TEMPERATURE AND HUMIDITY 



67 



This brief sketch of tropical climates must suffice for our present 

 purpose, and we now turn to consider the various climatic factors 

 and their effects upon man; but the reader interested in this subject 

 may find it useful to consult Chapter II., pp. 28 to 58, in our 

 second edition, in which more details will be found. 



TEMPERATURE AND HUMIDITY. 



Heat derived from the sun is capable of traversing the atmosphere 

 surrounding the world, and thus reaching the earth, because this 

 atmosphere is diathermanous to rays with short wave-lengths coming 

 from so hot a body as the sun. Dry air is diathermanous for all 

 rays, but aqueous vapour is almost athermanous for infra-red rays, 

 though largely diathermanous for other rays. 



In passing through the atmosphere, about 25 per cent, of the 

 heat which has entered it is absorbed before it reaches sea-level. 



The position of the sun is, however, of the utmost importance 

 with regard to the quantity of heat reaching the earth. If the sun 

 is quite vertical, probably only 20 per cent, is lost; whereas, if 

 the beam is nearly horizontal, probably none reaches the earth. 



The heat rays falling upon the earth are absorbed and converted 

 into dark heat — that is to say, into frequencies with longer wave- 

 lengths — and these are radiated back into the atmosphere (terrestrial 

 radiation). This dark heat is absorbed by the atmosphere, which 

 is, as already stated, almost athermanous to this kind of energy. 

 This atmospheric heat is one of the most important factors in 

 determining the nature of a climate, and shows daily and annual 

 periodic variations, of which the first is due to the sun's rays heating 

 the earth, and therefore this disappears at sea or during an arctic 

 winter, while the second depends largely upon the inclination of the 

 world's axis as it passes round the sun in its yearly orbit. As the 

 sun is vertical in the sky at noon twice a year in the tropics, there 

 is or ought to be a vs^et season at these times, and in their intervals 

 a dry hot season. 



It is impossible, however, to consider the effects of temperature 

 upon man without at the same time taking into account the humidity 

 of the atmosphere. 



Man can bear very high temperatures easily, provided the air is 

 dry, but not if there is much moisture or humidity in it. The 

 humidity of the atmosphere is, in fact, of the utmost importance 

 in the study of climatic effects upon man. This humidity is due 

 to aqueous vapour, caused by the constant evaporation which 

 takes place from the surface of all collections of water. 



The humidity of the atmosphere presents three problems: 

 atmometry, or the measurement of the quantity of water being 

 taken into the air ; hygrometry , or the determination of the quantity 

 of aqueous vapour present in the air at any given time; and hyeto- 

 metry, or the quantity of water being condensed from the 

 atmosphere. 



