REPORT ON ATMOSPHERIC CIRCULATION. 69 



America 30 "20 inches; and those of the Indian Ocean between South Africa and 

 Australia, to the west of South Africa and to the west of South America, are particularly 

 well defined, being separated from each other by pressure under the general average. 

 The result is the transference of a large mass of the earth's atmosphere from the 

 southern to the northern hemisphere, and from the ocean to the land surfaces of the 

 northern hemisphere ; in other words, the transference is from those regions of the 

 globe where temperature is relatively high to where, with respect to immediately 

 surrounding regions, it is relatively low. 



The cyclonic areas of low pressure in the North Atlantic and Pacific have now 

 virtually acquired their winter extension and depth. In November the difference in 

 pressure between the centres of the North Atlantic anticyclone and cyclone is 0'40 

 inch, but in December the difference increases to 0"60 inch, or a half more. With 

 this increased gradient, the Atlantic south-westerly winds increase in strength and 

 frequency, and precipitate over western Europe a much heavier rainfall, augmented by 

 meeting land of a relatively lower temperature than in the previous months. Thus the 

 maximum rainfall of the year occurs in this month and January following, when quite 

 similar meteorological conditions prevail over the whole of the strictly western division 

 of the Peninsula, the extreme north-west of France and south-west of England, and 

 the more western districts of Ireland and Scotland. 



Low-pressure areas also occur in South Africa, the valley of the Amazon, and from 

 the north-west of Australia to Java. A lower pressure now appears in the equatorial 

 region of the Atlantic between the two anticyclones of that ocean than prevails there 

 in any other month. 



A singular distribution of pressure is well seen this month in the Mediterranean, 

 illustrating the relation of the Italian peninsula to this area of low pressure. This 

 peculiarity is observed through all the winter months ; but as the isobars are drawn only 

 to half-tenths, it does not appear in so pronounced a manner in the other months. 

 The same peculiarity is shown in the relations of India to the isobars of that region as 

 compared with the isobars of the Arabian Sea and Bay of Bengal, — a feature still more 

 decidedly shown in this than in the previous month. 



Abnormal Pressures and Temperatures. — The isobars, isothermals, and winds 

 detailed are shown in the normal atmospheric conditions of the different months. It 

 not unfrequently happens, however, that the actual weather of individual months differs 

 widely from what these normals indicate. The most important weather changes, as 

 affects human interests, are those which depend on wind, temperature, and rain ; and 

 as these again are most intimately bound up with the actual distribution of pressure at 

 the time, it is the last that really furnishes the key to weather changes. 



As good an example as could be adduced to show these changes is offered by the 

 weather conditions of December 1878. This month was remarkable for unusually 



