Climate varies with latitude for two reasons. The first reason is that the spherical form of the earth results in an uneven distribution of solar energy with respect to latitude, as shown in Figure na. As the angle of incidence of the sun's rays approaches 90°, the area over which the energy is spread is reduced, so that there is an increased heating effect. In the high latitudes, energy is spread over a wide area; thus polar climates are cold. The precise latitude that receives sunlight at 90° at noon varies during the year; it is at the Equator during March and September, at the Tropic of Cancer (23- 45°N during June, and at the Tropic of Capricorn (23'45°S) during December. The effect of this seasonal fluctuation is more profound in some regions than in others.

The second reason is that variations also result from the pattern of movement of air masses. In an idealized picture that assumes a uniform surface to the Earth. Under these conditions air is heated over the equator, and therefore rises (causing a low pressure area) and moves northwards. As it moves northwards it gradually cools and increases in density until it descends, where it forms a sub-tropical region of high density. Air from this high pressure area either moves towards the equator, forming the Trade Winds, or else moves polewards. This latter air eventually meets cold air currents moving south from the polar region, over which air is cooled and descending (causing a high pressure area). Where these two air masses meet, a region of unstable low pressure results, in which the weather is changeable.

This idealized picture is complicated by the Coriolis effect (named in honour of the French mathematician Gaspard Coriolis, 1792-1843, who analysed it), which results from the east-west rotation of the Earth. This force tends to deflect a moving object to the right of its course in the Northern Hemisphere and to the left in the Southern Hemisphere.

The distribution of oceans and land-masses modifies this simple picture yet further. Because heat is gained or released more slowly by water than by land- masses, heat exchange is slower in maritime regions, while at the same time humidities are higher. In summer, therefore, continental areas tend to develop low- pressure systems as a result of the heating of land-masses and the conduction of this heat to the overlying dry masses. Conversely, in winter the reverse situation occurs, continental areas becoming cold faster than the oceans, and high pressure systems developing over them. Because most of the Earth's land areas occur in the Northern Hemisphere, the ideal situation shown in the diagram is disrupted to a far greater extent in the Northern than in the Southern Hemisphere.

In addition to the heating and cooling effects of land-masses, climate is also affected by altitude. On average the air temperature falls by up to 6° C for every 200-metre rise in height, but this varies considerably according to prevailing conditions, especially the aspect and steepness of slope and the wind exposure. Because of this tendency for temperature to fall with increasing altitude, the organisms inhabiting high tropical mountains—such as Mount Kenya in East Africa—may be more like the flora and fauna of colder regions than that of the surrounding lowlands. However, although in general temperature falls as one ascends such mountains, other environmental conditions do not mirror precisely those found at higher latitudes. For example, the seasonal variations in day-length typical of high latitude tundra areas are not found in the "alpine" regions of tropical mountains. Also the high degree of insolation resulting from the high angle of the sun produces considerable diurnal fluctuations in temperature which are not found in tundra regions. It is not surprising therefore that the altitudinal zonation of plants and animals should not reflect precisely the global, latitudinal zonation.