THE METEOROLOGY OF GLEN NEVIS 649 



is lowest from 1 to 3 p.m. at both places. At all hours the air in the Glen has a 

 lower relative humidity than at Fort- William, amounting on the mean of the day to 

 3 per cent. The hour of greatest difference is noon, and the hour of least difference 

 9 a.m., with values of 4'4 per cent, and 1"8 per cent, respectively lower than at Fort- 

 William. This, as we shall see later, is wholly a Fohn effect, and but for the com- 

 parative frequency of unusually cold periods, in which the air in the Glen was much 

 damper than at Fort- William, would have been even more pronounced. 



Fohn. 

 The name Fohn is applied to a class of weather the characteristic feature of which 

 is the high temperature of the air. It has been most frequently observed in the 

 valleys stretching in a northerly direction from the main summit-line of the chain of 

 the Alps, and takes the form of an abnormally warm wind blowing from the mountains 

 towards the plain. Continental meteorologists, especially Dr Hann, have made it 

 the subject of exhaustive investigation. Hann has shown that its high temperature 

 is due to its compression in descending from a great elevation, the air which is drawn 

 away from the base of the mountain towards the centre of a cyclone being replaced by 

 air from higher altitudes. The air is thus compressed and heated approximately at 

 the rate of 1° F. for every 200 feet of descent. Analogous phenomena are the Chinook 

 of Canada, the Nor'- westers of New Zealand, the east winds of Port-Nolloth, and the 

 Berg winds which blow along the south coast of Cape Colony at the base of the 

 mountain ranges between Swellendam and Storms River. Drygalski has shown that 

 Fohn winds are an important climatic factor in Greenland, and the phenomenon has 

 been recorded at all Antarctic stations where high mountains are in close proximity. 

 On the sub-antarctic island of South Georgia Fohn is often recorded. Until 1893, when 

 Mr J. Y. Buchanan, F.R.S., observed the true Fohn at Fort-William, # its occurrence 

 in this country had not been noticed. Hitherto it had been supposed that a great 

 absolute height of mountain chain was required for its production, but Mr Buchanan 

 showed that a relative height of 3000 to 4000 feet is quite sufficient. His observations 

 were made on 8th July 1893, the temperature being taken with a sling thermometer. 

 The Fohn on this occasion came in the form of hot blasts or puffs, " the sensation 

 produced being the same as when on the deck of a steamer the air passing the funnel 

 strikes the face." These hot blasts occurred at intervals of one or two minutes, and 

 lasted only for a second or two. They were observed during the heat of the day, and 

 Mr Buchanan was unable to measure their temperature owing to the thermal inertia 

 of the thermometer. The phenomena about to be described were first brought under 

 my notice on 31st October 1901. Leaving Achariach at 11 a.m. for Fort- William, 

 7^ miles away by road, a strong east wind was experienced for about three miles below 

 Achariach, but there it diminished greatly in force and soon after fell calm. The 

 air in the Glen was warm and dry, the temperature at Achariach at 11 a.m. being 49''8, 



* Proc. Roy. Soc. (1894), vol. lvi. p. 108. 



