5.2.2 WIND STRENGTH

5.2.2.1 Source and Variability

Winds occur as a direct result of differential solar heating of the atmosphere. These differences depend on latitude, land and sea, time of year, time of day, etc, and when combined with the rotation of the earth and local geographical features produce an extremely complex distribution of wind strength and direction over both time and space.

The major wind structures (e.g. 'trade winds') are the effect of hot air rising from the equatorial regions and cold air descending onto the polar icecaps. This creates three cells of large scale vertical circulation in each hemisphere (fig 5.6), which are then skewed horizontally by the earth's rotation (for instance, surface air travelling South to the equator is moving into a latitude where the surface is rotating eastwards faster, and will therefore blow from the North East).

 

Regional vortex instabilities occur as a result of the upward and downward air flows at the cell edges combined with interaction between the Easterlies and Westerlies, producing both cyclones and anticyclones, which then traverse across the surface and cause our complicated weather effects. In particular, a cyclone is an inward flowing vortex (similar to that observed when emptying a bath) and produces high tangential winds which change direction as it passes. Further interaction with surface irregularities (mountains and valleys, forests, large buildings etc) result in smaller scale gusts and turbulence.

It will be apparent that prediction of wind velocity and direction is difficult. Nevertheless it has been found that the statistical variations of wind speed at different sites are remarkably similar, and can usually be well described in terms of a universal distribution function (the Weibull distribution) and three experimentally determined quantities, the most important of which is the average wind speed.