xlvi METEOROLOGY OF BEN NEVIS. 



Islands disturbed by a series of Atlantic cyclones, that usually passed in a north- 

 easterly direction with the paths of their centres lying off our west and north-west 

 coasts. Thunder and lightning occurred at lower levels over the British Islands 

 on several of the nights that St Elmo's fire appeared on Ben Nevis. Only on one 

 occasion, on 4th February 1887, was thunder and lightning observed on Ben Nevis 

 about the times of occurrence of St Elmo's fire. On that occasion the thunder- 

 storm was two hours subsequent to St Elmo's fire. 



It deserves to be specially noted that well-nigh invariably St Elmo's Fire is 

 not a prognostic of any improvement in the weather, but in almost every case 

 another cyclone followed, ushering in another spell of bad weather.* 



Thunderstorms. — From the opening of the Observatory in December 1883 to 

 March 1890, 56 thunderstorms have been observed. These have been discussed 

 by Mr R. C. Mossman. They are phenomena of very irregular occurrence, 

 eight having occurred in 1887, while none whatever were observed during the two 

 previous years. On the other hand, five occurred during January 1890. Thunder- 

 storms, including lightning without thunder, are essentially autumn and winter 

 phenomena, 70 per cent, of the whole having occurred from September to February. 

 The winter maximum of thunderstorms is thus a well-marked feature in the meteor- 

 ology of Ben Nevis. Only two cases have been recorded in April, and none at all 

 in March, these being the months of minimum frequency in all parts of Scotland. 



Thunderstorms are rare in summer, only eight cases having occurred during the 

 four months from May to August. On the other hand, they are twice as frequent 

 during summer at Fort- William as at the Observatory, so that a considerable 

 number must be below the summit of the mountain. 



The hourly distribution of thunderstorms on Ben Nevis shows a distinct evening- 

 maximum, nineteen cases having occurred during the three hours ending 9 p.m., 

 while there were only two in the seven hours ending 2 p.m. A secondary maximum 

 occurs at 7 in the morning. 



The partition of the thunderstorms in relation to the hours of sunrise and 

 sunset gives very interesting results, it being found that all the summer cases, with 

 only six exceptions, which were cases of lightning only, occurred when the sun 

 was above the horizon ; while out of thirty-seven cases observed in the autumn and 

 winter, thirty-two took place when the sun was below the horizon. Only one case 

 of lightning without thunder was observed when the sun was above the horizon, 

 thus resembling the similar electrical phenomena observed over the open sea during 

 the " Challenger" Expedition. As regards sunrise and sunset, the thunderstorms at 

 Fort-William show a similar distribution to what obtains at the Observatory. 



Mr Mossman compared all these cases of thunderstorms with the Daily Weather 

 Charts at the times of their occurrence, and clearly showed that in winter they are 

 associated with a well-marked type of weather, having low pressure to the W. or 

 * " St Elmo's Fire on Ben Nevis," by Angus Rankin, Jour. Scot. Met. Soc, Third Series, vol. ii. p. 191. 



