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Unit 6: Local and General Winds

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General Circulation Between Highs and Lows

High Pressure Low Pressure High and Low Pressure

We need to make one more point regarding pressure cells. Air in a high-pressure cell moves clockwise and outward from the cell; air in a low-pressure cell moves counterclockwise and toward the center of the cell. (See the graphic above) Remember, clockwise in areas of high pressure, counterclockwise in areas of low pressure. This has a direct effect on the winds that occur at the surface.

Lee side winds
Lee waves cause updrafts and downdrafts

We'll give you a series of illustrations on how general winds are modified to help produce surface winds. The figure above illustrates the lee slope, eddy effects on a much larger scale. Tall mountain ranges can modify strong winds aloft to create waves and large eddies on the lee side of the mountains. Winds dip down due to the difference in pressure on the lee side, thus initiating wave actions in those strong winds. Lens-shaped clouds (altocumulus lenticularus) may develop in the tops of these waves. These clouds can easily be formed on the lee side of mountain ranges that are perpendicular to winds of 40 knots or more. The clouds are usually high, and the resulting winds may not be felt at the surface. However, occasionally these strong winds aloft may dip to the surface, or eddy winds may reverse the direction of usual winds. Depending on your location, surface winds can be significantly modified by this process.

Friction and air turbulence generated at the surface slow low level winds. There are two sources of turbulence--mechanical and thermal. The roughness of surfaces, usually due to vegetative cover, causes friction and results in mechanical turbulence. Surface heating during the day causes thermal turbulence as heat convection currents rise from the surface and mix with the air flowing over the surface.

Let's look at some effects of channeling and mechanical turbulence. Imagine a general wind blowing through a pass or saddle in a mountain range. Wind velocities might increase as they pass through the constricted area; then the air spreads out on the lee side with probable eddy actions.

Winds on the lee sides of ridges can shift in direction and speed, making them difficult to predict. We usually term eddy winds as gusty and erratic.

Vegetation Thermal turbulence
Vegetation may also cause updrafts and downdrafts Thermal turbulence caused by surface heating

Another factor which has a great deal of effect on low level wind is thermal turbulence caused by surface heating. (See figure above) Different land surfaces absorb, reflect, and radiate varying amounts of heat. Warm air rises and mixes with other air moving across the terrain. This mixing action has differing effects on surface winds, but often makes them gusty and erratic.

Turbulence
Turbulence from various sources modifies surfaces winds

We have given you a very brief explanation of what general winds are, how they develop, and how they are affected by terrain. There are excellent publications listed in the references for this course which give a much more detailed description of general circulation, pressure systems, and weather fronts.

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