60 THE VOYAGE OF H.M.S. CHALLENGER 



distinguish the more restricted regions of low pressure in the United States, Scandi- 

 navia, Spain, and North Italy. The point is of considerable importance in atmospheric 

 physics, as showing that when the sun's heat is strongest cyclonic areas of low pressure 

 are generated in dry climates ; whereas in winter, in the higher latitudes, cyclonic 

 areas are formed in humid and rainy climates. 



One of the most remarkable illustrations of the respective influences of land and 

 water on the courses of the isobars is seen at this season in the higher pressure main- 

 tained from the Straits of Gibraltar eastwards to the Sea of Aral by the extensive sheets 

 of water for which this region is so remarkable. The crowding, widening, and deforma- 

 tion of the isobars in the different parts of the region is curious and highly instructive. 

 On the other hand, the diminution of the pressure shown by the isobar of 29*80 inches 

 immediately to the north in eastern Russia, where there are no water surfaces, is equally 

 striking. 



As Australia is an island sufficiently large to show the climatic features of a 

 continent, it is interesting to note in connection with the anticyclone overspreading it at 

 this time, that on all coasts the winds blow outward from the land seawards. This, 

 therefore, is the dry season of Australia. 



One striking feature of the oceanic anticyclones deserves attention. The isobars 

 crowd more together on their eastern sides, where they press upon the continents 

 adjoining, than on their western sides, where they are prolonged through their respec- 

 tive oceans. The prevailing winds of continental coasts adjoining anticyclonic regions 

 are usually dry for two reasons ; they advance from higher to lower, and therefore 

 warmer latitudes, and they have traversed the evaporating surface of the ocean but a 

 comparatively short way since their descent from the higher regions of the anticyclones. 

 The dry climates of California, Peru, Morocco, and south-west of Africa at this time 

 may be referred to as illustrations. 



Quite different is it with the winds which blow through the West Indies, the Gulf 

 of Mexico, and thence northwards through the United States. These winds having 

 traversed a large extent of the ocean, distribute over these islands and States a generous 

 and rarely failing rainfall, thus rendering the United States one of the uniformly best 

 watered regions of the globe. Similarly the valleys of the Amazon and other rivers in 

 the north of South America have a large rainfall. 



As regards rainfall, southern and eastern Asia is, perhaps, the most remarkable 

 region of the globe. The isobars of July show this at once. If a line be drawn 

 marking the path of highest pressure from Durban in South Africa eastwards 

 through the Indian Ocean and Australia, and thence out through the Pacific by New 

 Caledonia to beyond the Sandwich Islands, then the whole of the Indian and Pacific 

 Oceans between this line and the continent is traversed by winds which blow home on 

 and into Asia during the summer months. Hence the prevailing summer winds arrive on 



