METEOROLOGY OF BEN NEVIS. xlv 



characteristics, not only on Ben Nevis, but also over the west of Europe generally. 

 Indeed, so well marked is the weather, and so notorious for its stormy character, 

 that the observers regard it as a distinct type of weather, and call it by the name 

 of St Elmo's weather. 



From 30 hours before St Elmo's fire is seen, the barometer falls steadily till 6 

 hours before, and thereafter rises till 16 hours after the appearance, with a slight 

 break in the rise at the time of occurrence of the phenomenon. The cases occur 

 during the well-defined depression of a cyclone, generally about 6 hours after the 

 centre of lowest barometer has passed. 



As regards temperature, a maximum is reached about 24 hours before St Elmo's 

 fire is seen; for between 24 and 16 hours before the display, temperature is on 

 the whole steady, but from 16 hours before, temperature falls continuously till 16 

 hours after, when a minimum temperature is reached. The rate of fall is more 

 rapid before than after St Elmo's fire is seen. It was by noting the variations 

 in the temperature before the display of this phenomenon that on several 

 occasions successful forecasts of the phenomenon were made some hours before its 

 appearance. 



The chief points to be noted with respect to the wind are that till from about 

 8 or 10 hours before the appearance of St Elmo's fire the wind is from a direc- 

 tion south of west, and thereafter from a direction north of west ; that from 

 18 hours till 6 hours before its occurrence, the wind steadily veers, from which 

 time till 10 hours after its appearance the veering nearly ceases ; and that the 

 mean winds are all in the western half of the compass. 



The rainfall observations show two distinct maximum periods, viz., between 10 

 and 6 hours before St Elmo's fire is seen, and again at the time of its occurrence ; 

 while there are also two equally defined minima, one between 30 and 26 hours 

 before, and the other 15 to 18 hours after its appearance. The maximum at the 

 time of its occurrence is wholly due to the heavy showers of snow and snow-hail 

 that accompany the displays. This snow-hail is in the form of small cones, with 

 spherical bases, and is hard and dry, but not icy. 



The Weather Charts at the times show that the depression of which St Elmo's 

 fire is an accompaniment, comes from the Atlantic, and that the centre passes 

 eastwards somewhere to the north of Ben Nevis, this being substantially the part 

 of cyclones where electrical displays are of most frequent occurrence. 



A more particular examination of the Weather Charts of the Meteorological 

 Office indicate that somewhere to the south or south-east of the British Islands, 

 usually over the south of France and the Spanish Peninsula, there was an anti- 

 cyclone at the time ; and to the west or north of Scotland the low depression of a 

 cyclone. Between the two regions the barometric gradient was chiefly for S.W. 

 or W. winds, and was generally a pretty steep one. So long as the anticyclone 

 maintained its position to the south-east, so long was the weather of the British 



