THE METEOROLOGY OF GLEN NEVIS 



659 



the early morning, as we have seen, did the sea-level pressure exceed that at Fort- 

 William, and this excess would have been slightly reduced if we had corrected the 

 Achariach barometer to sea-level by taking the mean temperature of Fort- William and 

 Achariach instead of using the actual temperature in the Glen. The percentage frequency 

 of the wind on Ben Nevis during the thirteen days under review was as follows, from 

 hourly observations :— 



N. 



N.E. 



E. 

 6 



S.E. 



S. 



s.w. 



w. 



N.W. 



Calm. 



43 



23 



15 



4 



3 



1 



1 



4 



Hence for two-thirds of the time the wind was from the north or north-east. 



Several ascents were made to determine the height of the lake of cold air which 

 filled the Glen during spells of intense frost. 



On 22nd December a partial ascent of Sgor-a-Mhaim was effected (see Table VHi.). 

 Leaving Achariach at noon with a temperature of 18°"8, the temperature rapidly rose 

 with height. At 12.30, 100 feet above Achariach, the thermometer stood at 21°*1 ; 

 at 1 p.m., at 735 feet above, it was 26 0, 9 ; and at 1.10 p.m., at a height of 913 feet above 

 Achariach, or 1063 feet above the sea, 28° *8. At this hour the temperature at Achariach 

 was 18 0, 7, and on Ben Nevis 26°'l, the reading on the summit being 2°7 lower than at 

 a height of 1063 feet, showing that the temperature fell above a certain height, which, 

 however, was not determined. Descending the mountain, the temperature 791 feet 

 above Achariach was 27°7 ; at 631 feet, 27°'5 ; at 426 feet, 25°-0 ; at 249 feet, 22°-9 ; at 

 107 feet, 21°'9 ; and at the base station, 20 o- 0. 



On 14th January an ascent of Sgor-a-Mhaim was made, but the temperature 

 inversions were not so well marked as on the previous occasion. At 12.45 p.m. the 

 temperature at Achariach was 23°'7 ; on Sgor-a-Mhaim (3601 feet), 22° - 5 ; and on the 

 summit of Ben Nevis (4406 feet), 19°'2. Temperature rose with height until a point 

 was reached 1346 feet above sea-level; during the next 850 feet temperature remained 

 steady, and then fell slightly, the rapidity of the fall apparently increasing above the 

 summit of Sgor-a-Mhaim, to judge by the Ben Nevis observations (see Table XII.). 



On 30th January an anti-cyclone of great intensity spread over Scotland, and 

 marked inversions of temperature were again observed. On this day an ascent of Ben 

 Nevis was attempted (see Table VIII.), but fresh snow was lying to a great depth, so 

 that the highest point reached was about 1260 feet. Here at noon the temperature was 

 found to be very variable, the sling thermometer giving readings varying from 25° *8 

 to 31°'5, the atmosphere seeming to be made up of sheets of air of very different tem- 

 peratures. At this hour the temperature at Achariach was 10° "7, showing an inversion 

 of 20°"8 in 1113 feet. On Ben Nevis the temperature was 22°'3, and at Fort- 

 William 18° '4. 



On 1 1th February an ascent of Ben Nevis was made (see Table VIII.), and inversions 



