NASA/TM—2014–218368 

 

Assessment of a Technique for Estimating 

Total Column Water Vapor Using 

Measurements of the Infrared Sky 

Temperature 

 

Francis J. Merceret 

NASA/Kennedy Space Center, Florida

Lisa L. Huddleston 

NASA/Kennedy Space Center, Florida 

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June 2014
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NASA/TM—2014–218368 

 

Assessment of a Technique for Estimating 

Total Column Water Vapor Using 

Measurements of the Infrared Sky 

Temperature 

 

Francis J. Merceret 

NASA/Kennedy Space Center, Florida 

Lisa L. Huddleston 

NASA/Kennedy Space Center, Florida 

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National Aeronautics and 

Space Administration

Kennedy Space Center 

Kennedy Space Center, FL 32899-0001 

June 2014

i 

Acknowledgments 

 Enter acknowledgments here, if applicable. 

The authors appreciate the time taken by Forrest Mims (GCO) to answer our questions 

regarding his instrumentation and procedures. Perry King and David Scitney calibrated our IRTs 

in the KSC Calibration Laboratory and Timothy Griffin provided us with training and workspace 

in the KSC Wet Chemistry Laboratory for additional calibrations. Kathy Winters (45WS) and Bill 

Bauman (AMU) assisted us in resolving significant differences between GPS and sounding IPW 

data during the summer of 2013. We appreciate the management support from Richard Mizell 

and John Madura that permitted us to undertake this informal, unfunded project. We also deeply 

appreciate the time and effort invested by the following in reviewing various drafts of this paper: 

David Brooks, Forrest Mims, Tim Garner, Dave Sharp, Pete Blottman, and William Roeder. 

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Executive Summary 

A method for estimating the integrated precipitable water (IPW) content of the atmosphere using 

measurements of indicated infrared zenith sky temperature was validated over east-central 

Florida. The method, suggested in a 2011 article in the Bulletin of the American Meteorological 

Society (BAMS), uses inexpensive, commercial off the shelf, hand-held infrared thermometers 

(IRT) costing less than $50 each. Two such IRTs were obtained from a commercial vendor, 

calibrated against several laboratory reference sources at KSC, and used to make IR zenith sky 

temperature measurements in the vicinity of KSC and Cape Canaveral Air Force Station 

(CCAFS). These measurements were compared with near simultaneous GPS-based IPW 

measurements from the National Oceanic and Atmospheric Administration’s GPS site at 

CCAFS. 

The calibration and comparison data showed that these inexpensive consumer-grade IRTs 

provided reliable, stable IR temperature measurements and that their zenith sky temperature 

readings were well correlated with the NOAA IPW observations. The regression equations 

between the IRTs and the NOAA IPW were consistent with those presented in the BAMS article, 

which were developed in Texas and Hawaii for the same make and model IRT several years 

ago. This consistency across geographic region and time suggests that the methodology is 

robust and stable enough to be applied in a variety of operational circumstances where 

inexpensive, field-deployable estimates of IPW are required.

Preface 

Integrated precipitable water has a variety of uses in weather observation and forecasting 

including identifying the passage of fronts and other boundaries and assessing the potential for 

convective or severe weather. There are several ways it is conventionally measured, all of 

which are costly, cumbersome, or both. In 2011, an article in the Bulletin of the American 

Meteorological Society (BAMS) described a method for measuring IPW which was neither costly 

nor cumbersome. The technique, developed during field programs in Texas and Hawaii 

measured the infrared sky temperature at the zenith using off the shelf consumer-grade handheld infrared thermometers available at retail for less than $50. Remarkably, under clear sky 

conditions, these simple instruments appeared to produce repeatable measurements wellcorrelated with GPS or sun photometer IPW data from instruments costing several orders of 

magnitude more. 
iv

Table of Contents 

Acknowledgements ...................................................................................................................... i 

Executive Summary ................................................................................................................... iii 

Preface ...................................................................................................................................... iii 

Table of Contents ....................................................................................................................... iv 

List of Figures ............................................................................................................................. v 

List of Tables ............................................................................................................................. vi 

List of Acronyms ....................................................................................................................... vii 

Chapter 1.Introduction ................................................................................................................. 1 

Chapter 2. Instrumentation and Data Collection ......................................................................... 2 

2.0 Overview. ......................................................................................................................... 2 

2.1 The IRTs. .......................................................................................................................... 2 

2.2 The Conventional Thermometers ...................................................................................... 3 

2.3 Calibration Procedures, Facilities and Fixtures ................................................................. 5 

2.3.1 The KSC Calibration Laboratory ................................................................................. 6 

2.3.2 The Wet Chemistry Laboratory ................................................................................... 7 

2.3.3 Office calibrations ......................................................................................................13 

2.3.4 Field calibration of the two IRTs ................................................................................19 

2.3.5 Overall Calibration of the IRTs ..................................................................................20 

2.4 Field Data Collection ........................................................................................................24 

2.4.1 GPS and Sounding Data Used as “Ground Truth” .....................................................25 

2.4.2 IRT Data ...................................................................................................................25 

Chapter 3.Results ..................................................................................................................... 27 

3.0 Overview .........................................................................................................................27 

3.1 Sky temperature regressions against GPS/sounding IPW ...............................................28 

3.2 Comparison of our regressions with those of the GCO team ...........................................31 

Chapter 4.Summary, Discussion and Conclusion ...................................................................... 35 

4.0 Summary .........................................................................................................................35 

4.1 Discussion .......................................................................................................................35 

4.2 Conclusion .......................................................................................................................36

References .............................................................................................................................. 38 
v 

List of Figures 

Figure 2.1-1. The IRTs used in this study with the thermal monitoring modifications. ................. 3 

Figure 2.2-1. The two Thermoworks RT600C thermometers with color-coded identifying cable 

ties. ........................................................................................................................................... 4 

Figure 2.2-2. The Oakton reference standard............................................................................. 5 

Figure 2.3.2-1. The body of the constant temperature bath. ....................................................... 8 

Figure 2.3.2-2. The constant temperature bath cap for thermometer intercomparisons. ............. 9 

Figure 2.3.2-3. The assembled thermometer intercomparison bath. ........................................... 9 

Figure 2.3.2-4. The IR “mini-cavity” for the constant temperature bath ..................................... 10 

Figure 2.3.2-5. The IR “mini-cavity” in the constant temperature bath. ..................................... 11 

Figure 2.3.3-1. IR calibration fixture made from an ice chest and the calibration plate. ............ 14 

Figure 2.3.3.1-1. IRT calibration plate time constant measurements ........................................ 15 

Figure 2.3.3.2-1. The IRT temperature time constant measurements. ...................................... 16 

Figure 2.3.3.3-1. IR and case temperature differences from ambient. ...................................... 17 

Figure 2.3.3.4-1. Wet Chem Lab and Office calibrations .......................................................... 18 

Figure 2.3.3.4-2. Wet Chem Lab and Office intercomparison of the IRTs ................................. 18 

Figure 2.3.4.1-1. IRT indicated sky temperature intercomparisons ........................................... 19 

