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Unit 4: Temperature-Moisture Relationship

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Temperature & Heat   ::   Temperature & Altitude   ::   Atmospheric Moisture   ::   Relative Humidity   ::   Clouds   ::   Exercises

Temperature Decreases With Increases In Altitude

Up to this point, we have been discussing air temperatures near the earth's surface. The temperature of the air at different levels in the atmosphere is a much different story. Those temperatures may not seem important to us on the ground, but you will see a little later just how much influence they have on fire behavior.

People who live in mountainous regions know that the high elevations are generally cooler than the low elevations. If you gain altitude in an air-plane or hot air balloon, you experience lower temperatures. On the average, this is true throughout the atmosphere.

There are natural processes responsible for the decrease in temperature at higher altitudes. In meteorology, we describe the temperature differences in terms of temperature lapse rates.

Temperature decreases with increases in altitude according to these temperature lapse rates:

  1. Dry : the lapse rate is 5.5 degrees per 1,000 feet. Dry air moving up or down. It is a function of atmospheric pressure, air density, and the molecular activity of a parcel of air.
  2. Wet : its lapse rate is 3 degrees per 1,000 feet. Moist air rising vertically. It is affected by the release of latent heat from the condensation of water vapor.
  3. Normal : its lapse rate averages about 3.5 degrees per 1,000 feet. An average throughout the lower atmosphere over time and space. It changes by time of day depending on air stability and the winds mixing the air at various levels.

Dry vs. Wet
Dry and wet temperature lapse rates in the atmosphere

Lapse rates at certain levels can be more than 5.5 degrees, thus unstable air, or less than 3 degrees which would be stable air.

The graphic above illustrates moist air moving up one side of a mountain range and descending on the lee side. Notice how condensation in the form of clouds and precipitation can occur as the moist air is cooled by lifting. As the same air descends on the lee side, it begins to warm and condensation stops. This air below the cloud will descend and warm at the dry lapse rate. The cooling and warming processes discussed here are known to meteorologists as adiabatic processes. Unit 7 will discuss how the adiabatic processes can affect fire behavior.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, October 27). Unit 4: Temperature-Moisture Relationship. 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_4__Temperature-Moisture_Relationship_3.html. This work is licensed under a Creative Commons License Creative Commons License