THE DIURNAL EANGE OF WIND DIRECTION ON BEN NEVIS. 711 



averaged for each hour of the day. The results showed for the whole clay a mean wind 

 from about W.S.W., with a velocity of 1^ miles per hour ; that superimposed on this 

 mean wind there was from 3 to 8 a.m. a northerly wind of about 2^ miles an hour, and 

 from 11 a.m. to 2 p.m. a south or south-south-east wind of about 3 miles per hour 

 velocity. At other hours the departures from the mean direction and force were small 

 and irregular, except about midnight, when there was a distinct increase in the velocity, 

 though the direction differed little from the mean. 



It is evident that this analysis of the lighter winds of two summers gives sub- 

 stantially the same result as the reduction of the direction alone of all the summer 

 winds for twenty years, for the latter also indicates that a northerly wind in the early 

 morning and a southerly one in the afternoon are superimposed on the W. by S. |- S. 

 wind, which is the mean direction ; the increase of strength about midnight is of course 

 not brought out by Lambert's formula, which takes no account of velocity. The 

 variation in the wind is most probably due to the effect of radiation on the sides of the 

 hill. To the north is a deep, steep-sided valley, into which sunshine scarcely penetrates, 

 except for a few hours after sunrise in summer, while the southern slopes lie freely open 

 to solar and terrestrial radiation. In the early morning the air over the south side of 

 the hill is chilled by terrestrial radiation, while that to the north gets what little solar 

 heat there is, and consequently a northerly wind passes over the hill-top. In the 

 afternoon ascending currents rising from the south side, now heated by the sun, cross 

 the summit as southerly winds and are drawn down into the colder northern valley. 



If this explanation is correct, the diurnal changes of wind direction on Ben Nevis 

 are not connected with any general circulation of the air over Scotland. They are not 

 variations in the wind due to the westward march of the diurnal fluctuations of baro- 

 metric pressure, nor are they land and sea breezes caused by the greater daily cooling 

 and heating of the land in comparison with the surrounding sea, but are due to local 

 winds that originate on the hill. Either of the other two agencies would, I believe, 

 cause throughout the day changes of direction quite different from those observed. 



