METEOROLOGY AND ALLIED SUBJECTS. 

 Maximum and minimum wind velocity at Vienna. 



293 



This increase in wind velocity up to a maximum in the warmest part 

 of the day is apparently coufiued to the lowest stratum of air and is to be 

 attributed to the descent of rapidly-moving upper currents of air replac- 

 ing that which is continually ascending from the heated surface of the 

 earth. The only other plausible hyi^othesis would seem to be that the 

 upper currents have a variable influence upon the lower strata, depend- 

 ing upon the variation of what is known as the internal friction of gases. 

 The coefiBcient of this friction increases with the temperature, whence it 

 might seem that stronger winds would be experienced at midday at sea- 

 level than during the night-time, but coefflcient (55) of friction, for the 

 temperature t is, according to O. E. Meyer, equal to (l+0.0025^)i;o, 

 where tjo is the coefficient at 0° C. 



It is, however, believed that the diurnal variation of the temperature 

 of the air at considerable altitudes is too slight to sensibly effect this 

 coefficient. Kopi)en, therefore, considers that the greater part of the 

 diurnal variation and wind force must be attributed to the descent of 

 rapidly moving upper currents of air. This result is confirmed by the 

 study of the relation of wind pressure and barometric gra'dient, and also 

 by the study of the daily and annual variation in humidity. Koppeu says, 

 since the atmospheric pressure is influenced by the warming of the lower 

 strata of air in the same direction as by the humidity, we can say that 

 the greater the difference of air temperatures in a vertical direction the 

 less is the difference in absolute humiditj^, barometric pressure, and 

 total horizontal movement, so that in the early afternoon hours, as far 

 as these elements are concerned, the inhabitants of the low plains 

 may be said to, in a certain sense, be transferred into an atmosphere 

 that belongs to a higher level, while the dwellers on the mountain tops 

 are on the other hand transferred to a lower level. The reason why the 

 trade-winds, and probably all winds, show on the open sea nothing or a 

 very little of the midday maximum is explained by considering that 

 the vertical circulation of air, as well as the increase of wind velocity 

 with the altitude, is much less than over the land. {Z. 0. 0. M., XIX, 

 p. 348.) 



Hann has, in a few words, set clearly forth the slight differences of 



