68 



TROPICAL CLIMATOLOGY 



Evaporation takes place most quickly in hot dry air, and causes 

 a considerable amount of heat to be rendered latent. This fact 

 can be made use of in hot climates, where it is a common practice 

 to wrap flannel or felt round a bottle of water, and after damping 

 the flannel, to hang the bottle in the breeze, so that the contents 

 may become cooled by the evaporation of the water from the flannel. 

 The same principle applies to the porous stone or earthenware 

 basins which are commonly used in Ceylon, Uganda, Egypt, and 

 other parts of the tropics to filter and cool water. Hence, also, 

 the value of collections of water in keeping the temperature of 

 places equable. 



The amount of aqueous vapour in the atmosphere is generally 

 spoken of as the humidity. Two kinds of humidity are recognized : 

 absolute humidity, which is the weight of aqueous vapour actually 

 present in a definite volume of air at a given temperature; and 

 relative humidity, which is the ratio of the weight of water actually 

 present in a known volume of air to the weight of water which is 

 required to saturate the same volume of air at the given tempera- 

 ture, and is generally expressed as a percentage. 



We have already stated that the humidity of the air is of the 

 greatest importance in considering the effects of a tropical climate 

 on man. The most trying climate would be one with a high air 

 temperature and a high relative humidity, for it is obvious that when 

 the air is full of aqueous vapour, evaporation from the surface of the 

 human body must be considerably diminished; and as this is one 

 of the methods of regulating the bodily temperature, it is apt to 

 rise and cause great discomfort, as will be described later. 



A warm place with a high humidity is less bearable than a much 

 hotter but drier place. A fairly high relative humidity can, how- 

 ever, be borne if there is a breeze, without which a much lower 

 humidity is most unbearable. In fact, Giles points out that 

 Abusher, in the Persian Gulf, in August, with a mean maximum 

 temperature of 96-5° F. (357° C), and a relative humidity of 65 

 per cent., with no rainfall during the month, and little or no breeze, 

 ■constiuutes one of the most unbearable climates in the world, 

 though neither the temperature nor the humidity by themselves 

 are high. 



When the aqueous vapour in the atmosphere condenses, the result 

 may be dew, fog, mist, cloud, rain, hail, or snow, of which the two 

 last are rare in the tropics. Clouds in the sky prevent to a great 

 extent the heating of the earth during the day, and at the same 

 time they prevent radiation of heat from the earth during the night, 

 and even reflect this back to it. Hence cloudy nights may be very 

 hot in the tropics. 



Effects. 



We must now consider briefly the effects of high air tempetatures 

 with and without high atmospheric humidity upon man, and we 

 will commence with a consideration of its effects upon the bodily 

 temperature. 



