xlii METEOROLOGY OF BEN NEVIS. 



2| miles an hour ; and from 11 a.m. to 2 p.m. S. or S.S.E. wind of about 3 miles an 

 Lour. At the other hours the differences are small and variable in direction, except 

 about midnight, when there is an indication of a moderate difference in the same 

 direction as the mean daily wind. This, however, may really be traced to the 

 well-marked increased velocity of the Ben Nevis winds at this time of the day. 



The most marked features in the diurnal variation are the northerly winds shortly 

 after sunrise, and the southerly winds about noon. Mr Omond is inclined to the 

 opinion that the shape of Ben Nevis affords the explanation of this variation. The 

 ridge of the hill top runs east and west ; the deep gorge on the north side is pene- 

 trated by the sun for only a few hours after sunrise in summer ; and on the other 

 side, a steep slope goes down into Glen Nevis at an angle of about 30°. In the 

 early morning, the sun's rays heat the northern valley, while the glen on the south 

 side is still in shade, thus the heated air from the northern gorge ascends and flows 

 over the summit as a northerly wind. On the other hand, as the day advances 

 and the sun acquires southing, the northern valley falls into shade, while Glen 

 Nevis, being now heated by the sun's rays, the current rising from it flows over 

 the- summit as a southerly wind.* 



Thermal Windrose. — This subject was investigated by Mr Rankin, from 

 the three years' observations from June 1884 to May 1887. The eight directions 

 of wind in the order of their temperature are — S., 32°'6 ; S.W., 32°"5 ; W., 31°'4 ; 

 N.W. and S.E., each 30°'2 ; E., 27°"8 ; N., 27 0, 6 ; and N.E., 26 0, 5. Thus the warmest 

 wind is S., 32° '6, and the coldest N.E., 26° "5, the difference between the two being 

 6°'l. The warmest point oscillates between S.W. in winter, passes through S. to S.E. 

 in July and September. Easterly winds (S.E., E., N.E.) have 20°*7, but westerly 

 winds only 15° '6 as their mean annual range, thus showing the greater influence of 

 the land than of the sea in raising the temperature of the winds. Winds from S. , 

 S.W., W. and N.W. indicate an annual minimum in January, and winds from N. 

 N.E., E. and S.E. in March. Winds from N.E. and N.W. have the maximum in 

 August, but all other directions in July.f 



Temperatures at different Heights above Ground. — The results of a month's 

 observations of two sets of temperature in Stevenson's screens, one at the normal 

 height of 48 inches, the other at 112 inches above ground, have been discussed 

 by Mr Omond. The observations extended over part of July and the whole of 

 August 1886. 



Along with these shade temperature readings were taken of a black bulb in 

 vacuo. The thermometer inside the glass jacket was a common thermometer, with 

 the bulb blackened in the usual way, showing therefore the solar radiation at the 

 time of observation, instead of, as in the usual maximum black bulb, giving only 



* " The Diurnal Variation in the Direction of the Summer Winds on Ben Nevis," by E. T. Omond, Proc. 

 /'oil. Hoc, vol. xiii. p. 839. 



t " The Thermal Windrose at the Ben Nevis Observatory," by Angus Rankin, Proc. Roy. Soc, vol. xiv. 

 j). 41(). 



