United States

Department of

Agriculture

Forest Service

Southern

Research Station

Research Note

SRS-4

Estimating Stability Class 

in the Field

Leonidas G. Lavdas

March 1997

Abstract

A simple and easily remembered method is described for estimating cloud

ceiling height in the field. Estimating ceiling height provides the means to

estimate stability class, a parameter used to help determine Dispersion

Index and Low Visibility Occurrence Risk Index, indices used as smoke

management aids. Stability class is also used as an input to VSMOKE, an

atmospheric dispersion model used to estimate prescribed fire smoke

effects on air quality and visibility.

Keywords: Cloud ceiling height, Dispersion Index, dispersion models,

Low Visibility Occurrence Risk Index, prescribed burning, stability class.

Introduction

Land managers must be able to assess whether a prescribed

fire will reduce visibility and increase driving hazards. In

the last 20 years, researchers have developed and improved

numerical indices and dispersion models that give those

responsible for prescribed burning this ability. Stability

class is one of three meteorological parameters used to

characterize atmospheric dispersion, both alone (Lavdas

1986) and within models that estimate smoke concentrations

from prescribed burning (Lavdas 1996b, Southern Forest

Fire Laboratory Personnel 1976). Unlike the other two

parameters, mixing height and transport windspeed,

estimates or forecasts of stability class are not generally

available from the National Weather Service. Without

stability class, field personnel cannot calculate Dispersion

Index (Lavdas 1986) or Low Visibility Occurrence Risk

Index (LVORI) (Lavdas 1996a), or use dispersion models

such as INPUFF (Petersen and Lavdas 1986), VSMOKE

(Lavdas 1996b), or VSMOKE-GIS (Jackson and others, in

press). This inability to obtain stability class operationally 

is in spite of the availability of algorithms for estimating

stability class from other commonly available meteorological parameters (Turner 1964) (summarized by 

Leonidas G. Lavdas, Research Meteorologist, Southern Research Station,

U.S. Department of Agriculture, Forest Service, Juliette, GA 31046.

Lavdas 1986). Stability class may be determined from

opaque cloud cover, cloud ceiling height, surface (20 feet)

windspeed, and solar elevation angle. Cloud cover and

windspeed are routinely given in weather reports and

forecasts. Moreover, both may be estimated in the field

after minimal instruction. Lavdas (1976) (summarized by

Southern Forest Fire Laboratory Personnel 1976) developed

a convenient way to determine solar elevation angle from

the length of shadows cast. Cloud ceiling height is not

given in National Weather Service forecasts or observations

routinely distributed to the general public. No formula

exists for translating ceiling height into parameters

accurately observable by field personnel. This single

parameter has remained an obstacle to field determination of

stability class.

Cloud Ceiling Height

Cloud ceiling height helps determine the incoming or

outgoing radiation which heats or cools the ground

surface—thereby heating or cooling the layer of atmosphere

nearest the ground. Accordingly, cloud influence on

radiation can be categorized by the action of particular

clouds on the sun's visible light. Cloud height categories are

related to cloud light transmission characteristics by the

following scheme:

Cloud height Cloud light

category Cloud height transmission

High >16,000 ft High

Middle 7,000 - <16,000 ft Medium

Low < 7,000 ft Low

Cloud light transmission characteristics may be estimated

for individual cloud elements (or groups of similar 

elements) by estimating the appearance of the sun or cast

shadows that would result if the sun were shining through

that particular cloud element. The following scheme may be

used:
 Cloud light

Solar/cast shadow appearance transmission

Sun is bright, casts distinct shadows. High

No distinct shadows, sun visible as a faint 

disk or bright area with its location in

the sky closely determinable. Medium

Sun not visible, cloud or cloud shadow is 

medium gray to dark. Low 

This method can be mastered by remembering three phrases: 

"There's the shadow"—high; "There's the sun"—medium;

and "There's no sun"—low.

The Turner stability class determination method specifies

cloud ceiling height—the lowest altitude for which opaque

clouds cover more than one-half of the sky. Field personnel

should determine if more than one-half of the sky is covered

by clouds of low light transmission, then medium, then

high. For example, if three-tenths of the sky is covered by

low clouds, three-tenths by medium (six-tenths low and

medium), and three-tenths by high (nine-tenths low,

medium, and high), then the ceiling is medium.

Estimates of cloud light transmission should be made as if

the sun were directly behind the cloud and fairly high in the

sky (i.e., a shadow cast by the sun in that position in the sky

should be no more than three times the height of the object). 

Clouds, sun, or both lower in the sky should be included in

the analysis, but regarded as if they were higher in the sky.

Puffy cumulus clouds appear dazzling white if in direct

sunlight but dark on the shadow side. Because cloud light

transmission is estimated as if the sun is attempting to shine

through the cloud, the appearance of a cumulus cloud's

shadow side should be used. These clouds normally yield

low light transmission.

Estimating cloud light transmission at night is more difficult

but can be accomplished by using a full moon as a

benchmark:

 Cloud light

Full moon's appearance transmission

Full moon "man in the moon" features 

evident. High

Full moon "watery," blurry, barely 

visible, or only apparent as a light blotch. Medium

Full moon not visible. Low

The nighttime method may be mastered by remembering: 

"There's the man in the moon"—high; "There's the moon"—

medium; and "There's no moon"—low.

Implications

Once fire personnel have obtained mixing height, transport

windspeed, opaque cloud cover, and surface windspeed

from the National Weather Service and calculated solar

elevation angle and cloud ceiling height in the field, they

can estimate stability class on site. With this information,

they can calculate Dispersion Index and LVORI in the field. 

In addition, they can use this information to run more

complex dispersion models.

Literature Cited

Jackson, William A.; Lavdas, Leonidas G.; Loberger, Dale. [In press]. 

How to use VSMOKE-GIS to predict smoke concentrations from a fire. 

U.S. Department of Agriculture, Forest Service.

Lavdas, Leonidas G. 1976. A groundhog's approach to estimating

insolation. Journal of the Air Pollution Control Association 26: 794.

Lavdas, Leonidas G. 1986. An atmospheric dispersion index for

prescribed burning. Res. Pap. SE-256. Asheville, NC: U.S. Department

of Agriculture, Forest Service, Southeastern Forest Experiment Station. 

33 p.

Lavdas, L. G. 1996a. Improving control of smoke from prescribed fire

using Low Visibility Occurrence Risk Index. Southern Journal of

Applied Forestry. 20: 10-14.

Lavdas, Leonidas G. 1996b. Program VSMOKE—users manual. Gen.

Tech. Rep. SRS-6. Asheville, NC: U.S. Department of Agriculture,

Forest Service, Southern Research Station. 147 p.

Petersen, William B.; Lavdas, Leonidas G. 1986. INPUFF 2.0—a

multiple source Gaussian puff dispersion algorithm–users guide. 

EPA/600-8-86/024. Research Triangle Park, NC: U.S. Environmental

Protection Agency. 96 p. Available from: National Technical

Information Service, 5285 Port Royal Road, Springfield, VA 22161;

PB86-242450.

Southern Forest Fire Laboratory Personnel. 1976. Southern forestry

smoke management guidebook. Gen. Tech. Rep. SE-10. Asheville, NC: 

U.S. Department of Agriculture, Forest Service, Southeastern Forest

Experiment Station. 140 p.

Turner, D. Bruce. 1964. A diffusion model for an urban area. Journal of

Applied Meteorology. 3: 83-91.

Southern Research Station, U.S. Department of Agriculture, Forest Service,

P.O. Box 2680, Asheville, NC 28802