_Effects of Amount and Distribution of Rainfall._--As already remarked, it is quite possible for a climate to be damp and yet have little or no rainfall, but such instances are rare, and on the other hand, a heavy rainfall necessarily brings about a coincident increase of relative humidity. Rain is, moreover, necessarily combined with a cloudy sky, whereby the heating of the soil during the day and its cooling by radiation during the night are alike impeded. The immediate effect of a shower of rain is to cool the air, and this for a double reason: first, coming as they do from the higher strata of the atmosphere, the temperature of the raindrops is necessarily much lower than that of the earth’s surface; and secondly, from the multiplied surfaces of the descending drops and from the wetted earth there necessarily occurs a rapid evaporation, whereby a further large amount of heat is absorbed, but unless showers recur at sufficient intervals to continuously diminish air temperature, the temporary remission is apt to be dearly paid for by a period of heat combined with high relative humidity, with its attendant discomforts of reduced evaporation from the surface of the body, prickly heat, and the other discomforts inseparable from tropical damp. The influence of rainfall on health necessarily depends to a great extent on the configuration of the land, but assuming that the latter admits of adequate drainage, more depends upon its distribution than upon its amount; for the sanitary influences of rain are, in the main, mechanical, and depend on the “laying” of the dust and the washing away of infective and otherwise deleterious material. A heavy shower of sufficient duration will carry away, _viâ_ the river to the sea, the deleterious products of human occupation during a preceding drought; but to do this the rain must be heavy while it lasts, for a prolonged drizzle in a warm climate simply turns the soil into a particularly efficient cultivation ground for the germs of infective diseases, and the attendant gloom of the sky stops entirely the beneficent germ-killing power of the sun’s direct rays. A prolonged drizzle, never exceeding the absorbtive and drainage capacities of a given site, marks the maximum of unhealthiness in all climates, and is possibly even more obnoxious when associated with heat than with cold: so that the most pleasant tropical climates are those that combine frequent short but heavy showers with intervals of bright sunlight, a continuously overcast sky being everywhere unfavourable to health.