METEOEOLOGY OF BEN NEVIS. xxxvii 



as causing a slight variation in the portion of the diurnal curve from 10 a.m. to 

 8 p.m., when the wind force is less than the daily mean. 



But further, the diurnal variation peculiar to truly high-level stations, or 

 those placed on mountain peaks, is shown by observations to occur on Ben Nevis 

 during continued periods of high winds. The longest continued storm extended 

 over seven days without intermission, from February 15 to 21, 1885, the 

 wind remaining steadily in the S.E., except on the 20th and 21st, when it 

 occasionally veered to S. and S.S.W. During that time, the mean hourly force 

 was 8*21, or about 75 miles an hour, the mean hourly maximum being 88 miles 

 from 5 to 6 a.m., and the minimum 65 miles from 4 to 5 p.m. Thus the diurnal 

 variation during this remarkable storm was well marked, the difference between 

 the hours of greatest and mean velocity being 23 miles. 



In explanation of this diurnal variation in the wind's velocity on mountain 

 peaks, we may revert to what has been said as to the high barometer main- 

 tained in deep narrow valleys during the night, as the result of cold air-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 

 furious blasts of wind blowing down the valleys during the night. 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 so long as the cold currents occasioned by terrestrial radiation continue to 

 be poured down to the bottom of the valleys. On the other hand, during the 

 warmer hours of the day, barometric pressure is abnormally low, owing to the 

 super-heating of these valleys, as contrasted with the temperature of the sur- 

 rounding 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 its 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. 



Diurnal Variation in the Amount of Cloud. — The hourly means of the 

 amount of cloud on scale to 10, during the four years, are given in Table VI. 

 The mean of the whole year is 8 "4 — the maximum being 8 "8 in January, and 

 the minimum 7 9 in April, the monthly amounts thus showing but little differ- 

 ence throughout the year. The curve of diurnal variation for the year is of 

 the simplest description, being above the mean from 8 a.m. to 7 p.m., and under 

 it from 8 p.m. to 5 a.m. These features of the curves appear in their most 

 pronounced form from April to September. The close agreement with the 



