WINDS OF BEN" NEVIS. 501 



Although the Ben Nevis winds differ so much from those at sea-level, the variations 

 in the relative frequency of each wind from month to mouth follow the same course, 

 showing an excess of easterly winds in spring, and of westerly in early winter, both of 

 which departures from the mean are more marked on Ben Nevis than at sea-level. The 

 average percentage increase of N.E., E., and S.E. winds for the months March, April, 

 May is 34 at Ben Nevis, and 31 at sea-level ; while the average increase for S.W., W., 

 and N.W. in November, December, January is 16 for the former, and 13 for the latter. 



During the harvest months of August and September there is a diminution of 33 per 

 cent, in the N.E., E., and S.E. winds at Ben Nevis, but of only 16 per cent, at sea- 

 level ; while the increase in the S., S.W., and W. winds amounts to 16 per cent, at the 

 former, and only 8 per cent, at the latter. Thus at this season, not merely is there an 

 increase of the warmer south-westerly winds, and a diminution of the cold easterly 

 currents, but both of these changes are more pronounced in the higher regions of 

 the atmosphere, a matter of great importance in the prevention of descending masses of 

 <x>ld, dry air, which, accumulating in flat or hollow ground, are so liable to cause 

 " radiation frosts." October presents a complete contrast. The S., S.W., and W. are 

 8 per cent, below the annual mean at sea-level, and 1 5 per cent, below on Ben Nevis ; 

 and the N., N.E., and E. winds are 13 per cent, above the yearly average at sea-level, 

 and 1 5 per cent, above on Ben Nevis ; showing an increase in the colder currents, 

 slightly greater at high levels, and a diminution in the warmer currents, markedly 

 greater in the upper atmosphere. 



In order to see how far the rainfall of Scotland is influenced by the winds, a pre- 

 liminary examination has been made of the fall in each month on the east and west 

 coasts. The annual fall on the west coast is greater than on the east, and its distribu- 

 tion throughout the year is different. At all purely western stations the maximum 

 rainfall is in the depth of winter, November to January, and the minimum in April 

 or May. At many eastern stations, however, especially in the southern half of 

 Scotland, the greatest rainfall is in July or August, the minimum being like the west in 

 April or May ; but there is also on the east coast a secondary maximum in winter. 

 November is a wet month on both coasts. 



Comparing the months with the wind tables, we see that the heaviest rainfall on 

 the west coast occurs during the winter maximum of westerly winds, and the least 

 during the spring maximum of easterly winds, and that the summer secondary 

 maximum of westerly and southerly winds in July and August does not affect the west 

 coast, but does largely increase the east coast rainfall. The winds of November have 

 this peculiarity : S.W. is in excess both on Ben Nevis and at sea-level, but E. and N. 

 are slightly below their mean at sea-level, and are above it on Ben Nevis, pointing to 

 the fact that some of the cyclones which cause the heavy rainfall of November have 

 not extended upwards to the height of Ben Nevis, but have there caused reversed or 

 outflowing winds. The month with perhaps the greatest difference between the east 

 and west coast rainfall is December. The winds of December show a marked increase 



