XXXIV 



METEOROLOGY OF BEN NEVIS. 





Inch. 







Inch. 



1 A.M. 



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1 



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3 „ 



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3 



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-•054 



4 „ 



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4 



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-•058 



5 „ 



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5 



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-•057 



6 „ 



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6 



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7 „ 



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8 „ 



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9 „ 



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10 „ 



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Noon 



-•015 



Mic 



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In such situations, during night, the whole surface of the region is cooled 

 by radiation below the air above it, and the air in immediate contact with the 

 ground becoming also cooled, a system of descending air-currents sets in over 

 the whole face of the country bounding the deep valley. The direction and 

 velocity of these descending currents are modified by the irregularities of the 

 ground, and, like currents of water, they converge in the bottom of the valleys, 

 which they fill to a considerable height with the cold air they bring down from 

 the sides of the mountains. This cold and relatively dense air rises above the 

 barometers which happen to be down in the valley, with the result that a high 

 mean pressure is maintained during the night. In this month at Gries, 

 pressure during the coldest time of the night is maintained about 0'40 inch 

 above what it would be in the open country. On the other hand, during the 

 day, these deep valleys become highly heated by the sun, and a strong ascend- 

 ing current is early formed, under which pressure falls unusually low. The 

 result is a diurnal range of pressure of 097 inch, — a range quite tropical in 

 amount. The same law of the diurnal distribution of pressure holds even in 

 such shallow valleys as the Thames, observations at Greenwich and Kew for 

 the five years ending with 1880 showing a relatively higher pressure at Kew 

 during the night and lower during the day than occurs at Greenwich ; and the 

 differences are greatest in the months of the year when the diurnal range of 

 temperature is greatest. From the position, this does not occur at the Low- 

 Level Observatory at Fort- William. The whole of this discussion has important 

 bearings on the diurnal variation in the wind's velocity on the top of Ben 

 Nevis, in common with all high-level observatories situated on true peaks. 



Diurnal Variation of the Force of the Wind. — The observations of the force 

 of the wind have been made hourly by estimation, the scale adopted being a 

 modified form of Beaufort's scale, in which represents a calm, and 12 the 

 highest wind which occurs at the Observatory. Much attention was given 

 from the first to secure that the estimations of force were made on the same 

 method by all the staff. These efforts towards real uniformity have been 

 unusually successful, as evidenced by the frequent comparisons made independ- 



