WINDS OF BEN NEVIS. 



539 



when an anti-cyclone or high-pressure area is on the other side ; the wind appears 

 to be a feeding current carrying the ascending air of the cyclone over to supply 

 the descent in the anti-cyclone. Why this action should only be observed when 

 the cyclone lies to the N. or E. is not very clear, but possibly may be due to 

 the fact that in this case the current is passing from a colder to a warmer atmos- 

 phere, and, displacing the lighter warmer air, sinks to the level of Ben Nevis, 

 while in the opposite case it is constantly meeting colder air, and rising higher passes 

 over it far above the level of the hilltop. If the wind on the hilltop is not 

 at a right or greater angle from the sea-level wind, it is generally very nearly the 

 same as it. The supposed veering of the wind at great heights, required by the 

 theory that a cyclonic storm is a whirling column drawing the air in spirally below 

 and pouring it out spirally above, is so seldom observed as to be the exception rather 

 than the rule. There is, on the whole, a slight tendency for the wind to blow more 

 nearly parallel with the isobars, and to have less indraught than at sea-level, but 

 this may be simply due to the greater freedom from ground friction, a freedom which 

 appears also in its increased velocity.* The complete change in average wind direction 

 with height is, as far as can be gathered from comparison with other mountain 

 stations, a peculiarity of cyclonic regions. The percentage frequency of the different 

 winds on the Santis (8094 feet), and the Puy de D6me (4813 feet), as given by 

 Dr Buchan in the Challenger Keport on " Atmospheric Circulation," are as follows, 

 Ben Nevis being added for comparison : — 





N. 



N.E. 



E. 



S.E. 



s. 



S.W. 



W. 



KW. 



Calm. 



Santis, .... 



2-7 



3-8 



4-9 



4-1 



8-5 



22-5 



27-7 



91 



16-7 



Puy de Dome, . 



8-2 



11-5 



7-9 



7-4 



96 



15-6 



27-7 



12-1 





Ben Nevis, 



18-8 



8-9 



7-6 



13-1 



13-0 



13-3 



11-5 



8-3 



5-5 



The low-level winds near the Santis and Puy de Dome are on the average of 

 the year westerly, the maximum frequency tending towards S.W. in winter, and N.W. 

 in summer ; but at the hill stations W. is the maximum point all the year. Here we 

 have mountain stations whose winds agree in direction with those blowing below ; but 

 Santis in Switzerland, and Puy de Dome in France, both lie in anti-cyclonic or high- 

 pressure regions, and are not comparable with a high-level station like Ben Nevis, 

 standing on the side of a great barometric depression, and close to the track of the 

 cyclones that sweep our north-western coasts. In connection with this, it should be 

 noted that May, the month in which the north or contra-isobaric winds are at a 

 minimum on Ben Nevis, is the month in which the depression in the North Atlantic 



* The average velocity of the wind on Ben Nevis is 17 miles per hour, exactly the same as that given in the Challenger 

 Report for the mean velocity in the open sea far from land. 



