REPORT ON ATMOSPHERIC CIRCULATION. 73 



high pressure north of the equator has a very irregular outline, and exhibits the 

 greatest differences as regards breadth and inclination to the equator. These irregu- 

 larities wholly depend on the peculiar distribution of land and water which obtains 

 in the northern hemisphere. The maximum breadth is reached over the continents of 

 Asia and America ; and, indeed, the area of high pressure may further be regarded as 

 stretching across the Arctic region from the one continent to the other. The highest 

 mean annual pressure, 30*20 inches, is attained in the anticyclonic region in the North 

 Pacific. On the other hand, the belt of high pressure falls to the minimum in the 

 Pacific immediately to the east of Japan, where it is less than 29 '95 inches. It is also 

 to be noted that pressure is nearly equally low in the east of the United States and 

 parts of the Atlantic adjoining. About the same latitudes, both north and south of the 

 equator, pressure is invariably high in the ocean a little to westward of all continents. 



These two belts of high pressure enclose between them the comparatively low 

 pressure of equatorial regions, through the centre of which runs a narrower belt of still 

 lower pressure, towards which the trade winds on either side blow. This intertropical 

 belt of low pressure exhibits several centres of still lower pressure. The most 

 important and extensive of these includes India, the southern half of Arabia, and a 

 large portion of Central Africa, where pressure falls below 29 '80 inches; and over a 

 considerable part of north-eastern India it falls under 2975 inches. Over the larger 

 proportion of the East India Islands pressure is also under 29 - 80 inches ; and there are 

 besides two small regions near the mouth of the Amazon and near Panama where 

 pressure does not quite reach 29"85 inches. 



Perhaps the most remarkable region of low pressure is in the Antarctic regions, 

 which, remaining low throughout the year, plays the principal role in the wind systems 

 bordering on and within the Antarctic Circle, with their heavy snows and rainfall, and 

 in the enormous icebergs which form so striking a feature of the waters of the Southern 

 Ocean. It is probable that over nearly the whole of the Antarctic regions mean pressure 

 is at least less than 29 - 30 inches. 



In the north polar regions pressure is lower than over the continents, but higher 

 than over the oceans immediately adjoining. In the temperate and Arctic regions 

 there are two strongly marked depressions — the larger covering the northern portion of 

 the Atlantic and adjoining lands, and the other the corresponding portion of the 

 North Pacific, the mean in each falling in the centre below 2970 inches. 



Now the whole of these areas of low pressure have the common characteristic of an 

 excessive amount of moisture in the atmosphere. The Arctic and Antarctic zones of 

 low pressure, and the equatorial belt of low pressure generally, are all but wholly 

 occasioned by a comparatively large amount of vapour in the atmosphere. But as 

 regards the region of low pressure in Southern Asia in summer, while the eastern half 

 of the depression overspreading the valley of the Ganges has a moist atmosphere and a 



(PHYS. CHEM. CHALL. EXP. — rAUT V. — 1889.) C ll 



