar beam.7,12,13 Reflected solar radiation is measured exactly like global radiation, except that the 

sensor is inverted so that only radiation from the ground reaches the thermopile. It is also possible to estimate 

reflected solar radiation by multiplying global radiation by the appropriate surface albedo.

Other radiometers use a polyethylene dome rather than glass to protect the sensor surface. The plastic is 

transparent to a broader spectrum (300–60,000 nm) of infrared radiation than glass; therefore, both short- and 

long-wave radiation are measured. By subtracting solar radiation from the total radiation, a value for infrared 

radiation can be obtained. 

A combination of pyranometers and net radiometers can be used to determine the incoming radiation components. A net radiometer consists of two radiation sensors mounted back-to-back with plastic domes. When 

mounted horizontally, one of the sensors measures ground radiation, and the other sensor measures radiation 

from the sky. The signals from the two sensors are electronically subtracted to obtain a net value. A second net 

radiometer with one side covered with a metal cup14 is used to determine the total sky radiation. From those 

four instruments, separate values for sky and ground infrared, plus direct, diffuse, and reflected solar radiation 

can be derived. Yet another radiometer with another filter is required to obtain a separate measurement for 

ultraviolet radiation. 

The standard height for a black globe thermometer is 1.2 m (4 ft). For pyranometers that measure radiation 

from the sun and sky, the important factor is exposure rather than height. The sun is 150 • 106 km from the 

Earth—a meter closer or farther away will not make much difference. The sensor element should not be shaded 

by any surrounding vegetation or structure, nor should light be reflected off a wall or other surface. 

Measurement of Wind Speeds

Outdoor wind speeds are often quite variable, and it is useful to record both the range and mean wind speeds. 

Another problem is the effect of objects and surface variations on wind speed. Wind speed varies with respect 

to height above ground, the texture or roughness of the ground surface, and any obstacles or surface variability 

that affect the flow of air. A variety of instruments are used to measure wind speed, including pressure, mechanical force, sonic, and convective or heated anemometers.15 The two most common types are electrically heated 

anemometers that are used to measure convective cooling and mechanical instruments (eg, propeller and cup 

anemometer; see Attachment Figure 8-A4) that are used to measure wind force in one plane. 

Hot-wire or heated-bead anemometers are often used indoors to measure lower air velocities, whereas the 

most common outdoor instruments are mechanical cup or propeller anemometers. Mechanical anemometers 

measure mechanical wind force in one plane, whereas heated anemometers measure convective heat loss to air. 

The inertia of mechanical anemometers generally limits their threshold sensitivity to low air velocities. From 

a physiological perspective, the heated anemometers measure the effects of turbulence and the more relevant 

factor of heat exchange. However, in an outdoor environment where air velocities are usually higher, the more 

rugged mechanical anemometers are the common choice.
235

Evaluation of the Thermal Environment

Air movement close to the ground tends to be turbulent relative to air measured at standard weather 

station heights (1.2–10.0 m). The two basic wind parameters are (1) wind speed and (2) wind direction. 

Direction is clearly important if the military population 

varies in its exposure to the wind. A change in wind 

direction can change a relatively comfortable sheltered 

position into an exposed position or vice versa. Wind 

direction would, of course, also be critical in the event 

of a nuclear, biological, or chemical attack. Mechanical 

wind vanes or simple observations of wind direction 

with reference to a compass will usually suffice. The 

basis of a mechanical wind vane can be modified with 

a contact sensor to sense electronically the direction of 

the wind vane. Aside from the overall bulk, the only 

real objection to wind vanes is that the motion of the 

vane tends to draw attention.

Like wind direction, wind speed can be estimated 

by observation. The Beaufort wind scale (Attachment 

Table 8-A1) uses observations of the effect of wind on 

common objects (eg, flags and dry leaves) to estimate 

wind speed. Wind speed is measured with an anemometer. One type of anemometer operates on the basis of 

the Bernoulli principle. This simple anemometer or 

wind gauge is a plastic tube with a small, light bead 

inside that moves up and down as the wind blows 

across the open top of the tube. Wind speed is read 

from a scale on the side of the tube. A pitot tube can 

be used as a much more sophisticated version of the 

same sensor.15,16 Other anemometers are based on the 

actual deflection of an arm against a known resistance. 

