REPORT ON ATMOSPHERIC CIRCULATION. 27 



question, viz., the observations recently obtained from such high-level observatories as 

 are situated on true peaks, as Ben Nevis, Santis, Obirgipfel, Sonnblich, etc. In such 

 situations, the curve of diurnal variation in the wind's velocity is precisely the reverse 

 of what obtains over what are substantially plains or plateaux. At these high-level 

 observatories the maximum hourly velocity occurs during the night and the minimum 

 during the day. 



Reference has been made to the high barometer maintained in deep narrow valleys 

 during the night, as being the result of the cold currents from the adjoining slopes which 

 the chilling effects of terrestrial radiation set in motion. These masses of cold air, 

 accumulated in the valleys, give rise to the well-known furious blasts of wind blowing 

 down the valleys of such mountainous regions as the Alps during clear and comparatively 

 calm nights. Now since these down-rushing winds must necessarily be fed from higher 

 levels than those of the mountain itself, it follows that the winds prevailing on the peak 

 of the mountain are really the winds of a higher level, and blow therefore with the 

 greater velocity due to that greater height ; and the increased velocity is kept up as 

 long as the cold currents occasioned by terrestrial radiation continue to be poured 

 down to the bottom of the valleys. This consideration serves to explain the apparently 

 anomalous direction of the winds in Greenland, which are in some degree modified by 

 the downflow from the adjoining high grounds. 



On the other hand, during the warmer hours of the day, the barometric pressure of 

 deep valleys is, as has been shown, abnormally low, owing to the super-heating of these 

 valleys, as contrasted with the temperature of the surrounding region. This gives rise 

 to a warm wind blowing up the valleys during the hottest hours of the day, and an 

 ascending current close to the sides of the mountain up to the very summit. Now 

 since no inconsiderable portion of this ascending current, whose horizontal velocity is 

 necessarily much retarded, mingles with the air-current proper to the level of the peak, 

 it follows that the prevailing wind on the peak must be retarded during the hottest 

 hours of the day. 



The explanation of the variation of the wind's velocity over comparatively flat 

 surfaces is more difficult. Whatever be the cause or causes, they are intimately, if not 

 immediately, connected with the temperature of the earth's surface over which the 

 winds blow. The Challenger observations on the five great oceans prove that, so far 

 as concerns any direct influence on the air itself, solar and terrestrial radiation exercise 

 no influence on the diurnal variation in the velocity of the wind, these showing 

 practically no variation in the velocity. The same observations prove that on nearing 

 land the velocity of the wind is everywhere reduced, but that the retardation is 

 greatest during those hours of the day when the temperature is lowest, and least 

 when the temperature is highest. The time of the day when the wind's velocity is 

 increased is practically limited to the hours when temperature is above the daily 



