THE METEOROLOGY OF GLEN NEVIS 651 



relative humidity was 12 per cent, lower, ranging from 24 per cent, at 7 a.m. to 2 per 

 cent, at 4 p.m. Accompanying this warmth and dryness was an increased barometric 

 pressure in the Glen, the mean excess, after reducing the two sets of observations to 

 sea-level, being - 007 inch. The maximum excess was 0'017 inch at 3 a.m., while at 

 noon the barometer in the Glen was 0'005 inch lower than at Fort- William. The 

 greatest temperature difference between the two places was from 6 to 9 a.m. of 

 31st October, being over 10° at each hour, and reaching 1 1 °*0 at 7 a.m. On 

 this day the temperature at 1 p.m. was 21° '2 lower on the summit of Ben Nevis 

 than at Achariach, being one of the largest differences observed on any day during 

 the five months. 



The diurnal range of pressure, temperature, and humidity at the three stations for 

 these four days is shown in Table V. It will be seen that the early morning minimum 

 of pressure is well marked on Ben Nevis, slightly marked at Fort- William, while at 

 Achariach, although the minimum is there, yet pressure remains above the mean of 

 the day. The morning maximum occurs at all three places at 9 a.m., and is much 

 greater below than above. The afternoon minimum is about half that of the morning 

 minimum on Ben Nevis, but at Fort- William and Achariach it is very pronounced. 

 On the summit, pressure gets above the mean of the day at 7 p.m., but at Fort- William 

 not till 9 p.m., and at Achariach at 11 p.m., the maximum phase occurring at 10 p.m., 

 midnight, and 1 a.m. respectively. As compared with the normal, the distinguishing 

 features of the Fohn curve are (1) the slight depth of the early morning minimum; 

 (2) the height of the morning maximum ; (3) the depth and long duration of the after- 

 noon minimum ; and (4) the all but complete absence (except on the Ben) of the early 

 night maximum. This is essentially a fine-day curve.* 



The diurnal range of temperature on Ben Nevis is very small. There is a curious 

 nocturnal increase of temperature about 5 a.m., associated with a rise in wind velocity. 

 After this hour the temperature rises to a maximum at noon, after which a fall occurs, 

 which is accompanied by an increase of cloud and of humidity, both absolute and relative, 

 showing that the strong ascending currents are bringing air from below which becomes 

 condensed in cloud on the summit. Temperature on the summit shows a distinct rise 

 from 9 p.m. to midnight during the evening pressure maximum, and this coincides with 

 a fall in humidity. The diurnal range of temperature at Fort- William is simply an 

 exaggeration of the normal, the greater amplitude being due to the solar and terrestrial 

 radiation accompanying the cloudless weather. Temperature falls to a minimum at 

 7 a.m., being then 5 0, 1 below the mean of the day, rising to 6 0, 5 above the mean of the 

 day at 3 p.m. At Achariach the minimum is reached at 11 p.m., being only 1 0, 9 below 

 the daily mean, while the maximum at 1 p.m. is 3° "4 above the mean, an amplitude of 

 only 5°"3, against 1 1°6 at Fort- William. The fall of temperature at Achariach at 

 7 a.m. and 8 a.m. is explained by a break-down of the Fohn at these hours on 

 2nd November. 



* See Trans. Roy. Soc. Edin., vol. xlii. p. 451. 



