NO. 17.] 



TEMPERATURE OF THE AIR. 



477 



With the sky clear, we have, in all the dark months, the lowest temperature 

 during the day, and the highest during the night. In the other months — 

 September excluded — we have the ordinary diurnal period. With the sky over- 

 cast, the diurnal period with a minimum in the early morning hours and maxi- 

 mum after noon, is very well developed in all the months except January. 

 The range is greatest in April, both with clear sky and with the sky overcast. 

 In April, May and June, the range is greater with a clear sky than with the sky 

 overcast. The most striking feature seems to me to be (PI. IV. 1 cm. = 1°) the 

 distinct diurnal period of the ordinary march in the winter and dark season, with 

 the sky overcast and relatively higher temperatures. The inverted period 

 with clear sky in the dark season seems to be due to the diurnal period of 

 the wind's direction, as pointed out above (p. 472). The dark-season period, 

 with its stronger, south-easterly winds, is hardly to be accounted for by the 

 radiation from the sun or sky. 



