RAINFALL 



<MEAN ANNUAL) 



S Summer Koins C Constant Rain 

 W Mfofe/- /fa//w N Ai 



Rainfall. 



Map showing the distribution of the rainiest regions and the hot deserts of the world in relation to the 

 season of maximum precipitation and the prevalent winds as indicated by arrows 



exerted by varied types of preci- 

 pitation it belongs to the science of 

 geography. 



Water vapour is always present 

 in the atmosphere ; warm air has a 

 greater capacity for water vapour 

 than cold air ; it rains whenever 

 warm wet. air is sufficiently chilled, 

 i.e. when the quantity of water 

 vapour exceeds the capacity of the 

 air at the lower temperature. The 

 main factor in producing rainfall is 

 loss of temperature. The air is 

 warmest at its lowest levels, conse- 

 quently rising air is chilled and is 

 likely to deposit" rain. Mountains 

 and other elevated regions are 

 rainier than the neighbouring low- 

 land ; the Pennines receive at 

 least 10 ins. more rain a year than 

 the valley of the Yorkshire Ouse ; 

 the wettest area in the world is on 

 the Khasi Hills in Assam. 



Very cold air, below freezing 

 point, is, however, almost perfectly 

 dry, and further cooling fails to pro- 

 duce a fall of snow. The rainless 

 areas of the world are the deserts, 

 both hot and cold. Over hot 

 deserts, such as the Sahara, the air 

 is so hot that it is never chilled 

 below its capacity for water vapour. 

 Over cold deserts the temperature 

 is usually too low for the air to con- 

 tain more than a very small 

 quantity of water vapour. There 

 are thus three rainfall belts in the 

 world, one N. and one S. between 

 the hot and the cold deserts and one 

 equatorial between the hot deserts. 



Rainfall is the complement of 

 evaporation, which, in turn, de- 



pends upon the oceans and the 

 great seas. The complete cycle de- 

 pends upon the carrying power 

 of the winds to move the wet air 

 from above the oceans landwards, 

 and upon gravitation which forces 

 the rain water ocean wards. Winds 

 from the sea are carriers of water 

 vapour, and, in general, coastlands 

 are rainier than inland areas. 

 Ireland is rainier than England, 

 and England is rainier than Russia. 

 Exceptions occur for two reasons ; 

 first, the winds may blow always 

 from the coast seawards, and, 

 secondly, the coastland may be so 

 much warmer than the sea that the 

 oceanic wind is not chilled, but 

 warmed at the coast. In both 

 these cases the coasts are almost 

 rainless. S.W. Africa has a coast- 

 land of the first type, Sind inN.W. 

 India of the second. Wet monsoon 

 winds carry water vapour across 

 Sind and the Thar desert, and 

 loose the rain on the distant 

 Himalayan slopes. 



The Rainy Belts 



Within the rainy belts, the 

 quantity of rainfall depends upon 

 the direction of the prevalent 

 winds in relation to the lie of the 

 coast, and the trend of the mountain 

 ranges and hills. The W. Ghats are 

 rainier than the Deccan, for the wet 

 monsoon comes from the Arabian 

 Sea ; British Columbia is rainier 

 than Alberta, since the prevalent 

 westerlies cross British Columbia 

 and the Rocky Mountains before 

 they reach Alberta. 



Moving air is more likely to be 



chilled than comparatively still air, 

 hence rain is frequently an accom- 

 paniment of storms and is char- 

 acteristic of the left front of a 

 cyclonic storm traversing the 

 British Isles. Areas crossed by the 

 regular storm tracks, such as Bri- 

 tain and Japan, are rainier than 

 areas where storms are less fre- 

 quent. In hot places such as tro- 

 pical regions or inland lowland 

 areas, such as Hungary or Mani- 

 toba, during the hottest summer 

 weather it frequently rains about 

 3 p.m., when the day's heat is near 

 the maximum ; such rains are of 

 short duration, but are heavy, and 

 are often accompanied by thunder. 



All these facts are summarised 

 in a map of the world showing the 

 annual precipitation by means of 

 isohyets or lines indicating equal 

 quantities of precipitation. From 

 any area of great rainfall the quan- 

 tity declines towards (i) the 

 deserts ; (ii) the lowlands ; and 

 (iii) places more remote from the 

 ocean. 



For practical purposes in relation 

 to farming and water supply for 

 human beings, or for stock, it is 

 essential that the rainfall distri- 

 bution through the year should be 

 investigated. It is found, in general, 

 that there are four types of dis- 

 tribution : summer rains, hot 

 rainy summers and cool or cold 

 dry winters ; winter rains, hot 

 dry summers and warm wet 

 winters ; constant rain or rain at 

 all seasons, with but slight varia- 

 tion between the seasons ; and no 



