METEOROLOGICAL OBSERVATIONS. 189 



The maximum elevating effect in winter amounted to 36° (November 28, 1860), 

 and the maximum depressing effect in summer to 9° (July 25, 1861). 



This annual variation is weU shown in the table given for Van Kensselaer Har- 

 bor, where the maximum effect was on the average in January +19°, and the opposite 

 effect on the average in June — 1°.3, and is, indeed, a most marked feature at either 

 locality. 



Effect of Clear and Cloudy Weather on the Temperature. 



To ascertain the effect upon the temperature of a serene and cloudy atmosphere, 

 the temperature observed on clear days (or at least three-quarters clear), and on 

 cloudy days (or at least three-quarters cloudy), was compared with the normal temp- 

 erature of the day ; a -I- difference indicates warmer, a — difference a colder day 

 than the normal ; for this investigation the year was again divided into two seasons. 



The clear days preponderate in the lointer season, the cloudy days in the summer 

 season; thus in 



r December f 18 ■\ 4 



A January there are i 19 clear days, and but y 1 cloudy days, and in June and 

 I February 117 J 1 



July there are 4 and 8 cigar days, and 16 and 15 cloudy days. 



In winter (October to March inclusive) on the average from 82 clear days the 

 temperature was loioer 3°. 5 than the normal, and in summer (April to September 

 inclusive) on the average from 41 clear days the temperature was Idglier 0°.8 than 

 the normal; a clear atmosphere consequently produces opposite effects in the sum- 

 mer and winter seasons. 



In winter on the average from 31 cloudy days the temperature was IdgUer 7°.0, 

 and in summer on the average from 48 days it was loioer 2°.l than the normal 

 value. 



The explanation of these results is obvious : In winter, under a clear sky, 

 radiation soon lowers the temperature, whereas a clear sky in summer by permitting 

 greater insolation, will increase the temperature. In cloudy weather in winter, 

 radiation is stopped, and with an atmosphere nearly or quite saturated with moisture 

 the temperature must rise ; in summer insolation is prevented, and consequently the 

 temperature will remain lower than its normal value. 



