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XIX. — The Winds of Ben Nevis. By R. T. Omond and Angus Rankin. 



(Read 1st June 1891.) 



Noting the direction of the wind is part of the regular routine of observations at 

 the Observatory on the summit of Ben Nevis. Owing to the climatic conditions, 

 especially the frequency of frozen fogs in winter, anemometers are practically useless, 

 but the record is made complete enough for many purposes by the system of hourly 

 observations ; at each hour of the day and night the wind, at the time of making 

 the observation, is noted, and the number of entries of each wind direction in the 

 day, month, or year must very nearly represent the relative frequency of such winds 

 for that time. Table I. is a summary giving the number of times each wind is 

 observed in each month of the year on an average of six years (1884-89) ; February 

 being given in duplicate, that in the body of the table containing 28 days, and 

 that at the foot 29, with the winds divided in the same proportion. The wind 

 is observed to 32 points, but the entries have been reduced to 8 points for this paper 

 by counting all the hys to their adjacent octant, and halving the 8 intermediate points 

 between the octants; thus all N. by E. count as N., all N.E. by N. as N.E., while 

 the N.N.E. entries are halved between N. and N.E., and so on. Perhaps a stricter 

 method of reduction would have been to divide the intermediate points in the ratio 

 of the relative frequency of the adjacent octants, but this would have been considerably 

 more laborious, and of doubtful utility ; still it should be borne in mind that the 

 method used tends to smooth out any sudden changes in the frequency of winds 

 of nearly the same direction. In Table II. the frequency of each wind is expressed 

 in percentages, thus equalising the varying lengths of the months, and making each 

 month easily comparable with the whole year; thus in January, out of the 744 hours 

 of the month, there are 120 hours or 161 per cent, of N. wind. 



A glance at these tables will show some interesting features. The most frequent 

 wind is N., next come S.W., S.E., and S., all about equal, and the least frequent 

 is E. N. is the point of greatest frequency in all months except May, when 

 S.E. takes its place; in May 1889 there were only 15 hours in the whole month 

 with N.. N.W., or N.E. winds, and during three-fourths of that month the air was 

 moving from the S. or S.E. This was, of course, an exceptional month, but in the six 

 years there are only two (1887-88) in which N. was the most frequent wind in May. 

 The exact determination of northerly winds is not very easy on Ben Nevis, owing to the 

 shape of the hill : the great cliff, 2000 feet high, that forms its northern face, breaks 

 these winds up, and makes them squally and uncertain ; some may be entered as N. 

 that should be really N.E. or N.W. If we group the percentages of N., N.E., and N.W., 

 and S., S.E., and S.W. together, we get 36 - for the former, and 39 - 4 for the latter, 



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