WINDS OF BEN NEVIS. 545 



ditions on the summit are purely and entirely characteristic of high-level observatories. 

 But we have already indicated that we do not hold that the veering of the wind on Ben 

 Nevis, relative to the sea-level winds, is the theoretical veering of the wind with altitude, 

 for reasons already given, and for the following. Since the Ben Nevis winds approxi- 

 mate so closely to the sea-level winds in the year of warmest months, and differ so much 

 from them in the year of coldest, we might expect to find, if this were a real veering 

 with altitude, similar relations between the upper and lower winds in the course of one 

 day, about the times of maximum and minimum temperature respectively, — that is to say, 

 a daily variation in the direction of the wind which would show on Ben Nevis a backing 

 from being in the early morning several points to the right of the sea-level wind, to being 

 almost coincident with it at the middle of the day, and a veering again towards night. 

 In other words, that from the time of the morning minimum temperature, the Ben 

 Nevis winds would back steadily till about the time of the afternoon maximum, and veer 

 again thereafter. That there is no such backing and veering in the Ben Nevis winds 

 is pointed out below, with an explanation of the peculiar changes that do occur ; 

 so that this difference in the excess above and below must be due to some other cause, 

 the most probable being that already given — the nature of the upper current, whether 

 polar or anti-polar. The thermal windrose, already referred to, shows that the warmest 

 point on the mean of the year is S. and the coldest N.E., so that in a year of warmest 

 months we would expect to find southerly winds in excess, and N.E. in excess in a year 

 of coldest months, which Table VI. shows to be the case. The warmest point on the 

 Ben Nevis windrose oscillates through 90 degrees from S.W. in winter to S. and S.E. 

 in summer, and we see that all these points are in a warm year in excess of what 

 they are in an average year. These points, again, are in a cold year in defect of what 

 they are in an average year, while the points of the northern half of the compass 

 are in excess. If we take the sea-level excess as an index to the distribution of pres- 

 sure at sea-level, we have in the Ben Nevis warm year an excess of pressure somewhere 

 to the east or south-east, and low pressure to the west or south-west, superimposed on 

 the normal excess in the south, and defect in the north-west or north. With this 

 distribution of pressure, then, we see that the excess winds above and below are 

 from about the same point of the compass. Similarly, in the cold year the sea-level 

 wind excess indicates an excess of pressure to the west or north-west, and a defect to 

 the east. Here we see that the Ben Nevis excess blows straight out from the indicated 

 centre of low pressure, — that it is, in fact, an out-flowing wind. 



In the year of driest months, N., N.E., E., and S.E. are in excess, and S., S.W., W. 

 in defect above and below, while N.W. is in defect above and in excess below. Both the 

 excess and defect are more pronounced below than above. In the paper on the " Winds 

 and Rainfall of Ben Nevis," already referred to, the wettest direction of wind is N.W. 

 and the driest E., both in cyclonic and anti-cyclonic areas. In the year of wettest 

 months we have, on the summit, an excess of W., N.W., N., andN.E., — N.W. being most 

 in excess ; and at sea-level, an excess of S.W., W., and N.W., — W. being most in excess. 



VOL. XXXVI. PART II. (NO. 19). 4 M 



