THE METEOROLOGY OF GLEN NEVIS 647 



Thus, on the mean of the five months, the Nevis was 0°'4 colder than the burn, 

 as was to be expected, owing to its receiving colder water from its more extensive and 

 higher drainage area. During the whole period the river was warmer than the burn 

 on 31 days, the greatest excess being 1°'2 on 15th January, during a condition of up- 

 bank thaw. The greatest negative difference was noted on 3rd January, during a high 

 flood, when the Nevis was 2°'8 colder than the burn. 



The Diurnal Period at Ben Nevis, Fort William, and Achariach. 



Table IV. shows the departure from the mean of the day for the five months of 

 observations of the barometric pressure, temperature, and humidity for the three 

 stations, adjusted for " midnight differences." As regards relative humidity, the 

 values for Ben Nevis are not given, as the curve is quite featureless, except for a 

 slight fall in humidity at 7 and 8 a.m., and an equally small departure in the opposite 

 direction from 11 p.m. to 2 a.m. 



A small table appended to Table IV. shows the actual difference between Achariach 

 and Fort- William, and at the foot is given the total sunshine recorded at Ben Nevis 

 and at Fort- William. 



Barometric Pressure. — The early morning minimum is well marked on the Ben 

 and at Fort- William, but at Achariach only a slight fall in pressure occurs. The 

 morning maximum takes place on Ben Nevis and Fort- William at 11 a.m. ; but in the 

 Glen, where it is less than half that observed at the other two stations, it occurs an 

 hour earlier. The afternoon minimum occurs at the three stations at 3 p.m., pressure 

 falling below the daily mean only O'OOl inch on Ben Nevis, while at Fort- William it 

 is 0'006 inch, and at Achariach as much as O'Oll inch. The evening maximum, which 

 is of much the same height at all the stations, occurs on Ben Nevis at 7 p.m., at Fort- 

 William at 8 p.m., and at Achariach not till 1 a.m., by which time pressure on the 

 Ben has fallen O'Oll inch below the maximum, and at Fort- William 0*009 inch below. 

 An important point in explaining the striking difference between the phases of the 

 curve at Fort- William and Achariach, is the direct effect of the higher temperature 

 and lower humidity which prevail in the Glen. If the ordinary laws governing the 

 fall of temperature for height held good, the temperature at Achariach should have 

 been o, 4 lower than at Fort- William at all hours — a difference which only obtained 

 from 3 p.m. until 6 p.m., when Fort- William was in sunshine, while in the Glen the 

 solar effect during the greater part of the five months was practically nil. At all other 

 hours of the day the temperature difference was less than the normal, and at many of 

 the hours the Glen was slightly warmer than Fort-William. In other words, the higher 

 barometric pressure, as compared with Fort- William, that prevailed in the Glen from 

 9 p.m. to 9 a.m., which is well shown in each of the five months discussed, was not an 

 effect of the accumulation of cold, dense air flowing from adjacent heights and filling the 

 Glen with a lake of humid and chilled air, but was due to air warmed dynamically by 



