It might be thought that wind flowed from a region of high pressure to one of low pressure, following the most direct path. This is not so. There are, in essence, three things which combine to produce the general pattern of wind flow over the globe. This pattern is complicated further by local effects such as the proximity of land and sea, the presence of mountains and so on. However, the three overall influences are:
(i) the unequal heating of land and sea;
(ii) the deviation of the wind due to forces arising from the rotation of the earth about its axis;
(iii) the conservation of angular momentum—a factor occurring because the linear velocity of air at low latitudes is less than at high latitudes.
The general picture of wind distribution is as follows. Over equatorial regions the weather is uniform; the torrid zone is an area of very light and variable winds with frequent calms, cloudy skies and violent thunderstorms. These light and variable winds are called the ‘Doldrums’. Above and below the Doldrums, up to 30° north and south, are the Trade Winds, which blow with considerable steadiness, interrupted by occasional storms. Land and sea breezes (mentioned later) also affect their behaviour.
Above and below 30° of latitude, as far as the sub-polar regions, the Westerlies blow. They are the result of the three factors mentioned earlier, but their behaviour is very much influenced by the development of regions of low pressure, termed cyclones, producing storms of the pattern familiar in temperate zones. This cyclonic influence means that the weather is much less predictable in the temperate zones, unless the place in question is in a very large land mass—for example in Asia or in North America. In temperate areas consisting of a mixture of islands, broken coastline and sea, as in north-western Europe, cyclonic weather is the rule and long-term behaviour difficult to forecast. The whole matter is much complicated by the influence of warm and cold currents of water.
Posted in Engineering Fifth Edition