CAUSES OF TROPICAL HEAT. 



7 



therefore conclude that other agencies are at work 

 which often completely neutralise the effect which 

 increased altitude must undoubtedly exert. 



There is another important difference between the 

 temperate and tropical zones, in the direct heating effect 

 of the sun's rays independently of altitude. In England 

 the noonday sun in the month of June rarely incon- 

 veniences us or produces any burning of the skin ; while 

 in the tropics, at almost any hour of the day, and when 

 the sun has an elevation of only 40° or 50°, exposure to 

 to it for a few minutes will scorch a European so that 

 the skin turns red, becomes painful, and often blisters 

 or peels off. Almost every visitor to the tropics 

 suffers from incautious exposure of the neck, the 

 leg, or some other part of the body to the sun's 

 rays, which there possess a power as new, as it is 

 at first sight inexplicable, for it is not accompanied 

 by any extraordinary increase in the temperature of 

 the air. 



These very different effects, produced by the same 

 amount of sun-heat poured upon the earth in different 

 latitudes is due to a combination of causes. The most 

 important of these are, probably, — the constant high 

 temperature of the soil and of the surface-waters of the 

 ocean, — the great amount of aqueous vapour in the 

 atmosphere, — the great extent of the intertropical 

 regions which cause the winds that reach the equatorial 

 zone to be always warm, — and the latent heat given out 

 during the formation of rain and dew. We will briefly 

 consider the manner in which each of these causes 

 contributes to the degree and the uniformity of the 

 equatorial temperature. 



