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The Diurnal Range of Wind Direction on Ben Nevis. By K. T. Omond. 



The weather conditions on the summit of Ben Nevis made it impossible to obtain a 

 continuous record from self-registering anemometers at the Ben Nevis Observatory, but 

 we have in the hourly observations of the wind made by the observers what approximates 

 to a complete and continuous record. The direction and strength of the wind were 

 noted at each hour by day and night, and these observations are to be found tabulated 

 month by month in extenso in this and the preceding volumes of observations. The 

 direction is recorded to 16 points (N., N.N.E., N.E., E.N.E., E., etc.), and the pressure 

 on a scale from to 12 closely akin to the well-known Beaufort scale ; # but the situation 

 of the Observatory rendered the record of the wind-pressure or velocity very un- 

 satisfactory. The ridge that forms the summit of Ben Nevis lies east and west, with 

 a steep slope to the southward and an almost vertical cliff to the north. Consequently 

 southerly winds blew across the summit with a locally accelerated velocity, while 

 northerly winds were broken up by impinging on the face of the cliff, and were felt at 

 the Observatory as a succession of squalls, often interspersed with swirls of back-draught 

 towards the cliff edge, and their velocity, whether recorded by an anemometer or 

 estimated by an observer, was probably considerably less than the speed that the air 

 was moving with before striking the cliff face. This breaking up of the winds from 

 northern directions introduced also a slight uncertainty in regard to their direction ; in 

 many cases the wind was noted as northerly, and is entered in the tables as N., when 

 its direction may have been anything between north-west and north-east. Hence in 

 the summarised tables there is a very large number of N. winds as compared with 

 N.W. and N.E., an excess which probably does not really exist; also an undue pro- 

 portion of the winds classed as variable, and included under calms in the summaries, are 

 northerly as compared with southerly winds. Neither of these two latter sources of 

 error, however, are of much importance as regards the directions of wind on Ben Nevis ; 

 they do not cause an uncertainty at all comparable with the discrepancies in the 

 velocities of northerly and southerly winds referred to before. The main fact stands 

 out that there is not on Ben Nevis the great preponderance of west and south-west 

 winds that is observed at sea-level stations all over the British Islands, but that the 

 most frequent directions are northerly and southerly in approximately equal pro- 

 portions^ The southerly winds probably represent the south-westerlies of lower levels 

 turned partly by the shape of the hill, but also with some alteration in direction due to 

 altitude, while the northerly winds are largely high-level outflowing winds from cyclonic 

 areas passing to the north and north-east of Scotland. J 



* See Tram. Roy. Hoc. Edin., vol. xliii. p. 483. t See Tables, Trans. Roy. Soc. Edin., vol. xliii. p. 501. 



% " The Winds of Ben Nevis," by R. T. Omond and A. Rankin, Trans. Roy. Soc. Edin., vol. xxxvi. p. 537, and 

 vol. xlii. p. 499. 



