WIND RESOURCE

Global winds


The Coriolis force
        Since the globe is rotating, any movement on the northern hemisphere is diverted to the right, looking it from our own position on the ground. In the southern hemisphere it is bent to the left. This apparent bending force is known as the Coriolis force, named after the French mathematician Gustave Gaspard Coriolis (1792-1843) .
        It may not be obvious that a particle moving on the northern hemisphere will be bending towards the right. Consider the first image below, where the red cone is moving southward in the direction of the tip of the cone. The earth is spinning, while the camera is fixed in the outer space. The cone is moving straight towards the south. In the second image, the same situation is shown with the camera locked on to the globe.
        In the third image the same situation is shown as seen from a point above the North Pole. The camera is fixed, so that it rotates with the earth. It can be noticed that the red cone is veering in a curve towards the right as it moves. The reason why it is not following the direction in which the cone is pointing is that now as observers we are rotating along with the globe. Finally, in the fourth image, the camera is fixed in outer space, while the earth rotates.


     The Coriolis effects     The Coriolis effects     The Coriolis effects     The Coriolis effects

        The Coriolis force is a visible phenomenon. Railroad tracks wear out faster on one side than the other. River beds are dug deeper on one side than the other. Which side depends on the hemisphere: in the northern hemisphere moving particles are bent towards the right. In the Northern hemisphere the wind tends to rotate counterclockwise (as seen from above) as it approaches a low pressure area. In the Southern hemisphere the wind rotates clockwise around low pressure areas.


How the Coriolis force affects global winds

Global winds circulation         The wind rises from the equator and moves north and south in the higher layers of the atmosphere. Around 30 latitude in both hemispheres the Coriolis force prevents the air from moving much farther. At this latitude there is a high pressure area, as the air begins sinking down again. As the wind rises from the equator there will be a low pressure area close to ground level attracting winds from the North and South. At the poles, there will be high pressure due to the cooling of the air.
        Keeping in mind the bending force of the Coriolis force, the following general results for the prevailing wind direction come:

 Latitude

90-60°N

60-30°N

30-0°N

0-30°S

30-60°S

60-90°S

 Direction

NE

SW

NE

SE

NW

SE


        The size of the atmosphere is grossly exaggerated in the picture above, which was made on a photograph from the NASA GOES-8 satellite. In reality the atmosphere is only 10 km thick, i.e. 1/1200 of the diameter of the globe. That part of the atmosphere is the troposphere, as already explained in the previous page. This is where all of our weather (and the greenhouse effect) occurs.
        The prevailing wind directions are important when siting wind turbines, since obviously it is preferable to place them in the areas with least obstacles from the prevailing wind directions. Local geography, however, may influence the general results in the table above, cf. the following pages.

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The geostrophic wind
        The winds that have been considering so far 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 winds is found at altitudes above 1 km above ground level. The geostrophic wind speed may be measured using weather balloons.





Video frames
- Global wind systems: general explanation