48 



NEW YORK STATE MUSEUM 



In the spring, rising temperature produces a modification and 

 shifting of pressure systems, the winds decreasing in velocity and 

 their direction being more variable than in winter. The frequent 

 showers occurring in April and May appear to be due more than 

 at any other time to the effect of an admixture of air having 

 different temperatures. 



In summer the Gulf of Mexico and ithe Atlantic ocean con- 

 tribute large supplies of moisture to northward-moving air cur- 

 rents, and, although cyclonic depressions are less frequent than 

 at any other season, the rainfall accompanying each storm is 

 heavy, and in New York the maximum seasonal precipitation, 

 amounting as an average for the whole State to 10.96 inches, 

 occurs in this season. 



As regards the fall months, the rainfall of September is usually 

 light in the region east of the Great Lakes, while in October the 

 maximum general rainfall occurs. As regards meteorological 

 conditions, winter may be considered as beginning in November. 



A study of the data shows that there are a number of contend- 

 ing forces which are distinctively operative in New York, and 

 which by modifying one another tend to produce numerous irregu- 

 larities of the rainfall. So irregular indeed is the precipitation 

 that frequently places only a short distance apart show wide 

 variation. 



In a general way it may be said that the amounts of annual 

 rainfall in different sections of New York are mainly determined 

 by proximity to sources of vapor or to vapor-laden air currents, 

 and by the character of the local topography. As regards the 

 latter statement, a more definite form would be that under similar 

 conditions the precipitation is in some degree proportionate to the 

 altitude. This rule, while generally true, does not apply to Hie 

 valley of the Hudson river, where the upper portion, including the 

 Chaniplain valley, receives a somewhat deficient rainfall as com 

 pared with the State as a whole. To the west, the Adirondack 

 plateau receives a marked increase of rainfall, while farther north- 

 west there is a decrease in the valley of the St Lawrence. This 

 is also true of the elevated region in the vicinity of Hemlock lake, 



