608 H. MOHN. METEOROLOGY. [NORW. POL. EXP. 



are accompanied by ascensional movements of the air, causing condensation 

 of aqueous vapours and the formation of clouds. The wind (PL II) being 

 fresher in the day than in the night, there is a probability of a similar period 

 in the amount of cloud. The winter is the season in which there is the 

 greatest number of cyclones (p. 581). In general we find the range of the 

 amount of cloud greater with fresh winds than with feeble winds; but in 

 summer the reverse is the case, and in this season the diurnal range of the 

 other elements is small with fresh winds. 



The annual period of the amount of cloud is very distinctly developed 

 with a minimum in December to January and a maximum in July to August 

 (p. 518, and PL VIII). The same holds good for the number of days with 

 an overcast sky. 



The probability of precipitation is generally a little greater in the day 

 than in the night (p. 527, and PL IX) and its diurnal period corresponds with 

 that of the temperature, the tension of vapour, the velocity of the wind, and 

 the amount of cloud. 



The annual period of the probability of precipitation comes out very well 

 (p. 528, and PL IX) with maximum in summer and minimum in winter, similar 

 to the periods of temperature, tension of vapour, and amount of cloud. Corre- 

 sponding to this, there is the annual period of the number of days with 

 precipitation (p. 531, and PL IX). 



The number of hours of precipitation in a day of precipitation (p. 534, 

 and PL IX) is generally greatest in winter and least in summer. The number 

 of observations are too small to show a regular annual period. 



The probability of fog (p. 536, and PL IX) has a diurnal period in sum- 

 mer, which is the reverse of that of the temperature. Fog forms more easily 

 at night than in the day. 



The annual period of fog (p. 537, and PL IX) is very regular. No fogs 

 in winter, and maximum of frequency in July. 