This type of simple gauge is often combined with a 

wind vane so that it is also orientated into the wind. 

The most familiar anemometers are cup or propeller 

anemometers that are rotated by the force of the wind. 

Fig. 8-A4. Cup anemometer, which measures wind speed in 

a horizontal plane. MET ONE model 014. 

Photograph: Courtesy of US Army Research Institute of 

Environmental Medicine (Natick, Mass).

The number of rotations is counted electronically to calculate wind speed. Like wind vanes, these anemometers 

tend to draw attention.

Electronic Sensors

Cup anemometers (see Attachment Figure 8-A4) are relatively rugged, and some can actually generate an 

electronic signal. However, the stall speed, based on inertia of the cup assembly, limits the sensitivity of the 

instrument to a threshold value of 0.2 m • s–1 to 0.5 m • s–1. For greater sensitivity, an electronically heated anemometer can be used. These anemometers operate by measuring the power required to heat a small wire or 

bead to a constant temperature. The power demand is proportional to the convective heat loss from the heated 

element, and convection is directly related to air velocity. Convection, which includes the effect of turbulence 

in addition to the mechanical force of the wind, is measured. A cup anemometer is a direct measurement of 

wind speed in a single plane of rotation. A heated anemometer is based on the more physiologically relevant 

property of convection. Beaded sensors are usually more rugged than hot-wire anemometers and tend to be 

more omnidirectional in terms of exposure.

As noted previously, wind speed varies in relation to height above ground, the texture or roughness of the 

ground surface, and any obstacles or surface variability that influence the flow of air. Turbulence and variation 

in air velocity are caused by friction or drag between the surface and air. Consequently, the height at which wind 

speed is measured is important. The best solution is to measure wind speed at two heights, 0.5 to 2 m apart. In 
236

Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

TABLE 8-A1 

BEAUFORT WIND SCALE

 Wind Speed

	Beaufort No.	 m • s–1 mph Description Specifications for Land Conditions

 0 < 1 0.5 Calm Calm; smoke rises vertically

 1 1 2 Light air Direction of wind shown by smoke drift

 2 2 5 Light breeze Wind felt on face; leaves rustle

 3 4 9 Gentle breeze Leaves and small twigs in motion; wind extends light flag

 4 7 16 Moderate Raises dust, loose paper; small branches moved

 5 10 22 Fresh Small trees in leaf begin to sway

 6 12 27 Strong Large branches in motion; whistling heard in overhead wires

 7 15 34 Near gale Whole trees in motion; some difficulty walking against wind

 8 18 40 Gale Breaks twigs off tree; impedes walking progress

 9 20 45 Strong gale Slight structural damage occurs

10 26 58 Storm Trees uprooted; considerable damage occurs

11 30 67 Violent storm Widespread damage

12 > 33 74+ Hurricane Massive and widespread damage to structures

Adapted with permission from Lutgens FK, Tarbuck EJ. The Atmosphere: An Introduction to Meteorology. Englewood Cliffs, NJ: Prentice-Hall; 

1979: 144. 

addition, objects in the path of the wind (or fetch) have an effect. The general rule is that the fetch should be 

10 times the height of an obstructing object.17 It is often simply impractical and unrealistic to obtain an unobstructed fetch. Wind speed fluctuates rapidly and can vary considerably within a small distance. In areas with 

any significant topography or tall vegetation, wind parameters are very local. The degree of correlation between 

a remote weather station and local wind conditions can be tenuous.

Measurement of Humidity

Broadly defined, humidity is the moisture content of the air. Measurement problems arise in part because a 

number of variables can be used to describe humidity, including absolute humidity, dew-point temperature, 

relative humidity, psychrometric or aspirated wet bulb temperature, natural wet bulb temperature, and water 

vapor pressure. The situation is not helped when many of the variables can be expressed in multiple units. For 

example, water vapor pressure can be expressed in units of millimeters of mercury or torr (mm Hg) or kiloPascals 

(kPa). Psychrometric charts (Figure 8-3) illustrate the third complication, the nonlinear relationship between 

temperature and humidity. However, psychrometric charts also demonstrate that, given air temperature and 

one measure of humidity, virtually any other humidity variable can be derived.