Figure 2.3.4.2-1. IR vs. case temperature departures from the mean ....................................... 20 

Figure 2.3.5-1. Combined calibrations of the FJM IRT ............................................................. 21 

Figure 2.3.5-2. Combined calibrations of the LLH IRT .............................................................. 22 

Figure 2.3.5-3. The LLH IRT readings as a function of the FJM IRT readings during calibrations

 ................................................................................................................................................ 23 

Figure 2.3.5-4. The LLH IRT readings as a function of the FJM IRT readings for all comparisons

 ................................................................................................................................................ 24 

Figure 2.4.2-1. Sample size as a function of hour of day (UTC) ............................................... 26 

Figure 3.0-1. GPS IPW vs. Sounding IPW. .............................................................................. 28 

Figure 3.1-1. The separate IRT measurements vs. composite IPW. ......................................... 29 

Figure 3.1-2. The combined IRT measurements vs. composite IPW ........................................ 30 

Figure 3.1-3. The combined clear sky IRT measurements vs. composite IPW. ........................ 31 

Figure 3.2-1. KSC and GCO indicated sky temperatures vs. IPW ............................................ 32 

Figure 3.2-2. Difference in indicated sky temperatures between GPS and Sun Photometer 

reference instruments for give GPS IPW. ................................................................................. 33 

Figure 3.2-3. KSC and adjusted GCO indicated sky temperatures vs. IPW .............................. 34 

Figure 4.1-1. GPS IPW and KSC IRT indicated sky temperatures for 23 November 2012. ...... 36 
vi

List of Tables 

Table 2.1-1. Kintrex IRT-0421 Specifications. ............................................................................ 2 

Table 2.3.1-1. Calibration Laboratory results for the IRTs. ......................................................... 7 

Table 2.3.2.1-1. Thermoworks comparison temperatures in the constant temperature bath. .... 12 

Table 2.3.2.2-1. The IRT Wet Chemistry Lab bath calibration results. ...................................... 13 

Table 2.4.2-1. Locations at which sky temperatures were taken. ............................................. 25 

Table 2.4.2-2. Sample size as a function of segment of day..................................................... 26 
vii 

List of Acronyms 

45WS 45th Weather Squadron 

AMU Applied Meteorology Unit 

BAMS Bulletin of the American Meteorological Society 

CCAFS Cape Canaveral Air Force Station 

EST Eastern Standard Time 

GCO Geronimo Creek Observatory 

GPS Global Positioning System 

IPW Integrated Precipitable Water 

IRT Infrared Thermometer 

KSC Kennedy Space Center 

KSCWO KSC Weather Office 

NASA National Aeronautics and Space Administration 

NOAA National Oceanic and Atmospheric Administration 

UTC Coordinated Universal Time 

1 

Chapter 1. Introduction 

In the October 2011 issue of the Bulletin of the American Meteorological Society, a group from 

the Geronimo Creek Observatory (GCO), NASA Langley Research Center and the Institute for 

Earth Science Research and Education presented a method for measuring integrated 

precipitable water (IPW) using inexpensive, commercially available hand-held infrared 

thermometers (IRT) (Mims et al., 2011). For convenience we will denote them by the affiliation 

(GCO) of the lead author. At the time, the Applied Meteorology Unit (AMU), managed by the 

KSC Weather Office (KSCWO), was beginning a study of the utility of GPS measurements of 

IPW as a tool for forecasting the onset of convective storms in central Florida, extending 

preliminary studies facilitated by the 45WS (Roeder et al., 2010) (Kehrer et al., 2008) (Mazany 

et al., 2002). Since they would be systematically collecting both local NOAA GPS IPW data and 

radiosonde soundings from Cape Canaveral Air Force Station, the KSCWO decided to leverage 

these data sets to evaluate how well the GCO methodology worked in the central Florida 

environment. 

In addition to professional curiosity, there was a practical motivation for the effort. The KSCWO 

works very closely with the 45th Weather Squadron at Cape Canaveral Air Force Station 

(CCAFS). If the technique could be verified, it might be of value to their personnel, and other 

U.S. military personnel, when deployed overseas in locations where the necessary precision 

GPS measurements are not available and soundings are not frequent enough to capture rapidly 

changing weather conditions. 

The authors each personally purchased one of the instruments specified in the BAMS article, 

the Kintrex IRT0421. These instruments are described in some detail in Chapter 2. The KSCWO 

provided calibration equipment also described in Chapter 2. We hoped that by having two 

identical instruments, and by performing as thorough a calibration as our limited resources 

would permit, we could quantitatively estimate the reliability and repeatability of the IRT sky 

temperature measurements under operational conditions. This would be done by comparing the 

results from the two instruments that would be used from different but nearby locations except 

when side-by-side comparisons were made. The locations from which IRT data were taken as 

well as the locations of the GPS and sounding sites are presented in Chapter 2. 

Because there was no formal funding for this project, measurements, including calibrations, 

were made on a non-interference basis with other work. In addition, it was essential that the sky 

near the zenith be clear or nearly so for the measurements to be valid. The project was 

conducted for more than a year so that wet season and dry season measurements could be 

made and readings were taken both during the day and at night, but because of the limited 

resources and clear sky constraints, the sample size is not large. Details are provided in 

Chapter 3.
2

Chapter 2. Instrumentation and Data Collection 

2.0 Overview. 

This project involved two IRTs, three conventional thermometers, and several calibration 

facilities or fixtures. These are described in sections 2.1, 2.2 and 2.3 respectively. Section 2.4 

describes the field data collection. Temperatures in this paper are generally reported in degrees 

Fahrenheit rather than SI units because all of our measurements were made in degrees F to 

gain additional resolution. The IRTs and in-situ thermometers all had readouts that would report 

either C or F as selected by the user, but, in either case, to one decimal place. By selecting the 

F option, we obtained a factor of 1.8 finer temperature resolution. Temperature measurements 

or specifications provided by others are reported in the units in which they were provided.

2.1 The IRTs. 

Shortly after reading Mims et al. (2011), one of us (FJM) ordered a Kintrex model IRT-0421 from 

an online retailer. This model was selected because it was one of the two providing the best 

results in the paper, and it was immediately available for less than $50. After experimenting with 

it for several weeks, he initiated discussions with others in the KSCWO and the AMU, including 

his co-author (LLH) who purchased a similar instrument from the same vendor several months 

later. The two instruments have serial numbers E1032026567 (FJM) and E1032026193 (LLH). 

Specifications for these instruments are presented in Table 2.1-1. 

Table 2.1-1. Kintrex IRT-0421 Specifications 

Quantity Value Units 

Measurement Range -60 to 500 (-76 to 932) deg C (deg F) 

Distance to spot ratio 12:1 N/A 

90% response time 1 seconds 

Emissivity 0.95 N/A 

Resolution 0.1 deg C or F 

Accuracy 1.0 to max of 2.0 or 2% of reading 

depending on ambient 

temperature and temperature of 

target 

deg C 

IR spectral response 5 - 14 microns 

Except for the IR spectral response, the data in Table 2.1-1 are taken from the vendor’s 

literature available online. The BAMS article noted that most of the manufacturers of the various 

IRTs they explored did not make their spectral response available, but cited figures for Kintrex. 

