METEOROLOGY OF BEN NEVIS. xli 



each wind is S.W. 25; W. 20; N.W. 16; N. 11; S. 9; N.E. 7; S.E. 5; E. 2; and 

 calms, 6. The non-cyclonic rainfall, on the other hand, is almost wholly confined 

 to N. 26, N.W. 23, and W. 22 ; in other words, 71 per cent, falls with winds from 

 these three directions. 



It is remarkable that when an anticyclone is to the north or east of Ben Nevis 

 the air is usually very dry ; but, on the other hand, if it lies to the south or west, 

 and its centre not very close to the mountain, fog, drizzling rain, or snow prevail. 

 This explains the remarkable dryness of the south-easterly winds of anticyclones at 

 this height, and the equally remarkable wetness of north-westerly winds. Both in 

 cyclones and in anticyclones the N.W. wind is the wettest, and the E. the driest. The 

 N.W. winds do not often blow ; but when they do occur, they are always accom- 

 panied by great precipitation, — actually great in cyclones, and relatively so in 

 anticyclones. On the other hand, S.E. winds, which are so frequently very wet 

 winds at lower levels, have on Ben Nevis a totally different character, being, with 

 the exception of E. winds, the driest of all the winds, both as regards the total 

 precipitations accompanying them and their mean hourly rainfall.* 



Diurnal Variation in the Direction of the Summer Winds. — The most cursory 

 examination of the wind records shows that there is no strongly-marked variation 

 in their direction ; nothing, for example, comparable to the land and sea breezes of 

 our coasts. In carrying out this inquiry Mr Omond reduced, by means of a trans- 

 verse table, to north-south and west-east co-ordinates each observation during the 

 summer months of 1884 and 1885, taking account of the velocity of the wind as 

 well as its direction, so as to obtain an approximation to the mean air motion at 

 each hour of the day. The scale for wind force at the Observatory is analogous to 

 the Beaufort scale, running from Oto 12. The estimations in use by the observers 

 were compared with the results when working the Robinson's anemometer at the 

 same times, from which it was ascertained that the equivalents in miles per hour 

 were the following : — 



Scale to 12, . 1 2 3 4 5 6 7 8 9 10 11 12 



Equivalent in miles 



per hour, . . 5 12 21 31 39 50 63 73 (85) (96) (108) (120) 



The observations from the Robinson's anemometer above force 8 could not 

 be used to determine the equivalents for the higher velocities with accuracy, but 

 by plotting the results on a diagram, and continuing the line into the higher values, 

 the bracketed numbers were obtained. The result indicates that the velocity of 

 the wind in a first-class Ben Nevis gale, such as occurred, for example, in February 

 1885, is about 120 miles an hour. 



When the observations were dealt with in this way the winds indicated a well- 

 marked diurnal variation. From 3 to 8 a.m. there is a northerly wind of about 



* "The Winds and Rainfall of Ben Nevis," Jour. Scot. Met. Soc, Third Series, vol i. p. 275 ; and vol. ii. p. 18. 

 VOL. XXXIV. J 