Instruments for measuring humidity include natural wet bulb thermometers, sling psychrometers, dew-point 

thermometers, and various electronic capacitance devices. Each presents its own problems. Humidity sensors 

that use water are of limited utility when the temperature range falls below the freezing point of water, although 

some of the electronic sensors claim a lower limit of –40°C. In other cases, a modified dew-point thermometer is 

also used below freezing. To some extent, the question is irrelevant, because even at saturation, the vapor pressure of water at –40°C is only 0.01 kPa versus 2.34 kPa at 20°C. When saturated cold air is warmed, either by 

contact with the body surface or inside the lungs, the warmer air is no longer saturated and has a much greater 

capacity to absorb water. Hence, in an apparent paradox for humans, even saturated cold air is very dry. From 

a physiological perspective, the effect is primarily on the respiratory tract, because thermoregulatory sweating 

is often minimal in extreme cold.

Natural wet bulb thermometers, which are a standard component of WBGT instruments, are as reliable as 

any liquid thermometers, as long as the wick covering the ball is kept clean and wet. Unfortunately, the natural 

wet bulb (or naturally aspirated temperature) has little application except for WBGT. The measured temperature 

is a product of humidity, air temperature, and wind speed. Unlike the other measurements of humidity, water 

vapor pressure cannot be readily derived from the natural wet bulb. Gagge and Nishi18 determined that, above 
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2.5 m • s–1, natural wet bulb and aspirated wet bulb are essentially equivalent. Most people are familiar with 

the sling psychrometer, which has wet bulb and dry bulb thermometers on a mount that is rapidly rotated or 

whirled to obtain a relatively uniform velocity. The reason for using a sling psychrometer is to eliminate the effect of variation in wind speed on the wet bulb temperature reading. Objections to using the sling psychrometer 

include the following: (a) wick management, (b) errors in reading the two thermometers, (c) susceptibility to 

damage, (d) time required to obtain a reading, and (e) difficulty in replicating the measurement. Relative humidity 

is computed from the difference or wet bulb depression between dry bulb and wet bulb temperatures. A third 

type of mechanical humidity sensor is the hair hygrometer, which senses expansion and contraction of human 

hair under tension in relation to changes in humidity.

An aspirated psychrometer uses a small electric fan rather than the sling to maintain a constant air velocity 

over temperature sensors. Dew-point thermometers cool a small mirror until water condenses on the surface. 

Dew-point thermometers are accurate, but often complex instruments. The term “frost point” is used when 

temperatures are below the freezing point. Neither aspirated psychrometers nor dew-point sensors are particularly well suited to remote field use because of fragility, weight, or power requirements for the fan or cooling. 

Electronic humidity sensors measure electronic signals generated in response to the effect of humidity on a 

variety of sensor elements. Properties measured include the absorption or conductivity of salt solutions, thin 

film capacitance, and differential cooling surfaces. Calibration and stability are the primary concerns with many 

electronic sensors, but the wide variety of sensors available makes it difficult to generalize about this category. 

The best solution, when feasible, is to use a second humidity sensor. The two sensors can be mounted as a pair, 

or one can be mounted 0.5 m lower than the other. Electronic humidity sensor elements are often wafer thin, 

and power requirements for some sensors can be low. Consequently, electronic humidity sensors can be readily 

incorporated into the small environmental sensor suites described in the main text.

Other Meteorological Parameters 

In addition to the four basic parameters that define the thermal environment—(1) temperature, (2) solar radiation, (3) wind speed, and (4) humidity—there are two other meteorological parameters that can significantly 

modify environmental effects on the soldier: (1) pressure, often an altitude effect; and (2) precipitation. 

Pressure and Altitude

The original barometer used a column of mercury with one open and one closed end, with a vacuum above 

the mercury column. The open end is sensitive to pressure, which pushed the column of mercury up into the 

end with the vacuum.19 Aneroid barometers use a metal diaphragm, which is usually a hollow disc of thin 

metal enclosing a cavity that is partially or completely evacuated with a spring inside. As pressure changes, the 

diaphragm expands or contracts. Changes in the diaphragm are electronically or mechanically translated into 

a pressure reading. 

