The conditions affecting the distribution of rainfall over the globe have next to be considered. In the lower layers of the atmosphere water exists in a gaseous form as water vapour. This is obtained by evaporation mainly from the sea, but to some extent also from the land, and the lakes, rivers, and vegetation upon its surface. The air, however, is only able to contain a certain amount of moisture at any given time, and that amount depends upon the temperature at which it then is. When the temperature is high it can hold a much greater amount of water as vapour than it can when the temperature is low. Hence, it is of importance to dis tinguish between the absolute and the relative humidity of the air. The former is the actual amount of water vapour in the air at any given time, while the latter is the ratio of that amount to the amount which the air could hold at the temperature at which it then is. When the temperature continues to fall below that point (called the saturation point), at which the air is just able to retain the vapour it holds at the time, condensation and precipitation follow. This process, however, is much facilitated by the presence of atmospheric dust around the particles of which the vapour more easily condenses.
The necessary cooling to effect condensation may be brought about in one or other of several ways. Within the equatorial belt of calms, warm air, containing much moisture evaporated from the ocean, ascends at all seasons of the year, but, cooling as it ascends, it is no longer able to retain that moisture and heavy rainfall ensues. This process of cooling by the ascent of convection currents also takes place to some extent over the heated interior of continents during the summer months. Winds blowing from the sea are frequently chilled when brought into contact with land, and their moisture is deposited. If the land is much warmer than the sea, however, this result may not follow, and in the absence of moun tains there may be comparatively little precipitation. Mountains, indeed, may play an important part in the distribution of rainfall, by deflecting upwards winds from the sea and causing the moisture which they contain to condense, either by directcooling or by cooling consequent upon expansion under decreased pressure. In such cases there is usually a wet and a dry side to a mountain range, as the air descending on the leeward side becomes heated, and is able to retain what moisture is left.