New York Weather Bureau. 411 



reversal of the above process takes place at that season, making 

 the pressure over the continents lower than over the oceans. 



Lastly, there is the very important class of shifting areas of 

 low and high pressure known as cyclones or storms and anti- 

 cyclones, respectively, and to these are due the abrupt weather 

 changes common over the central and easitern States. Areas of 

 this class present all gradations of size and intensity, in some 

 cases controlling the winds and weather over the greater part of 

 the continent, while in others their courses are barelv traceable. 

 Cyclones and anticyclones alike have a general eastern motion 

 across the continent from their point of OTigin; the former 

 usually bearing northward to the vicinity of the Great Lakes and 

 the St. Lawrence valley and Gulf, while the average course of the 

 latter is southeastward toward the general high pressure region 

 of the Atlantic which has already been referred to. The rate ait 

 which cyclones traverse the continent ranges from 600 to 900 

 miles per day; while the average velocity of anticyclones is 

 slightly less. 



The climatic effects of the several pressure systems mentioned 

 may now be considered; and firsit with reference to the condi- 

 tions which obtain in winter. At that season the high pressure 

 area of the central Atlantic extends also over the southern States, 

 and is joined to the " high " which develops in winter over the 

 interioir of the continent, and whose maximum pressure of 30.2 

 inches is found over the northwestern States. On the other 

 hand the low pressure area of the north Atlantic has reached 

 its greatest central depression of 29.5 inches, while the borders 

 of the system cover the great water areas which indent the 

 eastern coast of British America; and, acting with the con- 

 tinental high pressure, gives strong noTth westerly winds along 

 the entire northeastern portion of America. The frequent pas- 

 sage of cyclonic storms over the lower lakes and the St. Law- 

 rence valley also brings these regions into the low pressure sys- 

 tem; the line of demarcation between which and the high pres- 

 sure system of the Atlantic and southern States is found to pass 

 in the vicinity of this State. Thus, although our prevailing 



