660 MR R. C. MOSSMAN ON 



were well marked up to a height of 1209 feet, but from this point to the summit the 

 temperature fell rapidly. 



On 1st February an excursion was made to the farmhouse of Steall, about a mile and 

 a half to the east of the head of Glen Nevis (see Table VIII. ). Achariach was left at 

 10.47 a.m., temperature 18 0- 9 ; at the end of the carriage drive, at 11.37, it was 21 o, 0, 

 and at the cairn on the Meall Cumhann, 930 feet above the sea, the thermometer at 

 12 h 15 m read 26°'9. Descending some 200 feet to the strath, the reading was 22 o, at 

 12 h 27 U1 . Immediately after this reading had been logged a light puff of air came from 

 the south-east, which to the senses was appreciably very much warmer ; an observation 

 was at once made, and the temperature found to be 33°'7, or nearly 12° higher than that 

 recorded four minutes previously. Between this point and Steall, a distance of about a 

 mile and a quarter, the temperature was very variable, nine observations showing a 

 variation from 26° "0 to 31°"8. Eight observations were made on the return journey, the 

 highest temperature being 28*0 at the cairn on the Meall Cumhann. During the time 

 covered by this excursion the maximum temperature at Achariach was 20° '9. 



All these observations go to show that even in cold, calm radiation weather, of which 

 up-bank thaw is the most striking characteristic, Fohn effects are constantly taking place, 

 especially at the upper limit of the lake of cold air. Any wind that blows under these 

 conditions is a warm wind, even though it is the lightest breeze which the senses can 

 appreciate. The most stable conditions in such weather are found at the bottom of this 

 cold-air zone ; thus close to the river the temperature was found to be 2° or 3° lower 

 than at places situated only 40 feet above the river-bed. 



Typical distributions of pressure over Western Europe during anti-cyclonic and 

 cyclonic up-bank thaws are shown in figs. 4 and 5, p. 686. 



Observations on Excursions [Table VIII.). 



In the following tables are given a selection of the observations taken on special 

 excursions to determine the vertical extent of Fohn, up-bank thaw, and other 

 phenomena. The equipment consisted of two aneroids and a sling thermometer. The 

 aneroids were compared with the standard barometer at Achariach before and after the 

 ascents, and when the correction to be applied changed from the time of departure 

 to that of return, a progressive alteration in the correction was assumed. On some 

 ascents it was possible to check the indications of the aneroid against known heights, 

 and it was seldom that the height was out to a greater extent than 30 feet. The 

 temperature at the hour of observation at Fort- William, Achariach, and Ben Nevis is 

 given. In some cases interpolated values between the hours are given, these being put 

 in brackets. The temperature at Achariach at any time could be obtained from the 

 thermograph. Wet-bulb readings were made on one or two ascents by wrapping 

 moistened Swedish filtering paper round the bulb of the thermometer and whirling 

 rapidly. 



