NO. 17.] 



WIND. 



299 



The days with sky overcast show the ordinary period very well, particularly 

 November and March with ranges above one metre per second. 



The periods for the seasons give a better oversight. Plate II. (1 cm. = 

 1 m. p. s.). In winter there is almost no period; in spring the range with 

 sky clear is only one half of the range with sky overcast; in summer the range 

 with sky clear is twice as great as with sky overcast; in autumn, September 

 excluded, the range is much greater with sky overcast than with sky clear. 

 In the dark season there is no distinct period with sky clear. The relatively con- 

 siderable amount of range with sky overcast as great as the range with sky 

 overcast in the sunny season is due to the ranges of October and November. 

 Of the equinoctial months, March has a much greater range with sky overcast 

 than with sky clear. September is not suited for a comparison in this respect. 



The above tables indicate that the range of the diurnal period of the 

 velocity of the wind is greater with a greater velocity, and vice versa. A 

 further study of this relation seemed to me desirable, and the results of 

 such study are set forth in the following tables. From the tables giving 

 the velocity of the wind for each hour of observation, the days on which the 

 mean daily velocity (v) was less than 4'5 metres per second, and the days 

 when it was above 4'5 m. p. s., were taken out. For the months of the drift 

 of the Fram, the mean velocity for each of these two groups and for each 

 alternate hour, the corresponding diurnal mean, and the difference between this 

 mean and the bi-hourly means (d), were taken. 