Altimeters are barometers that use changes in atmospheric pressure to determine altitude. Most altimeters 

are calibrated for changes in atmospheric pressure at a known elevation. Because altimeters usually include 

barometric pressure as an output, they can be used to calculate changes in barometric pressure from standard 

temperature and pressure. For greater precision, barometric measurements should be adjusted for temperature 

effects. Making adjustments for barometric pressure is important if a unit moves to a different elevation. Usually, over short periods of time, changes in pressure from elevation are greater than changes from variation in 

atmospheric pressure associated with movement of air masses. Altimeters also aid in the navigation of mountainous terrain. 

Precipitation

Rain and snow have important effects on the overall operation of aid stations and medical evacuation operations. Simple rain gauges consist of a large collecting area, which funnels water into a smaller, scaled tube or 

gauge, so that even a small amount of rain can be read off the gauge. More elaborate instruments, including 

weighed or tipping rain buckets and raindrop detectors, are also available. Stream depth gauges are used to 

predict flooding and depth of immersion to monitor hazards for stream fording by both individuals and vehicles. Snow depth is sometimes read in a bucket or larger diameter gauge, but it is common to simply insert a 
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scale or gauge into the snow. Snow depth can be read easily by allowing it to accumulate on a flat bench, using 

a ruler to measure the depth, and then sweeping the bench clear for the next accumulation of snow. A more 

precise measurement of the weight of precipitation is to collect the snowfall, then melt it down to the liquid 

water equivalent. This can be more important in determining snow loading on tents, but snow removal is a 

function of both weight and volume.

REFERENCES

1. Santee WR, Matthew WT, Blanchard LA. Effects of meteorological parameters on adequate evaluation of the thermal 

environment. J Therm Biol. 1994;19:187–198.

2. Vernon HM. The measurement of radiant heat in relation to human comfort. Ergonomics. 1932;14:95–111.

3. Gagge AP, Nishi Y. Heat exchange between human skin surface and thermal environment. In: Lee DH, ed. Handbook 

of Physiology: Section 9. Reaction to Environmental Agents. Rockville, Md: American Physiological Society; 1977: 69–92. 

4. Olesen BW. International standards and the ergonomics of the thermal environment. Appl Ergonomics. 1995;26:293–302.

5. McIntyre DA. Indoor Climate. Essex, England: Applied Science Publishers; 1980.

6. Matthew WT, Santee WR, Berglund LG. Solar Load Inputs for Thermal Strain Models and the Solar Radiation Sensitive 

Components of the WBGT Index. Natick, Mass: United States Army Research Institute of Environmental Medicine; 2001. 

Technical Report T01-13.

7. Walsberg GE. Quantifying radiative heat gain in animals. Am Zool. 1992;32:217–224.

8. Underwood CR, Ward EJ. The solar radiation area of man. Ergonomics. 1966;9:155–168.

9. Breckenridge JR, Goldman RF. Solar heat load in man. J Appl Physiol. 1971;31:659–663. 

10. Breckenridge JR, Goldman RF. Human solar heat load. ASHRAE Trans. 1972;78:110–119.

11. Bla•zejczyk K, Krawczyk B. The influence of climatic conditions on the heat balance of the human body. Int J Biometeorol.

1991;35:103–106. 

12. Santee WR, Gonzalez RR. Characteristics of the thermal environment. In: Pandolf KB, Sawka MN, Gonzalez RR, eds. 

Human Performance Physiology and Environmental Medicine at Terrestrial Extremes. Indianapolis, Ind: Benchmark Press; 

1988: 1–43.

13. Campbell GS. An Introduction to Environmental Biophysics. New York, NY: Springer-Verlag; 1977.

14. Idso SB. Transformation of a net radiometer into a hemispherical radiometer. Agric Meteorol. 1971;9:109–121.

15. Unwin DM. Microclimate Measurements for Ecologists. New York, NY: Academic Press; 1980.

16. Sears FW, Zemansky MW, Young HD. University Physics. 6th ed. Reading, Mass: Addison-Wesley; 1982.

17. Platt RB, Griffiths J. Environmental Measurement and Interpretation. New York, NY: Reinhold; 1964.

18. Gagge AP, Nishi Y. Physical indices of the thermal environment. ASHRAE J. 1976;18:47–51.

19. Lutgens FK, Tarbuck EJ. The Atmosphere: An Introduction to Meteorology. Englewood Cliffs, NJ: Prentice-Hall; 1979.