500 



MESSRS OMOND AND RANKIN ON THE 



and dominates the surface winds over that area, does not in this country extend to a 

 vertical height of one mile. 



Individual observations show similar differences between the Ben Nevis and sea-level 

 winds. Thus, if a cyclonic storm of small area is lying to the north-eastward, the sea- 

 level winds are W. orN.W., but the Ben Nevis wind may be N.E., blowing straight out 

 from the centre of the low-pressure area. In larger storms, the Ben Nevis winds are 

 practically the same as at sea-level, indicating that in the latter case the disturbance 

 extends to a greater height as well as covers a larger area. But it is remarkable that, 

 with a cyclone covering perhaps Scotland, the North Sea, and Southern Norway, and 

 having the usual circulation of wind round it, the wind at such a low elevation as 

 Ben Nevis should be utterly different. This outflowing wind seldom or never occurs 

 when the centre of the storm is to the S. or W., but only when it lies to the N. or E., 

 and is most marked 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. 



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 Dome (4813 feet), as given by Dr Buchan in the Challenger Report on "Atmos- 

 pheric Circulation," are as follows, Ben Nevis being added for comparison: — 





N. 



N.E. 



E. 



S.E. 



S. 

 8-5 



s.w. 



W. 



27-7 



N.W. 



Calm. 



Santis, 



2-7 



3-8 



4-9 



4-1 



22-5 



91 



16-7 



Puy de Dome, . 



8-2 



11-5 



7-9 



7-4 



9-6 



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 is 

 most broken up and dispersed and the high-pressure area which lies off the N.W. of 

 Africa extends towards and partly includes the British Islands. 



