WINDS OF BEN NEVIS. 503 



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 according to some other yet unknown law. 



3. The small diurnal variation in wind direction on Ben Nevis will enable us to 

 study the changes of wind connected with variations of pressure, etc., independent of 

 the time of day of their occurrence. 



An examination of individual months that are higher or lower than their average 

 in temperature shows that, in the warmest months, southerly winds are in excess on Ben 

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

 northerly below. In the months below their average temperature, 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 the 

 centre of the compass, of those at sea-level, — eight in the case of the excess, and four in 

 the defect. It must be remembered we are dealing with average results : for in some of 

 our exceptionally warm weather, the conditions on the summit are purely and entirely 

 characteristic of high-level Observatories during anti-cyclones. 



The temperature of the different winds has been discussed, # and 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 all these points are in a warm month in excess of what they are 

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

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

 excess. If we take the sea-level wind excess as an index to the distribution of pressure 

 at sea-level, we have in the Ben Nevis warm months 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 dis- 

 tribution 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 months, 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 diminished pressure. Low temperatures on Ben Nevis are thus connected 

 with winds that indicate a different distribution of pressure there from what holds at 

 sea-level. 



There is no marked diurnal variation in the direction of the wind on Ben Nevis, 

 though the mean wind- velocity shows a distinct daily period. $ The latter, as at all true 



* See " Thermal Windrose at the Ben Nevis Observatory," by A. Rankin, Proc. Roy. Soc. Edin., vol. xiv. p. 416. 



