.~>44 MESSRS OMOND AND RANKIN ON THE 



lmt the following points should be borne in mind when studying the development and 

 behaviour of cyclones. 



1. The height of a cyclone is so small compared with its area, that any attempt to 

 represent a vertical section of it diagrammatically is useless and misleading. 



2. From the laws of fluid motion it is probable that the upper end of a rotating mass 

 of air, with or without an ascending motion in it, behaves in a very different manner in 

 the free atmosphere from its lower end, which is bounded by the surface of the earth. 

 It is unsafe to lay off isobars for the level of Ben Nevis from the winds observed there 

 according to Ballot's law ; they may blow directly from high to low pressure, or accord- 

 ing to some other yet unknown law. 



3. The small diurnal variation in wind direction will enable us to study the changes 

 of wind connected with variations of pressure, &c, independent of the time of day of 

 their occurrence. 



In Table VI. are given the percentage excess and defect from the six years means of a 

 year made up of (l) warmest months on Ben Nevis, (2) coldest months, (3) driest months, 

 and (4) wettest months. In each case the sea-level excess or defect is also given, for the 

 year made up of the same months as the Ben Nevis part. This table shows some inte- 

 resting points when studied along with results already discussed by the writers, viz., The 

 Thermal Windrose at the Ben Nevis Observatory (Proc. Hoy. Soc. Edin., 1886-1887, 

 p. 416), and The Winds and Rainfall of Ben Nevis {Jour. Scot. Meteor. Soc, vol. viii. 

 p. 18). In the year made up of warmest months, the southerly winds are most in excess 

 on Ben Nevis and also at sea-level ; while north-easterly are most in defect on the 

 summit, and northerly below. That is to say, in our warmest weather on the summit, 

 the average departures from the normal approximate to the same as the departures at 

 sea-level, more nearly in the case of the excess than of the defect. In the year of coldest 

 months, easterly winds are in excess on the summit, and northerly at sea-level ; and 

 the points of greatest defect are south on Ben Nevis, and south-east at sea-level. Thus 

 in both the excess and defect the directions are on the summit several points to the 

 right, looking out from centre of compass, of those at sea-level, — eight in the case of the 

 excess, and four in the defect. That is to say, in our coldest weather on the Ben, the winds 

 that are most in excess there are at right angles to, and to the right of, those in excess 

 at sea-level. If we look now at Tables IV. and V., and examine the excesses month by 

 month, we see that the following point is pretty well indicated, namely, that given an 

 excess at sea-level, an excess on the summit is, as a rule, found to the right of that 

 direction, this more particularly in the winter months than in the summer. The number 

 of points that the upper excess is to the right of the lower is not constant, but varies 

 from 1 to 8 or more. If this apparent veering of the wind with altitude is, as some 

 writers maintain, a law of the winds, then Table VI. shows that Ben Nevis Observatory 

 is more pronouncedly a high-level observatory in winter than in summer, in cold weather 

 than in warm. Here it must be remembered we are dealing with average results, for we 

 find that in some of our exceptionally warm weather, anticyclonic weather, the con- 