We have used their spectral response specification because we were unable to independently 

get access to that information. 

The BAMS article also indicated that no ambient temperature measurement was necessary 

because the instrument is internally compensated. We wanted to verify that this compensation 

actually worked, so we modified both instruments by attaching a 1/8 inch (3.2 mm) diameter 

hollow copper tube to the casing with thermally conductive epoxy as shown in Figure 2.1-1. The 
3 

tube was sized to accept the probe of a calibrated thermometer described below in Section 2.2. 

This permitted a rough measure of the ambient operating temperature of the instruments when 

IR data was taken. 

Figure 2.1-1. The IRTs used in this study with the thermal monitoring modifications. 

2.2 The Conventional Thermometers 

Since the two IRTs were to be used separately, it was necessary to acquire two case 

temperature thermometers. They had to be accurate and stable (after calibration), rugged, and 

inexpensive since they were going to be used in the field. After some market research, 

Thermoworks model RT600C was selected and two units ordered online. Because they did not 

have serial numbers or other identifying markings, colored cable ties were attached to identify 

them individually. A blue tie was attached to the unit assigned to accompany the FJM IRT and a 

red one for the LLH IRT. These are shown in Figure 2.2-1. 
4

Figure 2.2-1. The two Thermoworks RT600C thermometers with color-coded identifying 

cable ties.

These instruments cost less than $20 each, have a response time less than six seconds, and 

are accurate to within 0.5 C over the range -40 to 150 C. Calibrations against our laboratory 

reference instrument demonstrated that these instruments provided a stable and accurate 

reference for the case temperatures. 

Our primary reference instrument for laboratory temperatures was an Oakton Instruments Model 

Temp5, serial number 878341 with plug-in probe serial number 93X052912 192. It was NISTtraceably calibrated by Innocal with calibration report number 190264. The calibrations were 

performed at -38.8, 0.0, 50.0 and 120.0 C with deviations from the NIST standard not exceeding 

0.2 C. This instrument is shown in Figure 2.2-2. 
5 

Figure 2.2-2. The Oakton reference standard.

Unlike the field thermometers from Thermoworks, the Oakton was an expensive, laboratorygrade instrument which could serve as the primary temperature reference for the project. It has 

a specified resolution of 0.1 C with an accuracy of 0.2 C from -40 to 125 C. 

2.3 Calibration Procedures, Facilities and Fixtures 

There were several reasons that we undertook a significant amount of calibration work for such 

a small and informal project rather than simply using the data from the IRTs without additional 

inquiry. First, we wanted to be able to compare the two units we had purchased to see how 

much consistency there was between them. That required knowing how much of any difference 

between them was due to calibration differences as opposed to differences in taking the 

measurements. Second, we wanted to verify the GCO group’s assertion that ambient 

temperature measurement was not necessary because of the internal temperature 

compensation. This required the use of thermometers as described above for measuring the 

case temperature of each unit, and these two thermometers needed to be calibrated sufficiently 

that their measurements could be compared reliably. 

Calibrating IR instruments is difficult because the reference (whether plate or cavity) must have 

a known emissivity and temperature. The temperature is relatively easy to measure in the 

laboratory if it is uniform across the reference surface or cavity. The emissivity is not 

measurable without special capabilities that we do not have, but our Calibration Lab has a 
6

suitable device which approximates an emissivity of 1.0 to a sufficient level of accuracy for our 

purposes as long as the surface does not become contaminated. Unfortunately, even with a dry 

nitrogen purge, the reference device often developed a layer of dew or frost when the cavity 

was cooled below the ambient dewpoint, which in Florida was always above 10 C. 

Scheduling time in the Calibration Laboratory is difficult, and funding was not available for a 

significant amount of calibration by the lab. In order to collect some additional data under a 

variety of circumstances, the authors constructed a “mini-cavity” which is described below. It 

was temperature-controlled in a water bath monitored by the NIST-traceable Oakton 

thermometer. Like the device used by the Calibration Laboratory, our mini-cavity would rapidly 

become coated with water when cooled below the dewpoint. The bath was also used to 

calibrate the Thermoworks case temperature thermometers. Because of the convenience of 

having faucets and a sink at hand and the risk of spillage when working with a water bath, this 

work was performed in the Wet Chemistry Lab at KSC. 

In some cases, calibration measurements were made without the need for running water or 

significant risk of spillage. These were performed in our office using additional fixtures that are 

described below. They were primarily related to determining the time constants of the IRTs and 

attempting to assess their sensitivity to case temperature differences. 

Finally, sky temperature measurements were made with the instruments operated nearly 

simultaneously side by side for a direct intercomparison. 

2.3.1 The KSC Calibration Laboratory 

The KSC Calibration Laboratory calibrated both of our Kintrex IRT-0421 units with a Fluke 

model 9133 Portable Infrared Calibrator. Five test points were used: 5.0, 23.0, 41.0 and 59.0 F 

(-15.0, -5.0, 5.0, and 15 C). The temperature of the calibrator was monitored using a Fluke 

model 1502A thermometer readout with an RTD probe. The calibration report (Perry King, 

private communication) noted that there were difficulties with the test setup and that “even with 

a high rate of purge there was extensive frost buildup.” The results in that report are shown in 

Table 2.3.1-1 in the units in which the calibration was performed and reported by the calibration 

laboratory. 
7 

Table 2.3.1-1. Calibration Laboratory results for the IRTs. 

SN E1032026193 

Test Point 

°F (C) 

Indication 

°F (C) 

Error 

°F (C) 

Std Deviation 

°F (C) 

5.0 (-15.0) 10.0 (-12.2) +5.0 (+2.8) ±0.3 (0.2) 

23.0 (-5.0) 26.7 (-2.9) +3.7 (+2.1) ±0.4 (0.2) 

41.0 (5.0) 43.9 (6.6) +2.9 (+1.6) ±0.0 (0.0) 

59.0 (15.0) 59.6 (15.3) +0.6 (+0.3) ±0.2 (0.1) 

SN E1032026567 

Test Point 

°F (C) 

Indication 

°F (C) 

Error 

°F (C) 

Std Deviation 

°F (C) 

5.0 (-15.0) 9.4 (-12.6) +4.4 (+2.4) ±0.7 (0.4) 

23.0 (-5.0) 25.8 (-3.4) +2.8 (+1.6) ±0.4 (0.2) 

41.0 (5.0) 43.1 (6.2) +2.1 (+1.2) ±0.3 (0.2) 

59.0 (15.0) 59.4 (15.2) +0.4 (+0.2) ±0.1 (0.1) 

The results in bold italics are outside of the specifications of the Kintrex units, but they are also 

at temperatures below freezing where frost in the calibration device was a problem. This was 

one of the motivations for the additional calibration work we performed in the Wet Calibration 

Laboratory. It should also be noted that the factory calibration of the Kintrex 0421 assumes an 

emissivity of 0.95 over the range 5 – 14 m whereas the calibration device is specified to have 

0.95 emissivity over the range 8 – 14 m. It is not known what the emissivity of the calibration 

device is between 5 and 8 m, but that may affect the readings. Significantly, the two IRTs 

showed very similar error characteristics. 

