ix Climates of the World 143 



of which were mainly compiled by the American meteor- 

 ologist Professor Loomis, shows the rainfall on the land by 

 deepening blue tints according to the number of inches 

 which fall at each place in a year. It also shows, mainly 

 from the data of Mr. Buchanan of the Challenger, the 

 salinity of the ocean by deepening red tints ; salt areas in 

 the sea are produced by evaporation of* the water which 

 supplies the rainfall of the land, and they may be termed 

 the comparatively dry regions of the sea. They correspond 

 very closely with the centres from which the trade 

 winds blow. The lightest blue colour on the map denotes 

 regions where the rainfall is under ten inches per annum. 

 These correspond exactly with the regions of extreme range 

 of temperature, lying as a rule in the interior of continents. 

 The regions of greatest rainfall coloured in deepest blue are 

 all steep land slopes exposed to a sea wind. In North 

 America, for example, the trade winds blowing round the 

 Gulf of Mexico, and the south-west winds beating on the 

 coast of Oregon and British Columbia, ensure heavy rain- 

 fall. South America shows a very interesting relation. In 

 the trade- wind region vapour is carried up the flat valley of 

 the Amazon and condensed on the eastern slope of the 

 Andes, the western slope of which is rainless. In the 

 south of the continent the west winds of the Roaring 

 Forties dash perpetual showers against the western face of 

 the Andes, and descending sweep as drying winds across 

 Patagonia. In India and the Malay Archipelago the 

 heavy rains are produced entirely by the summer monsoons. 

 Attentive study of the rainfall map, along with those of 

 winds and configuration, will bring out similar reasons for 

 the local distribution of rainfall in all parts of the Earth. 



202. Winds of the British Islands. The British 

 Islands are usually covered by the edge of the North 

 Atlantic area of low pressure. The pressure being lowest 

 in the north-west, and highest in the south-east, corresponds 

 to prevailing south-westerly winds. In January the isobars 

 are closely crowded together ; in that month the average 

 gradient over the British Islands is steeper and the winds 

 are consequently stronger than in any other part of the 



