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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

Atmospheric Instability Affects Fire Behavior

Unstable , warm, rising air produces thermals, which help fires develop convective columns.

Fire behavior in an unstable atmosphere

Convective winds
Convective winds caused by surface heating

The above graphic illustrates some forms of instability. Strong surface heating produces several kinds of convective systems. Upslope winds develop along heated slopes and convection currents continue to rise. Superheated air in flat terrain escapes upward in bubbles or in the form of whirlwinds or dust devils. The height to which convective currents and bubbles of air rise will be dependent on the stability or instability of the atmosphere at various levels.

Unstable air produces strong gusty winds.

Increased Fire Behavior

Atmospheric instability can contribute to increased fire behavior by increasing:

  1. The chances of dust devils and firewhirls.
  2. Other convective wind activity at the surface.
  3. The heights and strengths of convection columns.
  4. The chance of firebrands being lifted by the column.

We'll look at each of these problem areas. Dust devils are rather common, sometimes erratic in their movement, and have been known to scatter fire and cause spotting across fire control lines. Firewhirls are much less common but present serious safety and security problems on the fireline. Firewhirl development will be discussed in detail in Unit 9 of this course.

Any convective lifting at the surface causes indrafts from adjacent areas to replace the rising air. These indrafts can be gusty and erratic.

Smoke convection columns rise much higher in unstable air. Chimneys, of a sort, develop with indrafts feeding the fire at the base of the column and strong convective currents rising through the column. The greater the instability and fire intensity, the stronger the indrafts and convection column updrafts. Of primary concern is the spotting potential of high, well developed convection columns due to the rise of firebrands in the column. Convection column development and spotting by aerial firebrands will be further discussed in Unit 9.

Thunderstorm Development Caused by Instability

The four lifting processes which can cause thunderstorm development are:

  1. Convection or thermal
  2. Orographic
  3. Frontal
  4. Convergence

Four common lifting processes in the atmosphere

Thermal lifting Orographic lifting
Thermal lifting Orographic lifting
Frontal lifting Lifting & convergence
Frontal lifting Relationship of lifting and convergence

Orographic lifting occurs in mountainous terrain when a mass of moving air is forced to rise because of the presence of slope. Air that is forced upward cools adiabatically. If this air reaches its saturation point, clouds develop. Orographic lifting and thermal lifting often work together to produce cumulus clouds in mountainous areas.

The third process is frontal lifting. Here a moving, cooler air mass pushes its way under and lifts a warmer air mass. Again, this lifting action can produce cumulus clouds if saturation occurs. Cumulus cloud development is usually associated with cold front passages, while stratus clouds generally accompany a warm front.

The fourth lifting process is convergence. Here you see the relationship of convergence to subsidence. In a high-pressure cell, air is piled very high, thus exerting more weight and pressure on air within the cell. The result often is "subsidence," and "divergence," or air flowing out of the a cell at the surface. Diverging air from the high-pressure area always flows toward lower pressure, which is a nearby low-pressure area. Air coming into the low converges to a central point and is then forced up in the center of the cell. Air that is lifted within the low-pressure area frequently causes cloudiness and sometimes precipitation.

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_3.html. This work is licensed under a Creative Commons License Creative Commons License