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Unit 6: Local and General Winds

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Examples of Foehn Winds

These figures give examples of two commonly known foehn winds. The two foehn winds shown are blowing in opposite directions, but they have similar origins. The chinook occurs on the east slopes of several large mountain ranges in the Western United States. Chinook winds are most prevalent on the east side of the Rocky Mountains during fall and winter.

Example 1

In the case of the chinook wind, air pushed up on the windward side is cooled adiabatically to the point that clouds and precipitation may occur. As that air passes over the mountains and descends on the lee side, it is warmed adiabatically at 5-1/2 degrees per 1,000 feet of fall. It also gains velocity as it passes through the constricted topography and accelerates as it flows downslope. The resulting foehn or gravity winds on the lee side of mountain ranges can be warm and dry, with moderate to high velocities.

Example 2

The Santa Ana creates the most critical fire weather situations in areas of Southern California during fall and winter. Foehn winds are produced when the large-scale circulation is sufficiently strong and deep to force air completely across a major mountain range in a short period of time. In many cases, there is a large stationary cell of high pressure over the land.

Subsidence or heavy air lowering within the pressure cell may push up against the mountain range. This heavy or stable air speeds up as it flows through passes and saddles, then down the lee slopes by gravity and pressure gradients

If you know of foehn or local gravity wind conditions existing in your locality, you should talk to a meteorologist or some other knowledgeable individual about them. There are likely to be peculiarities for each area, and knowing these can help you in recognizing or anticipating the effects of foehn winds.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, November 08). Unit 6: Local and General Winds. 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_6__Local_and_General_Winds_9.html. This work is licensed under a Creative Commons License Creative Commons License