The Geostrophic Wind
The Atmosphere (Troposphere)
The atmosphere around the globe is a very thin layer. The globe has a diameter of 12,000 km. The troposphere, which extends to about 11 km (36,000 ft.) altitude, is where all of our weather, and the greenhouse effect occurs.
Troposphere 1
Troposphere 2
On the picture you can see a stretch of islands 300 km (200 miles) across, and the approximate height of the troposphere. To look at it at a different scale: If the globe were a ball with a diameter of 1.2 metres (4 ft.), the atmosphere would only be 1 mm (1/25") thick.
The Geostrophic Wind
The winds we have been considering on the previous pages on global winds are actually the geostrophic winds. The geostrophic winds are largely driven by temperature differences, and thus pressure differences, and are not very much influenced by the surface of the earth. The geostrophic wind is found at altitudes above 1000 metres (3300 ft.) above ground level.
The geostrophic wind speed may be measured using weather balloons.
Surface Winds
Winds are very much influenced by the ground surface at altitudes up to 100 metres. The wind will be slowed down by the earth's surface roughness and obstacles , as we will learn in a moment. Wind directions near the surface will be slightly different from the direction of the geostrophic wind because of the earth's rotation (cf. the Coriolis force ).
When dealing with wind energy, we are concerned with surface winds, and how to calculate the usable energy content of the wind.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 1 June 2003
http://www.windpower.org/en/tour/wres/geostro.htm
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