2.3.2 The Wet Chemistry Laboratory 

Early in the planning of the project we realized that we would need to have a method of 

determining the relative calibrations of our three reference thermometers. It should be available 

as frequently as needed, cost very little, and be accurate within about 0.1F. Properly executed, 

an insulated water bath will easily perform that well, so we designed one using readily available 

off the shelf components. 
8

Figure 2.3.2-1. The body of the constant temperature bath.

For the body of the bath we used a high-end commercial stainless steel thermos bottle (Figure 

2.3.2-1) but replaced the removable top with two special fixtures depending on the type of 

measurement to be made. For comparing the three thermometers, a top made from a PVC pipe 

cap fit quite snugly to preserve the insulating properties of the bottle, while three 3.2 mm (1/8 in) 

holes (Figure 2.3.2-2) allowed us to insert the three temperature probes simultaneously (Figure 

2.3.2-3). 
9 

Figure 2.3.2-2. The constant temperature bath cap for thermometer intercomparisons.

Figure 2.3.2-3. The assembled thermometer intercomparison bath.

We also wanted to have some way to at least attempt some additional IRT calibrations of our 

own. We decided to try placing a cylindrical cavity in the constant temperature bath. We epoxied 

a copper tube 25.4 mm (1 in) diameter by 165 mm (6.5 in) long in the center of another PVC 

cap and sealed the bottom of the tube with metal and the thermally-conducting epoxy. When the 

epoxy had hardened, we sprayed the interior surfaces of the tube with Krylon 1602 “ultra flat 

black” paint which has very high IR emissivity. The completed cavity insert is shown in Figure 

2.3.2-4 
10

One 3.2 mm hole was drilled for a reference thermometer as shown in Figure 2.3.2-5. In 

operation, the IRT is aimed straight down the cavity. Its aperture is the same diameter as the 

tube so its entire field of view should be filled with the cavity and only the cavity. 

Figure 2.3.2-4. The IR “mini-cavity” for the constant temperature bath.
11 

Figure 2.3.2-5. The IR “mini-cavity” in the constant temperature bath.

2.3.2.1 The Thermoworks thermometers 

Several tests were performed to evaluate the comparability and time constant of the two 

Thermoworks thermometers. The first series of tests began with the water in the constant 

temperature bath cooled below or heated above ambient room temperature. The Thermoworks 

thermometers were allowed to come to equilibrium at room temperature, then plunged into the 

bath. After they had come to equilibrium in the bath they were withdrawn and again allowed to 

come to room temperature equilibrium. The room temperature and bath temperature were both 

measured with the Oakton reference thermometer. Room temperature ranged from 70.9 to 71.6 

F. The bath temperature ranged from 44 to 109 F. The equilibrium temperatures in the bath are 

shown in Table 2.3.2.1 – 1. 
12

Table 2.3.2.1-1. Thermoworks comparison temperatures in the constant temperature 

bath.

Oakton Reference , 

°F (C) 

Red-tagged Thermoworks, 

°F (C) 

Blue-tagged Thermoworks, 

°F (C) 

44.1 (6.7) 44.6 (7.0) 44.4 (6.9) 

53.7 (12.1) 53.8 (12.1) 54.2 (12.3) +/- 0.1 (unsteady) 

72.1 (22.3) 72.5 (22.5) 72.3 (22.4) 

88.1 (31.2) 88.2 (31.2) 88.0 (31.1) 

109.4 (43.0) 109.4 (43.0) 109.4 (43.0) 

Finally, a relatively crude “ice bath” test was performed by adding crushed ice and distilled water 

to the constant temperature bath and repeating the same test sequence just described. The 

resulting reference temperature was 31.9 F (-0.1 C). The red thermometer read 32.4 F (0.2 C) 

and the blue 32.0 F (0.0 C). When the location of the two Thermoworks thermometers was 

switched (in case of non-uniformity of the ice bath), the results were 32.5 F (0.3 C) (red) and 

32.0 F (0.0 C)(blue). 

The time constants for the Thermoworks thermometers going from air to water were always 

between ten and 15 seconds. The time to equilibrium when removed from the bath was more 

variable but was always less than 240 seconds and sometimes less than 200. 

Based on these results, it appeared the Thermoworks thermometers could be used as reliable 

case temperature monitors for the IRTs over the ambient temperature range at which 

measurements were likely to be made in central Florida. Their response time was fast and their 

accuracy was within 1C (1.8F) or better in reference to both the Oakton standard and each 

other. 

2.3.2.2 The IRTs 

The mini-cavity was placed in the constant temperature bath with the red-tagged Thermoworks 

thermometer as a reference as shown in Figure 2.3.2-5 above. Bath temperatures from 60.3 to 

105.1 F (15.7 to 40.6 C) were used. Colder temperatures were not attempted because we had 

no way of attempting a dry air purge to prevent condensation below the dewpoint which was in 

the mid-50s F. For the IRTs, the blue-tagged Thermoworks was used to measure the body 

temperature. Because small differences in the way the IRT was held when the measurement 

was made could result in slight differences in measured temperatures, four IRT readings were 

taken at each bath reference temperature for each IRT. Since this took several minutes, the 

bath temperature and body temperatures were recorded at both the beginning and end of the 

eight measurement sequence (four measurements for each of two IRTs). 

The bath temperature, the case temperatures and the IRT measurements were all very stable. 

In all cases but one, the bath temperature remained constant. In that case, the bath cooled by 

0.2 F between the first and last measurement. Case temperatures were, of course, always near 

room temperature which fluctuated a few tenths of a degree about 71 F (21.7 C). They ranged 

from 70.5 to 72.3 F (21.4 to 22.4 C) with the largest change during any one set of eight 

measurements being 1.2 F. The four IRT measurements within any single set for either 

instrument varied by at most 0.5 F and frequently by only 0.1 F. For that reason, we decided to 

throw out the highest and lowest of the four readings and average the remaining two to obtain 

the value for comparison with the reference. The results are presented in Table 2.3.2.2-1. 
13 

Table 2.3.2.2-1. The IRT Wet Chemistry Lab bath calibration results. 

Reference,°F (C) FJM IRT,°F (C) LLH IRT,°F (C) 

105.1 (40.6) 106.9 (41.6) 106.3 (41.3) 

96.8 (36.0) 97.9 (36.6) 97.8 (36.6) 

87.0 (30.6) 87.3 (30.7) 87.0 (30.6) 

77.0 (25.0) 76.5 (24.7) 76.9 (24.9) 

69.8 (21.0) 68.9 (20.5) 69.1 (20.6) 

60.3 (15.7) 58.7 (14.8) 58.9 (14.9) 

The results are very good at or near room temperature. These measurements are combined 

with additional measurements made in the office in section 2.3.3.4 below. 

