650 



MR R. C. MOSSMAN ON 



and the relative humidity 62. Near the entrance to the Glen a great fall in 

 temperature was felt ; the Curling Pond was found covered with a thin coating of 

 ice, and thick hoar-frost lay un melted on the ground. At Fort- William in the early 

 morning the temperature on the ground had fallen to 24° '6, while at Achariach the 

 lowest observed was 36 0, 6, or 12° higher. The air at Fort-William was cold, calm, and 

 humid, while in the upper part of the Glen there was a strong, warm, dry wind blowing — 

 in other words, a Fohn. This remarkable condition continued for the four days 30th 

 October to 2nd November, during which time pressure was high over Norway, Denmark, 

 and North Germany, and low to the south-west and south (see fig. 3, p. 686). On 

 1st November I made a series of observations from Achariach to Fort- William (see 

 Table VIII. ), and found the phenomenon just as marked as on the day previous, and 

 showing the same horizontal extent. On 2nd November observations were made to 

 determine the vertical height of the Fohn by means of an ascent of Ben Nevis (see 

 Table VIII.) from the head of the Glen, and on this occasion I found the Fohn well 

 marked up to a height of 1800 feet, the humidity on the mountain being somewhat 

 lower than at Achariach up to that elevation ; but higher up the hillsides it increased 

 rapidly, with a marked fall of temperature, showing that the upper limit of the Fohn 

 had been reached. At 3 p.m., at a height of 3817 feet, the temperature was found to 

 be very variable, ranging from 37° '3 to 34°'9 as puffs of air came from the south-east. 

 It is not unlikely that this was a modified Fohn effect. Wet-bulb readings were 

 obtained by means of wrapping moistened Swedish filtering paper round the bulb of 

 the thermometer and whirling rapidly for a few minutes. Had the observations been 

 made in the early morning, when, as we shall see, the Fohn influence is at a maximum, 

 instead of during the afternoon, when its influence is counteracted by the strong 

 ascending currents due to cumulative solar radiation, the results would have been 

 much more striking. Thus, two observations made on the southern slopes of Ben 

 Nevis on the afternoon of 30th October show that the Fohn was very feebly marked 

 at that time. The following were the data obtained : — 





Mountain. 



Achariach (150 ft.). 



Height. 



Temperature. 



Humidity. 



Temperature. 



Humidity. 



3 P.M. . 



4 P.M. . 



Feet. 

 1008 



1456 



43°0 

 39-6 



% 



68 



72 



45°6 

 45-0 



7 



/o 



69 



66 



At Fort- William at 3 p.m. the temperature was 0°'7 higher than at Achariach, and 

 at 4 p.m. o, 9 higher. 



During the four days on which the Fohn blew in the Glen the mean temperature 

 from hourly observations (see Table V.) was 4° "2 higher at Achariach than at 

 Fort- William, the difference ranging from 8°"8 at 7 a.m. to -0 o, 3 at 4 p.m. The 



