THE METEOROLOGY OF GLEN NEVIS 645 



It will be seen that at 9 a.m. and 10 a.m. the hills were in cloud for half the time. 

 At 2 p.m. (I4 h ) a slight reduction in the frequency of mist has taken place, which is 

 most marked below 3000 feet, and the clearest hour is 9 p.m. (21 h ). It will also be 

 noted that at the summit of Ben Nevis during 67 per cent, of these observations 

 mist prevailed. On the Cairn Dearg and Sgor-a-Mhaim the mean percentage is 50 ; 

 the Mullach nan Coirean, a little over 3000 feet in height, shows a mean frequency of 

 43 per cent. ; while on the Meall Cumhann (2306 feet) in only 20 per cent, of the cases 

 was the hill in mist. Taking only the 9 a.m. and 10 a.m. observations for the whole 

 period of 149 days from 3rd October to 28th February, and weighting the values for 

 the Mullach nan Coirean for the period 3rd to 24th October, for which no data are 

 available, we get the following mean percentage values : — 



Ben Nevis (4406 feet) .... 



. 76 



Cairn Dearg (3348 feet) 



. 61 



Sgor-a-Mhaim (3601 feet) . 



. 57 



Mullach nan Coirean (3077 feet) . 



. 53 



Meall Cumhann (2306 feet) 



. 26 



The Cairn Dearg, it may be recalled, is on the north side of Glen Nevis, being a 

 prominent spur of the mountain, while Sgor-a-Mhaim and the Mullach nan Coirean are 

 on the south side of the valley, and the Meall Cumhann closes the valley to the east. 



No systematic series of observations dealing with the actual height of the base of 

 the lower clouds was made. In January and February observations on 21 days at 

 9 a.m. give a mean height of 2100 feet as the lower limit of the cloud layer, and 16 

 days' observations at 10 a.m. give a mean of 2200 feet. Forty-eight observations 

 taken up to and including noon give a mean height of 2200 feet, and 32 observations 

 between 2 and 9 p.m. show a mean of 2300 feet ; but the results given in the tables 

 above indicate that in general the base of the clouds probably lies about 3000 feet. 

 The lower values shown above may, however, be taken as a close approximation 

 of the height of the base of the clouds on rainy days, to which practically all the 

 observations refer. 



Terrestrial Radiation. 

 The absolute maxima and minima recorded by a black bulb in vacuo and a freely 

 exposed terrestrial minimum thermometer are indicated by heavy type in the daily 

 values in the general tables (p. 668 et seq.). On 13 days, between the dates of 26th 

 January and 13th February, the actual temperature on the ground was read at frequent 

 intervals, as occasion offered, during the day. A comparison of these values with the 

 corresponding shade temperatures shows that in calm, clear, cold weather radiation in 

 such a place as Glen Nevis goes on actively during the whole day. The following table 

 gives particulars for the three days, 30th January to 1st February, when there was an 

 anti-cyclone of great intensity, and the last column shows the mean depression of the 

 ground temperature below the shade for the whole period of 13 days. From 17 to 

 21 h the observations are practically complete, but those at 9 h , 10 u , and 14 h are 



TRANS. ROY. SOC. EDIN. — VOL. XLIV. 4 N 



