CONDITIONS AFFECTING PEECIPITATION. 199 



distribution of the rain-foil with reference to the form of the land masses is 

 one of so much importance from a climatic point of view. 



Every large continental area, not situated within the tropies, by its exist- 

 ence not only diminishes the entire precipitation on the earth's surface, in 

 that it diminishes the area of the ocean, but it must also in its interior por- 

 tion be the recipient of less than the average amount ; and if the mass of 

 land be large enough its interior will be reduced to the condition of an 

 almost rainless country. This is the case with Asia, and to a very consid- 

 erable degree with North America, in both of which continents there are very 

 large areas where the precipitation falls below ten inches. The edges of 

 the land masses receive more than their proportion of the moisture evapo- 

 rated from the ocean, and the interior regions are in consequence robbed of 

 their share. 



The opposing wall of a high mountain range rising so as to face a wind 

 blowing across a warm ocean surface furnishes the conditions suited to 

 give rise to the largest possible amount of precipitation. This we see Avell 

 exemplified in the case of the Khassia Hills, which are so situated as to 

 receive upon their Hanks the hot and necessarily extremely moist wind 

 blowing across the Bay of Bengal, and which in consequence are the recipi- 

 ents of the largest rain-fall known anywhere in the world.* The same thing 

 is exemplified, only in a lesser degree, in many other localities; as, for in- 

 stance, along the northwest coast of England and Scotland, and on the west- 

 ern slopes of the Scandinavian Range. Similar conditions may be noticed 

 aloni>: the western coasts of New Zealand and of Pata<ronia, and also on 

 the Pacific coast of North America from California northward. In each of 

 the cases mentioned, with the exception of that of the Khassia Hills, the 

 shores of the land masses rise i)recipitous]y from the ocean to a very con- 

 siderable height, and face the return trade-winds, which are thus forced to 

 deposit the moisture which they have taken up in their passage across the 

 water surface. In the case of any one of these rainy belts we have to go 

 only a short distance inland, sometimes only a few miles, to find the precipi- 

 tation greatly diminished, or even reduced almost to nothing.! 



* At Cherapuiiji, in Assam, for instance, Professor Loouiis gives, on tlie aiitliority of 151anford's Euport on 

 the Meteorology of India, the mean preeipitation at 492.45 inches, the elevation of the station being 4,125 feet. 

 Of this quantity no less than ninety-five per cent falls during the six months from April to September inclusive. 

 He adds : " When the wind on the Bay of Bengal blows from the south the rain falls almost incessantly ; and 

 when the wind changes to the west or the northwest, the rain ceases almost entirely." 



t For abundant illustrations of this statement, see the detailed maps illustrating the precipitation of various 



