WINDS OF BEN NEVIS. 543 



of the cirrus clouds. A very careful series of observations on these clouds has 

 been made at Blue Hill Observatory, Massachusetts, by Mr Clayton, which may be 

 taken as typical for the North Temperate Zone. In the year 1887 over 1500 

 observations were made, giving the following percentages of frequency in the directions 

 of motion: — S.W., 17; W., 40; N.W., 26; all other directions, 17. In other words, 

 for two-thirds of the time the clouds were moving from the W. or N.W., and for 

 half of the remainder from the S.W., leaving only about one-sixth of the total 

 observations for the other five points. These observations were made on 290 days 

 during the year, there being 75 days on which no upper clouds could be seen, either 

 because there were none, or because the sky was overcast with lower clouds. Assuming 

 that the observations taken were fairly distributed over the 290 days, there might 

 have been about 390 observations made on the remaining 75 days had it been 

 possible to observe on them ; and even on the extreme hypothesis that on none of 

 these occasions were the upper currents S.W., W., or N.W., i.e., putting all the 390 

 into the ''other directions," it would not seriously reduce the preponderance of 

 westerly motion, the percentages being then S.W., 14; W., 31; N.W., 21; all other- 

 directions, 34; thus still leaving more than half of the winds W. or N.W., and more 

 than a third of the remainder S.W. Of course, it is not likely that none of the missing 

 observations were S.W., W., or N.W., and probably the percentages of the actual 

 observations given above are the nearer to the true average. These upper currents 

 have the same character as, and are an exaggeration of, the surface winds of Massa- 

 chusetts, where N. and N.W. are the points of greatest frequency, and the relationship 

 of the two is akin to that between the winds of Santis or Puy de Dome, and their 

 Jow-level winds as given above. But Massachusetts lies in the continuation of the 

 European belt of high pressure, and far removed from the North Atlantic depression 

 which influences the relation between the Ben Nevis and sea-level winds in Scotland. 

 The highest station from which data are available is Pikes Peak in Colorado, 14,134 

 feet, and the percentages of wind directions as given in the Challenger Eeport, page 

 165, are as follows :— N., 10; N.E., 9; E., 1; 8.E., 2; S., 5; S.W., 32; W., 20; 

 N.W., 18 ; calm, 3. Here we have a marked predominance of S.W. and W. winds, 

 but nothing like the excess from one direction of the cirrus cloud motion indicated 

 by the Blue Hill observations ; it would seem that even at this great height the 

 observers are not wholly in the upper currents, though the percentage of westerly 

 winds is in excess of that at any low-level station in that part of North America. 

 The measurements of the heights of clouds taken at Upsala confirm this (see Nature, 

 vol. xxxvi. p. 321). They show three layers of clouds — Lower, 2000 to 6000 feet, 

 in or near which most hill stations are; Middle, 12,000 to 15,000, in which Pikes 

 Peak is ; and Upper, 20,000 to 27,000, which are the true upper current clouds. 



Any attempt to elucidate the behaviour of the atmospheric currents in cyclonic storms 

 must be based on a careful study of individual cases, and not on general means, such as 

 are dealt with in this paper. For this we have neither had time nor opportunity as yet, 



