652 



MR R. C. MOSSMAN ON 



The outstanding feature of the curve of relative humidity on the Ben is the large 

 excess during- the afternoon hours. At Fort- William there is a slight fall in relative 

 humidity at 4 a.m., and a rise to a maximum at 7 and 8 a.m., which are the coldest 

 hours. The humidity then rapidly diminishes during the heat of the day, the greatest 

 dryness occurring at 3 p.m., the warmest hour. At Achariach the diurnal range is very 

 small, showing a maximum at 5 a.m. and a minimum at 3 p.m., the amplitude being 

 only 8 per cent., against 19 per cent, at Fort- William. 



During the Fohn the wind on Ben Nevis blew almost wholly from the south-east, 

 the following being the percentage values from hourly observations : — 



N. 



N.E. 



E. 



S.E. 



S. 



s.w. 



w. 



N.Wi 



Calm. 











5 



81 



11 



3 















Fohn in Rainy Weather. 



Another curious instance of a Fohn effect takes place at Achariach in a type of 

 rainy weather in which the rainfall diminishes from Fort- William to the head of 

 the Glen, instead of increasing, which, as we have seen, is the normal condition (see 

 fig. 2, p. 686). When a strong south-west or south-south-west wind is blowing, the 

 rain-bearing clouds are deprived of much of their moisture by the hills lying to 

 windward of Glen Nevis. At Achariach, under these conditions, the rainfall is con- 

 siderably less than at Fort- William ; the temperature and pressure are higher, and 

 the humidity lower, as is the case in the true Fohn. Owing to the rainfall at the 

 head of the Glen being measured only once a week, it is not possible to institute a 

 direct comparison between the daily falls there and at the other stations, but the 

 following totals for the three days 26th to 28th October show very clearly the 

 rainfall distribution in the Glen and on Ben Nevis : — 



Rainfall, 26th to 28th October 1901. 



Ben Nevis. 



Fort- William. 



Achariach. 



Head of Glen. 



4 - 82 inches. 



3*16 inches. 



2 - 2 2 inches. 



153 inches. 



These values, being deduced from the amounts measured at 9 a.m. and set down 

 to the previous day, are strictly comparable. The small rainfall at the head of the 

 Glen — less than half that recorded at Fort- William — is doubtless due to the rain- 

 bearing clouds at this place having to pass over Sgor-a-Mhaim (3601 feet). The 

 gauge at the head of the Glen is right under Ben Nevis summit, where, it will be seen, 



