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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


Upon completion of this unit you will be expected to:

  • Define atmospheric stability (stable air), and discuss its general effects on fire behavior.
  • Describe atmospheric instability (unstable air), and give three ways that it can contribute to possible extreme fire behavior.
  • List and discuss the four lifting processes which can cause thunderstorm development.
  • Describe thermal belts: when, where, and how they develop, and their effects on fire behavior.
  • Discuss subsidence, and describe two situations due to subsidence which can increase fire behavior.
  • Describe morning surface inversions and their effects on fire behavior.
  • Determine the stability or instability at various levels of the atmosphere based on visual indicators and/or temperature readings at different elevations or altitudes.


In earlier weather units of this course, we discussed the facts that the earth's gaseous mantle, called "the atmosphere," is very fluid, with air constantly moving and mixing, and that air changes in temperature, moisture, pressure, and other properties. By now you know that air moves horizontally or vertically in response to the earth's rotation, to large and small scale pressure gradients, to various lifting mechanisms, and to gravity.
Unit 6 concentrated on the horizontal movement of air or wind. In this unit, we will discuss vertical air movement, what causes it, and what it means to firefighters. This vertical movement, either upward or downward, is generally influenced by the degree of stability or instability of the atmosphere at any particular time.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, September 28). Unit 7: Atmospheric Stability and Instability. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: This work is licensed under a Creative Commons License Creative Commons License