THE METEOKOLOGY OF GLEN NEVIS 637 



was 20 per cent, above the average, and as in November northerly winds were in 

 excess and southerly in defect. The most pronounced difference at Fort-William, 

 as compared with the summit station, was in the rainfall, which shows a slight 

 deficiency. 



In January 1902, on Ben Nevis, pressure, temperature, sunshine, and rainfall were 

 all slightly above their respective normals ; wind force was markedly under the average, 

 and winds from north and west were in excess, and those from south and east in defect. 

 At Fort- William the observed values were in accordance with those on the summit, 

 except temperature, which' was practically the average. The relatively higher 

 temperature on the summit was thus due to the inversions accompanying the marked 

 anti-cyclonic conditions which were so conspicuous a feature of the meteorology of this 

 month. 



In February 1902, pressure, temperature, and rainfall were below the average both 

 on the summit of Ben Nevis and at Fort- William, the small rainfall — only one-fifth of 

 the average — being a noticeable feature at both places. Sunshine was markedly in 

 excess at both stations. Wind force was just the average at the summit station, where 

 there was an unusual prevalence of winds from the north-east and south-east, while 

 winds from north-west, west, and south-west were in defect. 



For the whole period of five months the most marked characteristic was the low 

 temperature at both places, and the reduced wind force on Ben Nevis summit. Taken 

 as a whole, the period under revieAV was a dry one, the deficiency of rainfall as well as 

 the excess of sunshine being more prominently exhibited at Fort- William than at the 

 Ben Nevis Observatory. Barometric pressure at both stations was in close accordance 

 with the normal. As regards wind direction on the summit, the data present 

 few features of interest, if we except the prominent excess of northerly winds in 

 November, December, and January, and the unusual frequency of south-east winds in 

 February. 



A comparison of the records at Fort- William and at Achariach for the five months 

 shows clearly : (l) lower minimum temperatures and consequent greater monthly range 

 of temperature in the Glen, associated with a slightly smaller mean daily range ; (2) an 

 increase in the solar and terrestrial radiation, associated with a lower relative humidity, 

 and this despite a higher rainfall, number of rainy days, and amount of cloud ; (3) a 

 mean wind force more than double that of Fort-William, due to the frequency and 

 strength of local squalls in the Glen ; (4) a marked difference in the observed wind 

 directions, there being a tendency for south-west and west winds to be deflected into 

 west and north-west. Calms are also more frequent, and this, in conjunction with the 

 higher mean wind force recorded, emphasises the statement as to the strength of the 

 gusts at Achariach. There is in the Glen what may be termed a natural tendency 

 to a condition of Fohn, and this, as we shall see later when the special features are 

 under discussion, is as marked a characteristic in overcast, rainy weather as in clear 

 weather. Another phenomenon frequently observed was up-bank thaw, in which a 



TRANS. ROY. SOC. EDIN. VOL. XLIV. 4 M 