2.3.3 Office calibrations 

There was another option for some of the instrument characterization and calibration tests we 

wanted to do which did not require access to water. It was more convenient to do these in our 

normal office environment. We needed a competent IR target with high emissivity and large 

thermal inertia so that its time constant was significantly longer than the time it takes to make a 

measurement, and a way of heating of cooling either the target, the IRTs, or both. 

For the target, we purchased a 305 cm (12 in) square aluminum plate 0.63 cm (1/4 in) thick and 

had three holes 0.32 cm (1/8 in) x 15.3 cm (6 in) deep drilled in one edge for insertion of one or 

more reference thermometers. The surface was painted with the same high emissivity paint 

used for the mini-cavity discussed above. The plate appears as part of Figure 2.3.3-1 which is 

discussed below. For cold-soaking or hot-soaking the plate or the IRTs, we used two ice chests 

and hot water bottles or bags of ice. Small holes were placed in the lids of the ice chests for 

reference thermometers. An IRT or the plate could be placed in an ice chest with hot water 

bottles or bagged ice and the chest allowed to come to equilibrium as indicated by the inserted 

thermometer. Then the instrument or plate could be removed for use as described later in this 

section. 
14

Figure 2.3.3-1. IR calibration fixture made from an ice chest and the calibration plate.

Near the end of the project, one of the ice chests was modified to make a dedicated IR 

calibration box by cutting slots in two sides to allow the plate to be inserted at an angle with the 

box sealed except for holes in the lid to insert the thermometers in the plate and a port in a third 

side through which the IRT would view the plate as shown in Figure 2.3.3-1.This allowed the 

use of ice bags to create a stable temperature reference for intercomparing the two IRTs 

essentially simultaneously against the same reference at cooler temperatures than we could 

obtain without this fixture. We could observe whether there was any dew or frost on the surface 

of the plate, but it was never cooled enough for that to occur. 

2.3.3.1 The time constant of the IR target plate 

The time constant of the aluminum plate to be used as a target for IRT comparisons and 

calibrations was determined by heating it above ambient temperature in one of the ice chests 

with a hot water bottle and then monitoring its temperature as it returned to ambient when 

removed from the chest. The plate was enclosed in an unsealed plastic bag to protect it from 

any possible leak in the hot water bottle. The ice chest was closed for at least an hour. Its 

internal air temperature was monitored with one of the Thermoworks thermometers to ensure 

that the enclosure had reached a stable temperature at least 10 F above ambient. 

After the temperature in the enclosure had stabilized and at least an hour had passed, the plate 

was removed from the plastic bag and set with its bottom on the rug and its top resting against 
15 

the ice chest with both the front (IR-blackened) and rear (polished aluminum) faces fully 

exposed to the ambient air. The red-tagged Thermoworks thermometer was inserted into the 

center thermometer hole in the plate and temperatures recorded at roughly one minute intervals 

for the first ten minutes, and then at increasingly longer intervals as the rate of change of 

temperature declined. After 6 minutes had elapsed, one of the IRTs was used to examine all 

four corners, the top and bottom edges and the center of the plate to determine how uniform the 

temperature was across the plate. The measurements always were within 0.2 F when 

comparing any two points on the plate within 15 seconds or so. 

The results are shown in Figure 2.3.3.1-1. The time constant calculated from the exponential 

least squares fit was 20.8 minutes. 

Figure 2.3.3.1-1. IRT calibration plate time constant measurements.

2.3.3.2 The time constant of the IRT instrument cases 

The process described immediately above for determining the time constant of the calibration 

plate was also used to determine the time constant of the IRT case temperatures. The two IRTs 

were processed separately. The red-tagged thermometer was used with the FJM IRT and the 

blue one with the LLH IRT. The thermometers were inserted into the copper tube affixed to the 

IRT case as soon as the IRT was removed from the ice chest. The IRT with its thermometer 

was laid on top of the closed ice chest. This exposed all but one side to the ambient air. The top 

of the ice chest is insulating Styrofoam, which was at or very near the ambient room 

temperature. Because the two IRTs were tested separately, their starting temperatures were not 

the same, but as shown in Figure 2.3.3.2-1, they had essentially identical slopes to the 

exponential least squares fit, equivalent to a time constant of about 28 minutes. 
16

Figure 2.3.3.2-1. The IRT temperature time constant measurements.

2.3.3.3 The sensitivity of the IRTs to changes in case temperature 

Our attempts to confirm that the internal temperature compensation circuitry in the IRTs worked 

acceptably were limited. The reference plate was allowed to come to room temperature 

equilibrium with the red Thermoworks thermometer in the center thermometer port. The case 

temperature of the IRTs was measured with the blue Thermoworks after the IRT has been 

allowed to come to equilibrium in one of the ice chests with a hot water bottle. Unfortunately, 

this methodology could only produce case temperatures less than 11 F above ambient because 

of the limited heat capacity of the hot water bottle. We were reluctant to try cooling the IRTs in 

the ice chest because of the possibility that we could cause condensation to form inside the 

units and thereby damage them. The results of these limited explorations are shown in Figure 

2.3.3.3-1. 
17 

Figure 2.3.3.3-1. IR and case temperature differences from ambient.

In our judgment, these results are inconclusive for several reasons. First, even taken at face 

value, the data show a negative slope for case temperatures near ambient, a positive slope for 

temperatures 4 to 8 degrees above ambient, and no temperature sensitivity from 8 to 11 

degrees above ambient, which provides no reliable indication of any real, consistent sensitivity 

given the small range and sample size. Second, the data cannot be taken at face value, 

because we noted what we called a “hand effect” when taking data. Specifically, the case 

temperature appeared to elevate by several tenths of a degree F when a reading was being 

taken. We strongly suspect that the heat from our hand as we held the IRT slightly warmed the 

copper tube in which the Thermoworks thermometer was placed. This effect differed in 

magnitude depending on the case temperature. 

We had tentatively planned to attempt measurements with case temperatures below ambient at 

the end of the project when damage to the IRTs would be least disruptive to the project. After 

examining the field measurements and especially the results presented in section 2.3.4.2 below 

we felt confident enough that the internal compensation was effective to abandon any additional 

efforts in the lab or office. 

2.3.3.4 Office calibrations and intercomparisons of the two IRTs 

In addition to the simultaneous calibrations of the two instruments described above in section 

2.3.2.2 above, we performed a number of measurements using the target plate in an office 

environment. These included ambient temperature measurements where the plate and both 

IRTs were allowed to come to equilibrium with the room temperature, and measurements made 
18

in the modified ice chest with the flat plate insert. The results are combined with the wet 

chemistry laboratory results and shown in Figures 2.3.3.4 -1 and 2.3.3.4-2 below. There were 

also several plate calibrations made at the home of one of the authors (FJM) that are not 

included in this figure, but are included in the data shown in section 2.3.5. 

Figure 2.3.3.4-1. Wet Chem Lab and Office calibrations.

