The proportion of moisture present in the air has at least as important a bearing on health as its temperature. It is obvious that when the air is actually saturated with watery vapour evaporation from the surface of the body must necessarily be stopped, and with it the natural provision for preventing an undue rise of the temperature of the body. Actual saturation combined with high temperature is, however, fortunately rare for anything but short periods, as the absolute amount of water requisite to saturation increases rapidly as temperature rises; and hence warm air, but partially saturated, and therefore still active as an absorbent of evaporation from moist surfaces, may contain a far larger absolute amount of water than saturated air at a lower temperature. In practice, saturated air is only to be found in situations where it is brought into contact with colder surfaces, such as that of the earth, cooled by radiation during the night, as is seen in the production of dew; the dew point being, in fact, the temperature at which the amount of water present in the air suffices to saturate it. Apart from its diminution by the formation of dew, the absolute amount of moisture present in the air, depending as it does on but slowly changing conditions, can naturally also change but slowly, but the relative moisture, or the percentage of the amount required to produce saturation, which is actually present always varies greatly during the twenty-four hours in all places where the diurnal range of temperature is at all considerable; so that relative moisture, when given for any day or other period, always refers to an average. As a rule it is only the stratum of air of a few yards in thickness that is cooled by contact with the soil during the night and is hence concerned in the formation of dew, a fact which is prettily illustrated by the low-lying bands of vapour which hang over the landscape after a clear night on any fine morning in the Tropics, and which clear off as if by magic as the returning sun once more warms up the soil and air. Wherever radiation is impeded by the shelter of trees, by artificial shelter, or by sufficiently dense masses of cloud, the temperature of the soil and air falls but little, and hence, under such circumstances, dew does not fall.