NO. 17.] THE METEOROLOGICAL PERIODS IN THE ARCTIC SEA. 601 



It is evident that other factors besides the radiation are instrumental in 

 modifying the range of the diurnal period of the velocity of the wind. The 

 most prominent factors seem to be the amount of cloud and the absolute 

 velocity of the wind. 



Comparing the range (pp. 297 and 298) with a clear sky and with an 

 overcast sky, we find that the first is less than the second in all those 

 months in which, in the first column of the table, we have the arguments 

 and 10. In the months June to September we have no quite clear days, 

 and the range with overcast days comes out smaller than on half-clear days. 

 Hence the cloudiness does not seem to have any appreciably lowering in- 

 fluence upon the range, except in the summer months. 



This indicates a difference between the nature of the summer clouds and 

 the clouds of the other seasons. The amount of cloud (p. 518; PI. VIII) is 

 greatest — above 8 — in the four months, June to September, and consider- 

 ably greater than in the other months. This almost holds good for the 

 probability of precipitation (p. 528; PI. IX) and for the number of days with 

 rain (p. 582). The tension or quantity of aqueous vapour has its maximum 

 in the summer months (p. 502; PI. VI). These facts seem to indicate that 

 the summer clouds are thicker and denser than the clouds of the other 

 seasons. If this be true, then the clouds must intercept the radiation from 

 the sun in the summer months in a greater proportion than at the other 

 seasons, and thereby cause a diminished range of the diurnal period of the 

 velocity of the wind. ^ 



The Table on p. 302 and PI. II, shows that the diurnal range of the 

 velocity of the wind in all months and seasons is greater with a high velocity 

 of the wind than with a low. The reason may be, as I have pointed out 

 (p. 599), the increase of the force of the convectional currents caused by the 

 greater resistance exerted by the rugged surface of the earth to the stronger 

 winds of the day, than to those at night. 



We have seen (p. 294) that the velocity of the wind is greater in cloudy 

 weather than with a clear sky, and (p. 304) that the greater velocity of the 



' Exact observation of the amount and density of clouds is difficult in the night and 

 dark season. The error is presumably in the direction of relatively higher figures 

 being noted then than in the day-time. The transparency of the clouds in the dark 

 time is probably greater than one would suppose from the amount and density noted. 



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