658 MR R. C. MOBSMAN ON 



Up-bank Thaw. 



Under normal conditions the temperature of the air falls as we ascend, but in anti- 

 cyclonic conditions in winter inversions of temperature are common. To this pheno- 

 menon the name of up-bank thaw is given, because the temperature high up on the hill- 

 sides is often above the freezing-point, while hard frost prevails in the valleys, which are 

 filled with cold, dense, damp air produced by the effects of radiation on the sides of the 

 hills, whose summits are in the warm, dry current descending out of the centre of the 

 anti-cyclone. Up-bank thaw is, however, observed in cyclonic weather, or rather it may 

 occur with a low pressure associated with slight gradients and light winds ; but under 

 these conditions the temperature falls rapidly with height above the neutral zone. 



Before giving the results of several excursions made to determine the vertical distri- 

 bution of temperature under conditions of up-bank thaw, it is of interest to summarise 

 the specific features of the diurnal range of the climatic elements at the three places. 

 Table VII. gives the mean values for thirteen days, the hourly departures from the 

 mean of the day, and the differences between Fort- William and Achariach. The diurnal 

 range of pressure is much the same at all the stations, but the morning minimum occurs 

 earlier at Achariach and Fort- William than on Ben Nevis ; the morning maximum is re- 

 tarded, taking place on Ben Nevis and at Fort- William at noon, and at Achariach an hour 

 earlier. The afternoon minimum occurs on Ben Nevis at 3 p.m., and at Fort-William 

 and Achariach at 4 p.m., being much more prominent in the G-len than at the other two 

 places. The diurnal range of temperature is of a simple character ; the minimum occurs 

 on Ben Nevis at 6 a.m., but at Fort- William and Achariach at 8 a.m. The maximum 

 occurs at 2 p.m. at Ben Nevis and Achariach, but at Fort- William an hour later. The 

 humidity curves offer considerable differences. On Ben Nevis there is a primary 

 maximum at 2 a.m. and a slight secondary maximum at 2 p.m., while the most pro- 

 nounced minimum occurs at 11 a.m., and a secondary minimum at 8 p.m. At Fort- 

 William and Achariach the curves are much the same, the hours of lowest humidity 

 being from 1 to 4 p.m., while in the early morning the humidity is high. 



The table of the actual difference between Achariach and Fort- William shows that 

 during the thirteen days under review the mean pressure in the Glen was 0'004 inch 

 lower than at Fort- William, the temperature 3 0, 5 lower, and the relative humidity 

 13 per cent, greater. As regards pressure, the values in the Glen were above Fort- 

 William from midnight to 7 a.m., the mean excess for these hours being 0'007 inch, 

 reaching a maximum of 0*009 inch from 2 to 4 a.m. During the hours from 8 a.m. to 

 11 p.m. pressure was lower in the Glen, the maximum being 0*020 inch at 3 p.m., which 

 was also the warmest hour at Fort- William. Relative humidity was greater in the 

 Glen than at Fort- William at every hour of the day, the excess ranging from 17 per 

 cent, at 1 a.m. to 7 per cent, at 5 p.m. 



These results are of interest as they show that pressure is not raised in deep valleys 

 during cold spells to the extent that was hitherto supposed. Only for a few hours in 



