New York Weather Bureau. 



455 



The prevailiug westerly winds at midday, while mainly due 

 to the general atmospheric circulation, must be oonsiderably 

 strengthened by the updraught of air on the heated hill slopes. 

 At night, when -the motion of the upper currents isi (nio loinger 

 imparted to the surface air by oonvectional action, the downflow 

 from the hills proceeds unchecked; but, owing to the diistance 

 of the city from the highlandis, the eaisterly wind does not become 

 fully estiablished there until after the evening observation, and 

 is much more apparent in the early morning. The large per- 

 centage of cloudiness in winter evidently tends to decrease the 

 frequency of easterly winds at that season. 



The valley winds have a eonsiderable climiatic importance, 

 since they bring a cool and refreshing air at night which is not 

 felt on the plains or upper hill slopes. Also-, in the autumn and 

 winter the same condition gives earlier froists and lower noc- 

 turnal temperatures in the valleys than obtain on the hills or 

 plains, as long as the skies remain clear. 



The following temperature readings were obtained at hill and 

 v^alley stations of central New York during the clear weather 

 accompanying anticyclonic conditions in January and February, 

 1892: ' 



Note. — The rapid rise of the g^eneral surface of the eastern plateau brings the valley bottom 

 at Brookfleld above the level of Oxford. The distance between the stations is about thirty 

 miles. 



The inversion of temperature which occurred in northern New 

 York on December 8, 1891, has already been described (page 444). 

 In that case there was apparently a cold stratum of surface air 

 and a slight increase of temperature with vertical height within 

 the mass of the anticyclone itself. Such a condition must be less 

 likely to occur south of Lake Ontario, as the surface air for the 

 time being loses its intense oold in passing over the water. 



