The variable

Wind is caused by imbalances in the atmosphere due to temperature and pressure differences. The movement of air is an attempt to attain equilibrium but, owing to solar heating, this is never achieved. Although air movement is three dimensional, the horizontal component is usually by far the greater and it is this that is normally meant by the term ‘wind’. However, vertical motion also occurs, both at a small scale near to the ground, as eddies caused by turbulent flow and convection, and on a large scale as a result of solar heating in the tropics, which powers the general circulation of the atmosphere.

For the first 100 metres or so above the ground, wind speed increases approximately logarithmically (Fig. 5.1), but with increasing height the influence of the surface has progressively less effect and at an altitude of somewhere between 500 and 2000 metres, depending on surface roughness and other factors such as latitude, the speed becomes constant and equal to the geostrophic wind (the wind blowing parallel to the isobars). The altitude through which the earth's surface has an influence on the wind is known as the planetary boundary layer.

Wind direction is also affected by altitude. At the top of the planetary boundary layer, the direction is the same as that of the geostrophic wind. But descending through the layer, the wind blows at an increasingly oblique angle across the isobars with a component towards the lower pressure region. Plotted from above, the movement of the line of direction marks out a spiral, known as the Ekman spiral (Lockwood 1974).