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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

## Inversions

There are several aspects of stable air conditions that should be understood by the firefighter. One is the relationship of surface inversions to thermal belts. In the graphic below, we again illustrate the nighttime drainage of cool air into a valley. Air in contact with the upper slopes cools and flows downslope like water, always seeking the lowest elevation. This drainage is most prominent in side canyons and draws.

Nightime cooling creates stable air

Depending on the size of the valley, the pooling of cool air may be several hundred feet deep. An inversion develops above the pool of cool air. Remember, an inversion is a layer of air in which the temperature increases with increase in altitude.

Inversion : A layer of air in which the temperature increases with increase in altitude.

 Example of an inversion Inversion and thermal belt relationship

Where the inversion layer contacts the mountain slopes, we have a relatively warm area called the thermal belt. (See above graphic) At night, the temperature in this region is actually warmer than on the slopes above or below. The elevation of the thermal belt varies by locality and depends on the time of night and the size of the valley below. Its depth also varies.

Thermal belts can, and often do, have a significant effect on fire control efforts. To the firefighter, the thermal belt is an area on a mountainous slope that typically experiences the least variation in diurnal temperature, has the highest average temperature, and has the lowest average relative humidity. Overall, this area can have the highest average fire danger. Most important is the continued active burning during the night, while areas above and below the thermal belt are relatively quiet.

Thermal Belt : An area on a mountain slope that typically experiences the least variation in diurnal temperature, has the highest average temperatures, and the lowest average relative humidity.

Fire behavior can remain extreme at night in thermal belts

## Subsidence

Subsidence : A slow, sinking motion of high level air over a broad area occurring in high pressure areas. The subsiding air is warmed by compression and becomes more stable.

Effects of subsidence

Subsidence is a slow process that occurs over a period of several days. During summer and autumn, somewhat stationary, deep high-pressure cells often develop over relatively large areas of the land.

If the high-pressure system persists for a period of days, a subsidence inversion aloft slowly lowers toward the surface. The cold, dry air at very high altitudes, which is lowering, becomes warmer and drier as it reaches lower altitudes. The tops of mountain ranges will experience the warm, very dry air first. If this condition persists, fuels are dried out and burning conditions become severe.

Foehn winds caused by areas of subsidence

Another important effect of subsidence can be foehn winds. (See figure above) Foehn winds often occur on the lee slopes of prominent mountain ranges when the windward sides of the mountains are exposed to areas of subsidence. Heavy, stable air within the high-pressure cell pushes out in all directions from the center of the high but is restricted in its horizontal movement by the presence of the mountains. Eventually, this heavy air pours over the ridges and through canyons, creating strong, warm, dry winds at lower elevations on the lee side of the mountains. The character and effects of foehn winds were discussed in Unit 6.

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_2.html. This work is licensed under a Creative Commons License