THE BEN NEVIS AND FORT-WILLIAM OBSERVATORIES. 703 



air transported from the summit to the base of Ben Nevis, without any gain or loss of 

 heat from external sources, would be raised 24° in temperature by the work done in 

 compression through the increase of atmospheric pressure. Similarly, air raised from the 

 base to the summit would be cooled 24° if it did not contain sufficient vapour to induce 

 condensation by that cooling. Thus a difference of 24° between the summit and base 

 is a limiting or critical value, for with any greater difference the atmosphere is in 

 unstable equilibrium, the air at the summit being then so much colder than that at 

 the base that if the former descended it would still be lower in temperature, and 

 therefore heavier than the low-level air. It is of interest to consider those cases in 

 which the difference exceeds 24°. In the thirteen years over which these hourly records 

 extend there were 205 instances in which the difference of temperature amounted to 

 25° or more, that is, less than 0'2 per cent, of the whole. The longest series of 

 hourly readings in which a difference of 25° or more was noted were, on 9th June 1897, 

 from 9 a.m. to 5 p.m., nine hours, and on 30th May 1899, from 1 to 6 p.m., six hours. 

 There were five cases between 8 a.m. and 2 p.m. on 22nd May 1899, though they were 

 not consecutive, but on no other day were more than four instances of a difference of 

 25° and upwards recorded. On each of the three days mentioned there was, during the 

 time of the large difference of temperature, south-easterly wind, moderate to strong, 

 large amounts of sunshine at Fort- William and little at the summit of Ben Nevis, and 

 no rainfall except a very slight shower at Ben Nevis on 30th May 1899. The air was 

 dry at Fort- William, the humidity ranging from 48 to 56 per cent., unusually low values 

 for that station, but saturated or nearly so at Ben Nevis. These conditions are all 

 favourable to a rapid decrease of temperature with height : the south-east wind crosses 

 many ranges of hills and descends on Fort-William with a Fohn-like effect, raising the 

 temperature, while the strong sunshine would also locally heat the air there. The 

 great relative dryness of the air at Fort- William and the absence of rain ensure that 

 there would be no condensation of aqueous vapour in air ascending from the warm 

 stratum over Fort- William, and consequently no liberation of latent heat, and that the 

 temperature of such ascending air would therefore fall 24° during its ascent to the 

 level of the summit of Ben Nevis. 



Other cases of differences of 25° and upwards were mostly on summer afternoons, 

 and under similar conditions of weather. In Table II. is given the number of hours 

 at which a difference of 25° and upwards was observed in each month of each year. 

 It will be seen that May and June are the months with most cases, and that the 

 annual totals range from 46 in 1891 to 1 in 1893. The records for 1890 and 1903 

 are for only parts of these years. 



The limitation introduced by the adiabatic heating of descending air prevents the 

 indefinite cooling of the higher air relatively to that at sea-level, but there is no 

 corresponding limit the other way, and under favourable conditions of weather the 

 temperature of a high-level station may rise considerably above that of neighbouring 

 low-level places. 



