f)40 MESSRS OMOND AND RANKIN ON THE 



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. 



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 month follow the same course. 

 In Table IV. is given the percentage excess or defect of each wind on Ben Nevis in each 

 month from the mean of the year; heavy type indicating an excess, and italic type a 

 defect. This table is constructed from Table II.; thus the mean of the )^ear for N. 

 winds is 18" 8, but in January they are 16"1, i.e., 2'7 or 14 per cent, below their 

 annual mean. Table V. is drawn up in identically the same way for the sea-level 

 winds. The most obvious features of these tables are the 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. At the foot of the tables are given 

 the extreme departures from the annual mean, with the range or sum of the positive and 

 negative differences. It will be seen that the maxima and minima for the various winds 

 occur at much the same time of the year on Ben Nevis and at sea-level, except 

 the maxima of the N. and W. winds, and the minimum of the N.E., which are the 

 three directions generally affected in cases where the Ben Nevis and sea-level winds 

 differ largely in direction as described above. The difference of the winds from month 

 to month is, on the whole, rather greater on Ben Nevis than at sea-level, the average 

 range of the eight winds — excluding calms — being 71 for the former, and 65 for 

 the latter ; thus indicating that the seasonal changes in w T ind direction affect the 

 whole stratum of the atmosphere up to at least the height of Ben Nevis, and are 

 not merely local or surface currents, and that their causes must be looked for in 

 correspondingly great and widespread changes of pressure and temperature. The 

 easterly winds of spring are not a mere spilling over of cold air from Central Asia, 

 extending by virtue of its density as a thin film under the lighter and moister western 

 air, but are a great bodily movement through a considerable depth of the atmosphere ; 

 they begin earlier on Ben Nevis, February showing an increase, but last longer at sea- 

 level. 



It is interesting to compare the high and low level winds during the harvest months 

 of August and September. During these months there is an average 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 the increase and the decrease are doubled in the 

 higher regions of the atmosphere, a matter of great importance in the prevention of 

 descending masses of cold, dry air, which, accumulating in flat or hollow ground, 



