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Unit 6: Local and General Winds

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Wind Effects   ::  General Winds  ::   Local Winds   ::   Winds of Concern   ::   Wind Input   ::   Exercises

General Winds

All winds are the result of temperature differences, thus, pressure gradients between different areas. This can be on a very large scale, such as over many hundreds of miles, or on a very small scale, such as a few inches.

General Winds

Here we see that upslope local winds, produced by warm air rising over the terrain, meet and mix with the general winds. The general winds in the lower layers of the atmosphere will be further modified by locally induced convective winds.

Local Winds

In the above figure, we see the general wind blowing across the terrain from an area of high pressure to an area of lower pressure. Close to the terrain, the general air flow will become modified by the roughness of the surface.

General circulation due to earth's rotation and unequal heating

Circulation Circulation

To better understand general winds, we should discuss the general circulation of air around the earth. This mass of moving air is our atmosphere--a gaseous mantle encasing the earth, held there by gravity, and rotating with the earth. See the graphics above. Several natural forces interact to produce continual movement of this air, thus producing wide and varied weather patterns.

The surface of the earth is not heated uniformly by the sun, and the resulting unequal heating of the atmosphere causes compensating air motions which tend to reduce the horizontal temperature differences. As the air moves from the equatorial regions toward the polar regions, it is affected by the earth's rotation. In the Northern Hemisphere, the air is deflected to the right as the earth rotates on its axis. This deflection force-is called the Coriolis force.

The uneven heating and cooling at all latitudes, due to seasonal and day-night cycles, modifies the large-scale circulation pattern. Uneven heating and cooling, due to the distribution of land and sea areas, is another important factor. These and other factors are responsible for producing a series of wind belts that circle the earth at various latitudes. One of these wind belts is called the prevailing westerlies. Closer to the North Pole, high pressure is maintained, and easterly winds occur. We are primarily concerned with latitudes between 30 degrees and 60 degrees North. In the graphic above, arrows indicate air flowing out of the south and west. This is the region where we have the prevailing westerlies.

At some latitudes the air tends to "pile up" to cause belts of high surface pressure. These belts are never uniform, but instead consist of a series of rather large pressure cells. Some of the pressure cells are relatively fixed, such as the polar high, while others are migratory. Our weather is closely related to the location and movement of these primary pressure cells and other smaller scale pressure patterns.

Pressure Systems
Large scale pressure systems and fronts.

On the graphic above illustrates a surface pressure map over the United States for a particular time. There are cells of high pressure and cells of low pressure on this weather map. The lines that you see on the map are called isobars which are drawn through points of equal air pressure. These isobars outline the areas of high and low pressure.

Typically, pressure cells move from west to east across the United States. However, at times they move in other directions, which further complicates weather patterns. The circular isobars over Mississippi indicate a low pressure cell, or in this case, a hurricane that has moved inland from the Gulf of Mexico.

A boundary between two air masses or different temperatures and other characteristics is called a weather front. The figure shows both a short warm front extending to the east coast and a longer cold front extending into Texas. We will discuss the weather associated with weather fronts a little later.

General Circulation Between Highs and Lows

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Measuring Winds Aloft and Wind Profiles

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Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, November 07). 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_2.html. This work is licensed under a Creative Commons License Creative Commons License