Figure 2.3.3.4-2. Wet Chem Lab and Office intercomparison of the IRTs.
19 

2.3.4 Field calibration of the two IRTs 

In addition to calibration in laboratory and office environments, additional assessments of the 

performance characteristics of the IRT instruments were made using the indicated sky 

temperature measurements. Measurements from the two instruments at or near the same time 

and place were compared to provide an estimate of measurement uncertainty. An assessment 

of the effects, if any, of case temperature on the measured sky temperature was also attempted. 

2.3.4.1 Field intercomparisons of the two IRTs 

In thirteen cases, we made simultaneous indicated sky temperature measurements with the two 

IRTs at the same location, and we had one additional case where the two measurements were 

nearly simultaneous but from locations separated by about 10 km. The agreement is very good 

as shown in Figure 2.3.4.1-1. 

Figure 2.3.4.1-1. IRT indicated sky temperature intercomparisons.

2.3.4.2 Temperature sensitivity 

Because the laboratory examination of the effects of case temperature were inconclusive as 

described in section 2.3.3.3, we used the field measurements to provide some additional 

assessment of this concern. In 60 cases where we had case temperatures and valid sky 

temperature measurements, we made at least two measurements within a few minutes from the 
20

same location. Specifically, we had 20 sets of two measurements, 33 sets of three, 5 sets of 

four and one each of six and nine measurements within 10 minutes or less. For each of those 

cases we averaged both the case temperature readings and the IRT readings and computed 

the differences from the average for each of the separate measurements within the average. 

The departure of the IRT reading from the mean is plotted against the departure of the case 

temperature in Figure 2.3.4.2-1. 

Figure 2.3.4.2-1. IR vs. case temperature departures from the mean.

The sky temperature is assumed to be constant during the period because even with the 

relatively small field of view of the IRTs, the volume of atmosphere sampled is quite large and 

unlikely to have significant rapid changes in IPW. The scatter in the IR reading is assumed to 

represent random sampling error plus the effect, if any, of changes in the case temperature. The 

case temperature can vary for a variety of reasons including but not limited to the conduction 

from the atmosphere or the hand holding the instrument, internal heating due to activation of the 

instrument, or inadequate time allowed for the instrument to reach equilibrium with the 

environment. The figure shows that the IRT departures are completely uncorrelated with the 

case temperature departures. This is consistent with the assertion in BAMS that the internal 

thermal correction is effective. 

2.3.5 Overall Calibration of the IRTs 

As noted earlier in this report, there were several different sources of calibration data for the 

IRTs. These include the Calibration laboratory data, the Wet Chemistry Laboratory data, 
21 

comparisons in the office and plate calibrations at the home of an author. When combined, 

these show a consistent level of reliability and accuracy that is quite gratifying given the informal 

and sometimes crude methodologies used and the inexpensive consumer-grade 

instrumentation being calibrated. The complete calibrations for the ITRs are shown in Figures 

2.3.5-1 and 2.3.5-2. 

Figure 2.3.5-1. Combined calibrations of the FJM IRT.
22

Figure 2.3.5-2. Combined calibrations of the LLH IRT.

If these same data are used to compare the LLH IRT against the FJM IRT for mutual 

consistency, we obtain the result shown in Figure 2.3.5-3. 
23 

Figure 2.3.5-3. The LLH IRT readings as a function of the FJM IRT readings during 

calibrations.

If the sky temperature comparisons are added to the data in Figure 2.3.5-3 we have the overall 

composite of all of the comparison data as shown in Figure 2.3.5 -4 
24

Figure 2.3.5-4. The LLH IRT readings as a function of the FJM IRT readings for all 

comparisons.

Although the parameters of the linear least squares regressions are slightly different, the overall 

difference between the two instruments appears to be on the order of one degree F or less over 

the entire range from near -20 F to over 100 F. This is of the same order of the scatter in the 

measurements and indicates that for practical purposes, the two instruments may be treated as 

identical.

2.4 Field Data Collection 

This section describes the methodology for acquiring the GPS and balloon reference integrated 

precipitable water data and for taking the IRT sky measurements. The IRT sky readings will be 

referred to as “indicated sky temperatures” because the IR thermometer calculates a 

“temperature” assuming it is looking at a surface with black body characteristics and an 

emissivity of 0.95. The sky is not a surface nor is it a black body. The measurement by the 

zenith-directed IRT is of radiation emitted by a volume rather than a surface and any 

“temperature” varies within that volume. In the spectral region of interest, the emissivity for 

water vapor, in particular, varies significantly with wavelength (Petty, 2006). For that reason, the 

temperature indicated by the IRT cannot really be said to be the temperature of anything 

specific: it is just the temperature “indicated” by the device.
25 

2.4.1 GPS and Sounding Data Used as “Ground Truth”

NOAA’s CCV6 GPS IPW site located on Cape Canaveral Air Force Station near 28.46N, 

80.55W and soundings from the CCAFS balloon facility (ID = XMR) near 28.48N, 80.56W were 

used as the data against which regressions of IRT sky temperatures were generated. Sounding 

data was only available once or twice daily, but GPS data was usually available every 30 

minutes. When the GPS data was available, the GPS IPW reading closest in time to each sky 

temperature reading was used as the “ground truth” reference for that reading. If timely GPS 

data was not available, the nearest XMR sounding in time was used instead. 

Although the agreement between the soundings and the GPS data was generally excellent, 

there was a period during the early summer of 2013 where the soundings often appeared to be 

drier than either the GPS or nearby soundings. It is believed that there was a possible defect in 

the batch of sondes used at XMR to make those measurements, and during that period, 

sounding data was ignored. 

2.4.2 IRT Data 

IRT measurements were made at five sites as shown in Table 2.4.2-1. 

Table 2.4.2-1. Locations at which sky temperatures were taken. 

Description N. Latitude W. Longitude CCV6 distance (km) 

KSC Headquarters Building 28.52 80.65 12.6 

KSC Operations Support Building I 28.58 80.65 17.4 

Morrell Operations Center 28.43 80.59 6.3 

Author’s Home (FJM) 28.45 80.71 15.8 

Author’s Home (LLH) 28.40 80.75 21 

AMU PM’s Home 28.20 80.71 32.9 

Mims et al. (2011) used sun photometers as their primary reference because they found they 

correlated better with the sky temperatures than GPS readings. They speculated that this may 

have resulted from the fact that their sun photometers were essentially co-located with the IRT 

measurements whereas their GPS measurements were more than 30 km away from the IRT 

measurement site. Most of our comparisons involved separation distances of half that or less as 

shown in Table 2.4.2-1. 

We followed the protocol from Mims et al. (2011) for taking field measurements. It can be 

difficult to know if the IRTs were truly pointed to zenith. To compensate for this difficulty we 

made multiple measurements at the same time and place. We then recorded the lowest 

temperature observed based on the assumption that the straightest path to zenith would give 

the lowest temperature. We made measurements during most hours of the day except for the 

period from 11 PM through 4 AM EST as shown in Figure 2.4.2-1. In the figure, as in our data 

set, the time is recorded in UTC. EST is equal to UTC – 5 hours. Our local solar time is 

approximately EST – 20 minutes. 
26

Figure 2.4.2-1. Sample size as a function of hour of day (UTC).

