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Unit 7: Atmospheric Stability and Instability

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Stability & Instability   ::   Inversions & Subsidence   ::   Instability & Fire Behavior   ::  Stability or Instability  ::   Summary   ::   Exercises

Determining Atmospheric Stability or Instability

There are generally two methods that can be used in the field--visual indicators and temperature measurements at different elevations or altitudes. We will discuss each of these.

Visual Indicators of Stable Air

  1. Clouds in layers, no vertical motion
  2. Stratus type clouds
  3. Smoke column drifts apart after limited rise
  4. Poor visibility in lower levels due to accumulation of haze and smoke
  5. Fog layers
  6. Steady winds

Stratus clouds Smoke column
Stratus clouds Smoke column
Haze and smoke Fog layers
Haze and smoke Fog layers

Using visual indicators is the easiest way to recognize air stability and instability, and the firefighter should be observant of these at all times. However, there will be times when limited visibility will not permit observations at various levels, or perhaps a combination of stable and unstable indicators may confuse the field observer. For these reasons we offer an alternative method. Measuring temperatures at various altitudes can help you determine temperature distribution and evaluate the degree of stability or instability of the atmosphere.

Visual Indicators of Unstable Air

  1. Clouds grow vertically and smoke rises to great heights
  2. Cumulus type clouds
  3. Upward and downward currents
  4. Gusty winds
  5. Good visibility
  6. Dust whirls

Smoke rises Cumulus clouds
Smoke rises Cumulus clouds
Good visibility Dust whirls
Good visibility Dust whirls

Using Temperature to Determine Stability

Measuring temperatures at various altitudes can help you determine temperature distribution, and evaluate the degree of stability or instability of the atmosphere.

Determining stability from temperature distributions

Unstable
Unstable, temperature decreases with altitude

The graphic above gives an example of atmospheric temperature distributions. This situation has temperature readings from three elevations--90° at the fire, 78° at a lookout, and 66° from an aircraft. There is a difference of 2,000 feet elevation between each, and 12° difference between each location. The temperature lapse rate is 6° decrease per 1,000 feet increase in elevation. This lapse rate is greater than the dry adiabatic rate; thus the atmosphere is unstable.

Stable with Inversion
Stable with inversion, temperature rises with altitude

This illustration gives four temperature readings taken at different locations on a slope. Elevations are not given, but we can see that there is a plus lapse rate below midslope and a minus lapse rate above midslope. From this we can conclude that an inversion exists at the 50 level. The air in the canyon below that level is very stable.

Determining actual temperature lapse rates from your own temperature readings is not difficult. Simply divide the temperature difference by the elevation difference expressed in 1,000's of feet. Usually you will have a minus lapse rate, i.e., where temperature decreases with altitude increase. However, under inversion conditions, you may have a plus lapse rate where temperature increases with altitude increase.

Actual Temperature Lapse Rate = Temperature difference / Elevation difference (1000's ft)

Minus Lapse Rate = Temperature decrease with altitude increase

Plus Lapse Rate = Temperature increase with altitude increase

Weather Monitoring : Constant or frequent observations of the weather elements to detect changing conditions that could influence fire behavior.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, November 09). Unit 7: Atmospheric Stability and Instability. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/Wildland_Fire_Management_and_Planning/Unit_7__Atmospheric_Stability_and_Instability_4.html. This work is licensed under a Creative Commons License Creative Commons License