416 Eighth Annual Report of the 



not necessary to their occurrence, especially in mountainous sec- 

 tions, v.here only a moist air and the convectional currents pro- 

 duced by high temperature appeairs to be necessary to local 

 thunderstorm formation. The relative frequency of local showers 

 in different parts of the State, as bearing upon the distribution 

 of summer rainfall, will be referred to later. 



Considering now the pressure conditions of autumn, we find 

 that in September the anticycloinic area of the Atlantic has 

 moved northward and that the maximum pressure, 30.1 inches, 

 extend wesitward over the central Atlantic States, maintaining 

 southerly winds along our coast. The western limits of the anti- 

 cyclonic system are found beyond the Mississippi valley, and dur- 

 ing October the pressure remains high over the central and south- 

 ern States, while diminishing over the ocean. The transient anti- 

 cyclones of October and the early days of November show a strong 

 tendency to drift very slowly over the region of average maximum 

 pressure extending from the middle States to the central coast; 

 and under such conditions the warm southweisterly winds and 

 bright weather of " Indian Summer " prevail in the northern 

 States. 



The rainfall of September is usually light in the region east of 

 the Upper Lakes, although no marked variation from the normal 

 distribution lof pressure characteristic of the month is ^required 

 to bring about a heavy precipitation. For example, in Septem- 

 ber, 1890, the anticyclones moved to the northeastern rather than 

 to the central part of our coast; and in one case, such an area, 

 acting with a depression to the westward, gave easterly winds 

 and, phenomenally heavy rains over Central New York for several 

 days; the pressure over the State being meanwhile unusually high 

 (30.4 inches). 



The maximum general rainfall of the autumn season occurs in 

 October, accompanying the shifting of prevailing winds and a 

 decided increase of storm frequency which obtains during the 

 month. The northern winter commences soon after the middle 

 of November; at which time the continental high pressure area 

 ard the depression of the North Atlantic are strongly developed, 

 and northerly winds again prevail over the eastern States. 



