CLIMATE. 293 



of a clock, and commonly produce rain in their east portion. 

 A high-pressure wave or center, from which the air naturally 

 flows toward the outside, will usually bring about an " anti- 

 cyclone " area with fair, and in winter cold ("blizzard") 

 weather, the direction of the whirl being, in this case, the re- 

 verse, or in the same direction as the hands of a clock. Both 

 cyclones and anti-cyclones move in North America from west 

 to east, mostly entering from the Pacific Ocean off the north- 

 west coast and traversing the continent with a slight south- 

 east (or in the case of cold weather almost south) trend, with 

 a velocity of twenty to thirty miles an hour ; until upon reach- 

 ing the region of the Great Lakes they generally turn north- 

 eastward and pass into the Atlantic Ocean from the New Eng- 

 land and Canada coasts. It is upon these general facts, 

 roughly outlined here, that the weather forecasts are in the 

 main based ; taking into consideration, of course, the local or 

 regional conditions, topography, etc., which modify the appli- 

 cation of the general rules. 



In the southern hemisphere, the air-movements substantially 

 correspond to those observed in the northern, so far as not 

 modified by mountain chains ; as is especially the case in South 

 America. 



INFLUENCE OF TOPOGRAPHY. 



Rains to IVindzvard of Mountain Chains. The surface fea- 

 tures or topography of the regions traversed by the air cur- 

 rents or winds may materially modify both their direction and 

 their physical condition, especially as to moisture and temper- 

 ature. Mountain chains may deflect them, or, causing the air 

 currents to rise on their slopes, and thus to cool by expansion, 

 the moisture these bring with them from the sea may be 

 partially, or sometimes almost wholly, deposited in the form of 

 rain or snow ; chiefly on the windward slopes. Then, continu- 

 ing across the range, the air deprived of most of its moisture 

 cannot readily yield up more; hence the scarcity of rain 

 "arid climate" under the lee of mountain chains; as in the 

 Great Basin between the Sierra Nevada and Cascade ranges 

 on the one hand and the Rocky Mountains on the other, and 

 also on the Great Plains under the lee of the latter. The 



