64 THE VOYAGE OF H.M.S. CHALLENGER. 



except to eastwards of the New England States as far as long. 45° W. ; and over 

 Australia, and nearly the whole of Africa and South America. The more remarkable 

 of the resulting changes is the reappearance, in the neighbourhood of Spitzbergen and 

 North Greenland, of the high pressure which overspreads nearly the whole of the Arctic 

 regions during the colder half of the year. In Spitzbergen, pressure is now slightly 

 above the general average of 29*95 inches. The high pressure characteristic of 

 continents during the winter months has disappeared from South Africa ; but is still 

 shown in Australia, reduced, however, about half a tenth of an inch. 



The anticyclone of the North Atlantic covers nearly the same extent as in the 

 previous month ; but as pressure over it is generally half a tenth of an inch less, and 

 as in Asia to the north of the high tableland of the interior it has risen a tenth and a 

 half, the gradient for westerly wdnds is greatly reduced. Climatologically this is, 

 perhaps, the most important change, next to that of the temperature, that occurs in 

 August. The gradient for westerly winds from the Atlantic being reduced, these 

 winds are correspondingly lessened in force, and the amount of the rainfall is diminished 

 to the east of long. 20° E. 



September. — In this month the low temperature of the Arctic regions spreads and 

 deepens, and in the interior has fallen to 20°. The highest isothermals are now 90° in 

 Asia and Africa, and 85° in North America and the north of Australia. The compara- 

 tively high temperature of the extensive tropical regions of the Pacific and Atlantic, 

 where the difference of temperature is very slight, is still maintained. The other 

 outstanding feature of the temperature is a greatly more rapid fall now in progress over 

 the land as compared with that over the ocean. Thus, while in mid- Atlantic about 

 lat. 52° 30' N. and long. 32° 30' V?., the mean temperature of August is 57° "6, and of 

 September 55°"2, on the continent the means are 64° - 9, and 58°7 at Berlin, 62° - 5 and 

 51°"2 at Barnaul, and 48°7 and 32° 7 at Werkojansk. These changes altogether alter 

 the temperature relations of the different regions of the northern hemisphere to each 

 other, and it is these changed relations which bring about the vital change in the 

 peculiar distribution of the pressure which sets in with the autumn months. 



Over the whole of Asia and Europe, except the British Islands and north-western 

 Norway, pressure has risen, and most largely where temperature has fallen to the 

 greatest degree, unless the region is situated so as to be affected by an extensive low 

 pressure area now being formed ; the whole of North America, except Labrador, and 

 Alaska, and British Columbia ; the northern half of Africa ; and in the south of 

 Australia and south of South America. Everywhere else pressure has fallen, notably 

 so in the north-eastern part of the Pacific and of the Atlantic, and part of the 

 continents adjoining, where the winter cyclonic low pressure of the regions are rapidly 

 forming, the isobars of the North Pacific now showing a pressure of 2970 inches, and 

 of the North Atlantic of 2975 inches. 



