METEOROLOGY OF BEN NEVIS. xlvii 



N.W., with high pressure over the Iberian Peninsula; in other words, the identical 

 weather conditions under which Mr Rankin has shown the instances of St Elmo's 

 fire have occurred. The winter thunderstorms invariably occur on the S.E. side 

 of the cyclone's centre, with the easterly passage of which they appear to be 

 intimately connected. On several occasions, when the lowest barometric reading 

 of the cyclone occurred many hours before or after the thunderstorm, a slight fall 

 in pressure took place at the time of the thunderstorm. 



Temperature is little affected either by the winter or summer thunderstorms. 

 Such rapid falls of 5° or 10° in an hour, as frequently accompany or immediately 

 follow the thunderstorms of lower levels, have not been observed to occur at the 

 height of the Observatory. Strong squalls, from 60 to 80 miles an hour, frequently 

 accompany the thunderstorm in a manner more uniform and marked than lower 

 down in the plains below. 



The thunderstorms at the High-Level Observatories on the Continent — at Santis, 

 8215 feet; the Rigi, 5905 feet; and the Great St Bernard, 8131 feet — have also 

 been examined during the same years. These all show the thunderstorms to be 

 restricted to the period of the year between April and October, thus proving that 

 these Observatories are quite outside the sphere of winter thunderstorms. Further, 

 90 per cent, of the recorded cases occur in the day time between the hours of sun- 

 rise and sunset, being thus in marked contrast to Ben Nevis, not only as regards 

 the season of the year, but also as regards the hours of the day. Indeed, thunder- 

 storms in winter are not met with at any of these continental High Level 

 Observatories.* 



Daily Mean Temperatures. — These have been calculated by Mr Omond, both 

 for the Observatory and Fort- William, for the six years ending 1889. The coldest 

 day (19°*1) is March 3, and it is evident from the general trend of the temperature 

 curve, that the true annual minimum occurs in the beginning of March, being 

 from four to six weeks later than in the low country, but a little before the time 

 of the annual minimum of the water of the Atlantic. 



The time of maximum temperature is not so clearly marked, the day of highest 

 mean temperature (45°'5) is July 2, but there is a secondary maximum (43°"2) on 

 August 8, thus agreeing with most land stations in having a double summer maximum. 



It is thus plain that while the time of the winter minimum is delayed at this 

 high level, the time of occurrence of the summer maxima is nearly a fortnight 

 earlier than at lower levels, — features in the annual distribution of temperature 

 which Ben Nevis has in common with other High-Level Observatories. 



The most striking feature in the annual curve of temperature is the sudden rise 

 in the middle of June. Thus on the 15th the mean 34°'8, on the 18th, 44 0, 9, or an 

 increase of 10°'l in three days. This rise is well marked in each of the six years. 



* " Thunderstorms at Ben Nevis Observatory," by E. C. Mossman, Jour. Scot. Met. Soc, Third Series, 

 vol. iii. 