The times of sunrise and sunset vary throughout the year but sunrise is always within about an 

hour of 11 UTC and sunset roughly within an hour of 23 UTC. As the figure shows, our 

observations were biased towards the hours just after sunrise and just before sunset. The local 

noon hour was deliberately avoided because the sun would be too close to the zenith and might 

get into the field of view of the instrument. Otherwise, we tended to make our measurements in 

the morning before or just after arriving at work or in the evening before or just after leaving the 

office for the day. Table 2.4.2-2 shows the sample size distribution I categorical form. 

Table 2.4.2-2. Sample size as a function of segment of day. 

Segment of Day Count Percent 

Night 17 7.7 

Near Sunrise 37 16.8 

Daytime 104 47.3 

Near Sunset 62 28.2 

Total 220 100 

If the zenith sky is cloudy, then the IRT essentially measures the cloud base temperature rather 

than the IPW, so it was important to note the sky conditions when making observations. We 

printed prepared forms using a spreadsheet which contained fields for the date, time and 

location of each observation, the case temperature, the sky temperature, and notes on the sky 

condition and anything else that might affect the representativeness of the reading. This served 

the dual purpose of reminding us of the need to pay attention to the sky conditions before 

committing to a measurement and to document that we had done so. 
27 

Chapter 3. Results 

3.0 Overview 

As noted in Chapter 1, the reference data for evaluating the performance of the IRTs as IPW 

instruments was to be measured by comparison with nearby NOAA GPS IPW data and 

soundings from Cape Canaveral Air Force Station. The GPS IPW data are available at 15 and 

45 minutes past the hour UTC from NOAA site CCV6 through the NOAA GPS data access 

gateway at http://gpsmet.noaa.gov/cgi-bin/gnuplots/rti.cgi. The sounding data are available for 

11:15 UTC for CCAFS (XMRB) and are also available on the NOAA GPS website. Unless it was 

unavailable or suspect, we used the GPS IPW observation closest in time to each IRT 

measurement as the reference value for that measurement. 

In some cases, the GPS data were not available. In those cases if there was a sounding less 

than four hours from the time the IRT data were collected, it was used as the reference. We 

compared the soundings with the GPS data when both were available in order to assure that the 

two data sources were comparable. The simple bias between the two data sources is -0.032 

and the mean average error (MAE) is 0.186. The results presented in Figure 3.0-1 demonstrate 

that the soundings and the GPS are mutually consistent data sources. In the discussions that 

follow, where no distinction is made between the GPS and sounding data it is referred to as the 

“composite” IPW.
28

Figure 3.0-1. GPS IPW vs. Sounding IPW.

Based on the calibration results in section 2.3.5 above showing that the two IRT instruments 

could be treated as identical for practical purposes, we sometimes combined the sky 

temperature measurements from the two instruments into a single composite sky temperature 

database called “KSCIR” to increase the sample size over what was available from the FJM and 

LLH instruments separately. 

3.1 Sky temperature regressions against GPS/sounding IPW 

All of the IRT sky temperature measurements collected with both IRTs are plotted in Figure 3.1-

1. 
29 

Figure 3.1-1. The separate IRT measurements vs. composite IPW.

The data are fitted to a logarithmic curve which is equivalent to the exponential fits used by 

Mims et al. with the dependent and independent variables reversed. Note that the difference 

between the two IRTs is everywhere within the scatter of the measurements. The results when 

the data are combined are presented in Figure 3.1-2. 
30

Figure 3.1-2. The combined IRT measurements vs. composite IPW.

Some of the data shown in Figure 3.1-2 were taken in circumstances where the possibility may 

have existed for some minor contamination of the reading by nearby or thin overhead cloud. A 

subset of the data was selected where it was certain that the sky was completely clear overhead 

in the field of view of the IRT. As seen in Figure 3.1-3, this did not significantly affect the results, 

and the effects of any possible contamination of the readings by cloud in the field of view appear 

to have been negligible. 
31 

Figure 3.1-3. The combined clear sky IRT measurements vs. composite IPW.

3.2 Comparison of our regressions with those of the GCO team 

In addition to determining that there was a dependable relationship between the IR sky 

temperature and IPW, we also wanted to see how closely that relationship agreed with the 

relationship reported by the GCO team. Since the relationship depends on the field of view and 

spectral response of the IRT, we compared our results only with the GCO results for the Kintrex 

0421. In Mims et al. (2011), Figure 7, the following equation is given: 

y = -0.141 + 3.333*exp(-x/-29.851) (3.2-1) 

where y is the IPW (cm) measured by the Microtops instrument described in the article and x is 

the zenith sky temperature measured by the IRT0412 (C). This equation may be inverted to give 

the equivalent GCO IRT reading as a function of IPW since the relationship between them is 
32

monotonic. The initial intention was simply to plot the computed GCOIR along with the KSCIR 

as a function of IPW and see how they compare. It turned out to be more complicated than that. 

Figure 3.2-1 shows the comparison of the computed GCO IR temperatures computed from 

equation 3.2-1 with the measured KSC IR temperatures for the same values of IPW. The data is 

a subset of all of the data obtained by eliminating balloon data and eliminating essentially 

redundant data. As previously noted in section 2.3.4.2 above, at any given time we usually took 

multiple measurements. These were not independent. In the figures in this section, each of 

those near-simultaneous sets of measurements has been replaced by the average of all of the 

values in the set. 

Figure 3.2-1. KSC and GCO indicated sky temperatures vs. IPW.

While the general shapes of the curves are the same, the agreement is not as good as we had 

hoped, and the difference between the two curves clearly exceeds the scatter in the 

measurements. A more careful analysis indicated why. 

There is a critical adjustment that needed to be made to the GCO equation in order for the 

results to be comparable with ours. Our IPW reference was GPS, but the GCO reference for 

equation 3.2-1 is a sun photometer called “MICROTOPS II”. It does not give the same values 

for IPW as the GPS does. To cross calibrate the MICROTOPS II with GPS, we used Mims et al. 

Figures 3 and 4 which compared a different IRT (an Omega OS540) with the MICROTOPS II 

and GPS respectively. The equivalent figures were not available for the Kintrex units, but we are 
33 

willing to assume that the OS540 response is sufficiently similar to permit it to be used as a 

transfer standard for determining the relation between the GPS and the sun photometer for our 

use. 

Combining the data from the two figures, we generated a regression between the difference in 

sky temperatures using GPS and MICROTOPS for a given GPS value of the IPW. The result 

was a second order polynomial that fit the data perfectly as shown in Figure 3.2-2 

Figure 3.2-2. Difference in indicated sky temperatures between GPS and Sun Photometer 

reference instruments for give GPS IPW.

