704 MR R. T. OMOND ON THE LARGE DIFFERENCES OF TEMPERATURE BETWEEN 



Although most of these large differences of temperature occurred on summer after- 

 noons, yet the largest difference of all, 28° "8, was recorded on a winter night, namely, at 



2 a.m. on 19th December 1890. The records at the two Observatories on the night of the 

 18th-19th December show that the sea-ievel barometer fell till 2 a.m. and thereafter 

 rose, that the temperature at Ben Nevis reached its lowest point at 2 a.m. and thereafter 

 rose more rapidly than it had fallen, and that the temperature at Fort- William was 

 falling till 4 p.m., remained fairly steady till 7 p.m., rose till 2 a.m., fell quickly till 

 4 a.m., and continued to fall slowly and rather irregularly thereafter till noon. The 

 difference of temperature exceeded 25° at the four hourly readings from midnight to 



3 a.m. inclusive. There was fog on the summit till 10 p.m., the hill top was clear and 

 the sky partly clouded till 4 a.m., but fog again thereafter and drizzling rain came on 

 about noon. No rain or snow fell at either station during the night. The air on the 

 summit was saturated except at 4 a.m., when there was a difference of o, 2 between 

 the dry and wet bulbs ; but at Fort- William the air was dry throughout the humidities 

 at the four hours of greatest difference, being 54, 64, 47, and 58. During this time a 

 strong S.E. wind, rising to the force of a gale at times, was blowing on the summit. 

 The difference of temperature between the two stations was 20° or more continuously from 

 7 p.m. on the 18th to 8 a.m. on the 19th, and the humidity at Fort-William during this 

 time ranged from 74 to 47 per cent. This mass of relatively dry air moving rapidly 

 over the hilly regions to the S.E. of Ben Nevis would be thoroughly mixed vertically, 

 and would thus tend towards the maximum adiabatic gradient of 24° for 4400 feet. 

 But there must have been a slightly warmer stream included in the general current of 

 the air, which stream flowed over Fort-William for four or five hours without affecting 

 the summit of Ben Nevis, and thus causing the larger difference of temperature from 

 midnight to 3 a.m. Where this special stream of air obtained its heat we cannot say 

 definitely, but it is very likely that its higher temperature was due to the latent heat 

 liberated by a fall of rain in some locality to the south-east, that is to windward, of 

 Fort-William. 



