648 MR R. C. MOSSMAN ON 



compression — in other words, to Fohn, which is a normal condition in the Glen, and 

 which, as we shall see, is more pronounced during the night than during the day, being 

 least apparent at those hours when the cumulative effect of solar radiation is at its 

 maximum, that is, in the afternoon. It is obvious that in seeking for an explanation 

 of the high pressure which has been found to obtain during the night in closed valleys 

 we must keep in view the possibility of the region being one in which Fohn, and not 

 radiation effects, is the dominating factor in this connection. 



Temperature. — On comparing the actual difference of temperature between Fort- 

 William and Achariach, it would appear that the higher temperature in the Glen was 

 largely occasioned by the exceptional nature of the months of October and November, 

 in which the excess from hourly observations was 1°*1 and 0°'3 respectively, whereas 

 in December and January the Glen was 0°'4 colder than Fort- William, and in February 

 as much as l o, colder. But, as we shall see later, those months in which the Fohn 

 effect was apparently absent were each characterised by a number of exceptional 

 days, in which the Glen was very much colder than Fort- William, which affect the 

 means and eliminate the Fohn influence, which in a normal or mild winter would have 

 been much more conspicuous. 



The diurnal range of temperature is of a simple character. On the summit of 

 the Ben the coldest hours are from midnight to 2 a.m., when the values are only 0°*3 

 below the mean of the day. From 3 to 9 a.m. the values are practically the same, 

 and must be looked upon as relatively high. This nocturnal rise of temperature is a 

 characteristic feature of the meteorology of Ben Nevis in winter, and it is also well 

 marked in Polar regions, especially at stations in the highest latitudes in both Arctic 

 and Antarctic. Mohn has ascribed it to changes in the direction of the wind. 



On Ben Nevis the maximum temperature of the day is reached at 1 p.m., at Fort- 

 William about 2.30 p.m., and at Achariach at 2 p.m. In the Glen temperature falls 0°'9 

 from 2 to 4 p.m., but at Fort- William only o, 5, and it is this prolongation of the solar 

 effect at Fort- William during the afternoon hours, combined with the weakening of 

 Fohn conditions in the Glen through the increased strength of ascending currents, 

 that causes the difference in temperature between the two places to be greater than 

 at any other period of the day, 



The contraction of the atmosphere, which begins at 10 p.m. on Ben Nevis, as shown 

 by the fall of the barometer on the hill-top, is accompanied by a rise of pressure and 

 increased warmth in the Glen as compared with Fort- William, the conditions then 

 being eminently favourable for Fohn, owing to the system of descending currents 

 brought about by the shrinkage of the atmosphere between the valley floor and the 

 hill-tops adjacent. 



The amplitude of the variations or difference between the warmest and coldest hour 

 is '9 on Ben Nevis, 3°1 at Fort- William, and 3 o, at Achariach. 



Humidity. — At Fort- William the air is most humid from 5 to 8 a.m., and at 

 Achariach this also obtains, except that it is slightly prolonged. Relative humidity 