When this adjustment is applied to the GCO computed values, the result is significantly closer to 

ours and within the scatter of the measurements as shown in Figure 3.2-3. The remaining 

differences may include not only the effects of sampling error and natural variance, but also the 

effects of local site variations that are not adequately captured in the GPS retrieval algorithms 

(Bevis et al., 1992). One such local variation that is likely is azimuthal anisotropy in low level 

atmospheric moisture due to the coastal location of the GPS site. Azimuthal isotropy is assumed 

in the GPS IPW calculation (Wolfe and Gutman, 2000; Bevis et al., 1992) and violation of that 

assumption can lead to errors in the results (ibid). Since the effects of this anisotropy are almost 

certainly dependent on the mean wind direction and vertical profiles of water vapor onshore and 

off shore, it probably increases the scatter in our data as these properties may differ for any 

given IPW value.
34

Figure 3.2-3. KSC and adjusted GCO indicated sky temperatures vs. IPW. 
35 

Chapter 4. Summary, Discussion and Conclusion 

4.0 Summary 

We have demonstrated that the use of inexpensive commercial off the shelf handheld infrared 

thermometers to estimate IPW from IR sky temperature measurements as suggested by Mims 

et al . (2011) can be replicated in our sub-tropical environment. The instruments as delivered 

from the vendor gave consistent, repeatable results without special adjustment, tuning or 

handling. Our results were quantitatively within the scatter of the data of those of the GCO team 

(ibid) even though their instrument and ours were purchased from separate sources several 

years apart and operated in different natural environments that could contribute some sitespecific variations to the GPS measurements against which they were compared. 

4.1 Discussion 

The IRT instruments cost less than $50 (latest on-line price at the time this is being written) and 

they are extremely portable. GPS IPW stations cost between 10 and 20 thousand dollars per 

receiver/antenna a decade ago (Wolfe and Gutman, 2000) and are still cost prohibitive for highdensity applications. Radiosonde systems can’t be carried in your briefcase and costs for 

radiosonde measurements will vary with the amount of flight elements needed (~$250 to $500 

per flight not including ground equipment costs.) Satellite observations may not be available 

when or where they are needed. The IRT sky temperature measurement technique opens the 

door to IPW measurements when and where there are needed at extremely low cost. 

There are, of course, limitations. While most of the standard methods work under a wide range 

of ambient weather conditions, the IRT method requires clear sky near the zenith. This can be 

an extremely restrictive constraint, especially in some climates during some seasons. 

Nonetheless, the effectiveness of the IRT method poses some very interesting opportunities. 

One of the areas of ongoing research where these instruments may be of assistance is 

improving our understanding of the spatial and temporal scales of IPW variability, especially 

where influences from mountains or coastlines may be important. Most GPS measurements 

lack the density and resolution to characterize mesoscale fluctuations in IPW which may reach 

0.5 cm or more in less than 20 km (see, e.g., Bastin et al., 2007, Figure 6). The power spectrum 

of IPW fluctuations is continuous and has been observed to have characteristics similar to wind 

and temperature fluctuations in the mesoscale range (Zhu et al., 2008). One way of 

approaching this would be through the use of structure functions (see, e.g., Haase et al., 2003). 

The key to observing these small scale features and being able to compute spatial structure 

functions or spectra is the ability to make many measurements with horizontal spacing close 

enough to resolve the scales of interest. We envision the possibility of a university or other 

research program acquiring dozens of these IRTs and hiring students or others to operate them 

at selected locations during intensive observing intervals. Even including the cost of hourly 

labor, a line of 20 or more optimally spaced measurements could be made for a small fraction of 

the cost of obtaining similar resolution in space and time from conventional measurements. 
36

Another application we envision is for campers, hikers, prospectors, hunters or others who 

spend significant time in areas with low population density and no convenient access to local 

weather data beyond what they can see for themselves. Hanssen et al. (1999) noted the utility 

of high resolution IPW for forecasting the local onset of storms. An IRT instrument can provide 

individuals with a personal sensor for detecting changes in the state of the atmosphere that 

might not otherwise obviously manifest themselves. We once observed a 20 degree F drop in 

the sky temperature between two readings taken about three hours apart. When the second 

reading was taken, we initially suspected that we had incorrectly recorded the first reading. 

When we obtained the GPS data for the period we saw that over the three hours between the 

two IRT readings, the IPW had dropped from about 1.2 cm to less than 0.7 mm as shown in 

Figure 4.1-1. A subsequent examination of synoptic surface data showed that some sort of dry 

boundary not otherwise accompanied by significant weather had passed over us during the time 

between the readings. Such a feature could be important to severe weather warnings under 

different weather conditions.

Figure 4.1-1. GPS IPW and KSC IRT indicated sky temperatures for 23 November 2012.

4.2 Conclusion 

This limited and somewhat informal investigation has replicated the methodology presented in 

Mims et al. (2011) and demonstrated its general utility. The inexpensive, portable nature of the 

hand-held IR thermometers opens a variety of interesting opportunities for both operations and 

research related to integrated precipitable water. 

Small networks of these hand-held IR thermometers could be used to augment GPS-IPW, 

RAOBs, and satellite-derived IPW for a variety of tasks. For example, they could be used in the 

vicinity of incident/emergency sites to enhance high resolution local weather modeling or by 

students and scientists to monitor longer term water vapor trends in climate studies. They could 
37 

even be used by military personnel in the field where GPS, RAOBs, and satellite coverage is 

sparse.
38

References 

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Water Vapor as Documented by a Dense GPS Network in a Coastal Area during ESCOMPTE 

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Bevis, M, S. Businger, T.A. Herring, C. Rocken, R.A. Anthes and R.H. Ware, 1992: GPS 

Meteorology: Remote Sensing of Atmospheric Water Vapor Using the Global Positioning 

System, J. Geophys Res., 97, 15787-15801. 

Haase, J., M. Ge, H. Vedel, and E. Calais, 2003: Accuracy and Variability of GPS Tropospheric 

Delay Measurements of Water Vapor in the Western Mediterranean, J. Appl. Meteor.,42, 1547 –

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06/30/2014 Final Jan 2012 - Jun 2014

Assessment of a Technique for Estimating Total Column Water Vapor Using 

Measurements of the Infrared Sky Temperature

Francis J. Merceret 

Lisa L. Huddleston

Kennedy Space Center Weather Office 

Mail Code: GP-B 

Kennedy Space Center, FL 32899 

NASA/TM-2014-218368

Unclassified, Unlimited

A method for estimating the integrated precipitable water (IPW) content of the atmosphere using measurements of indicated 

infrared zenith sky temperature was validated over east-central Florida. The method uses inexpensive, commercial off the 

shelf, hand-held infrared thermometers (IRT). Two such IRTs were obtained from a commercial vendor, calibrated against 

several laboratory reference sources at KSC, and used to make IR zenith sky temperature measurements in the vicinity of 

KSC and Cape Canaveral Air Force Station (CCAFS). The calibration and comparison data showed that these inexpensive 

IRTs provided reliable, stable IR temperature measurements that were well correlated with the NOAA IPW observations.

weather, meteorology, hand held infrared temperature (IRT) measurement, infrared (IR) zenith sky temperature, integrated 

precipitable water (IPW), east central Florida, Kennedy Space Center, Cape Canaveral Air Force Station 

U UU UU UU 50

Lisa L. Huddleston

(321) 861-4952