440 



NA TURE 



[Sept. 5, 1889 



Fire were noted at the Observatory, and these occurred 

 all in the night-time, and all during the winter months, 

 beginning with September and ending with February. 

 The feebleness of the light it gives forth, in comparison 

 with ordinary daylight, makes it difficult if not impossible 

 to see it during the day, which is probably the reason for 

 its being a nocturnal phenomenon, and partly for its being 

 a winter phenomenon, for the short summer nights greatly 

 lessen the chances of seeing it. Other reasons for its 

 being a winter phenomenon, and not a summer one as 

 well, are that the high temperature in summer is not 

 favourable to its appearance, and that the weather type in 

 which it appears is far more common in winter than in 

 summer. The fifteen cases are distributed throughout 

 the winter months as follows : two in September, three 

 in October, five in November, two in December, one in 

 January, and two in February. An investigation of these 

 cases, and of the meteorological observations made before 

 and after each appearance of St. Elmo's Fire, has shown 

 that the weather which precedes, accompanies, and follows 

 it has very definite characteristics, not only on Ben Nevis 

 but also over the whole west of Europe. What these 

 characteristics are will be briefly explained in what 

 follows. 



To arrive at a knowledge of the conditions obtaining 

 on Ben Nevis, the observations of pressure, temperature, 

 wind direction, and rainfall, made at the Observatory, were 

 tabulated for each hour from thirty hours before to twenty- 

 four hours after the appearance of the phenomenon in 

 each case. The general averages thus obtained for the 

 whole fifteen cases for each element give very decided 

 curves. They show that, as regards pressure, the baro- 

 meter, from being 24"993 inches at thirty hours before St. 

 Elmo's Fire is seen, steadily falls till the sixth hour before, 

 when it is 24771 inches, and thereafter rises till the 

 twenty-fourth hour after the display, when it is 24*979 

 inches. At the hour at which St. Elmo's Fire is seen and 

 the following hour, however, a slight dip is indicated in 

 the ascending curve. Taking all the pressure averages 

 into account, they indicate a well-defined depression, in 

 which St. Elmo's Fire is seen six hours after the centre, or 

 point of lowest barometer, has passed. It is important to 

 note that all the pressure averages come out below 25'ooo 

 inches, because this indicates that the depression occurs 

 while Ben Nevis is in an area of general low pressure — 

 the mean barometric pressure at the Observatory for the 

 four years ending 1887 being 25'296 inches. 



The temperature averages show a broad maximum from 

 twenty-four to sixteen hours before and a minimum sixteen 

 hours after St. Elmo's Fire is seen — the range being 37 ; 

 and between these hours there is a continuous fall in the 

 temperature. The rate of fall is greater before than after 

 the display. In some of the cases the range is much 

 greater than 3°7 — in one it is I3°'4. It was chiefly from 

 the observations of temperature that we were able on 

 several occasions to give successful forecasts of the 

 appearance of the phenomenon several hours before it 

 occurred. 



The chief points shown by the wind-direction averages 

 are, that they all belong to the western half of the 

 compass ; that till the tenth hour before St. Elmo's Fire is 

 seen the wind blows from a south of west direction, and 

 thereafter from north of west ; and that from the twenty- 

 fourth hour before St. Elmo's Fire is seen, when the 

 direction is south-west, the wind steadily veers till the 

 fourth hour before, when it is west-north west, which it 

 continues to be till after the appearance of the pheno- 

 menon, and goes on veering again till the tenth hour 

 following, when it is north by west. This veering of the 

 wind before St. Elmo's Fire is seen is well marked in all 

 the cases. 



The rainfall averages show two distinct maxima, the 

 first between ten and six hours previous to the display, 



and the other at the hour St. Elmo's Fire is seen and th® 

 following. The latter maximum is wholly due to the 

 heavy showers of snow and snow-hail that accompany 

 the displays. This snow-hail differs from the usual flaky 

 snow crystals in being of the shape of small cones with 

 spherical bases, and being hard and dry. During 

 the finer displays this kind of snow was always 

 present. 



Thus, as far as local observation goes, we see that St. 

 Elmo's Fire is seen on an average six hours after the 

 lowest reading of the barometer has been recorded, in a 

 depression that occurs in a general low-pressure area ; 

 that it is preceded, accompanied, and followed by a falling 

 temperature ; that before it is seen the wind has veered 

 considerably, and goes on veering for some time after its 

 appearance ; and that it is attended by heavy precipitation 

 in the form of snow-hail. 



The averages are of some interest apart from their 

 connection with St. Elmo's Fire, for they show the 

 relations existing in this class of storms between the four 

 elements here discussed. The averages of pressure and 

 of the direction of the wind are sufficient to show that 

 the depression comes in from the Atlantic, and that the 

 centre passes eastwards somewhere to the north of Ben 

 Nevis. Then it is seen that the temperature is at a maxi- 

 mum when the barometer is falling, and is still falling 

 when the barometer is at its lowest ; that is, at a time 

 when the atmosphere is very unstable and ascending 

 currents at their strongest, the temperature is falling on 

 Ben Nevis, a state of matters that must necessarily result 

 in a rapid condensation of vapour, and a consequent 

 copious precipitation. The rainfall averages quite agree 

 with this. 



As to the prevailing weather over the British Isles and 

 west of Europe in general, about the times of St. Elmo's 

 Fire being seen on Ben Nevis, the weather charts of the 

 London Meteorological Office show that, in almost all the 

 cases, the following conditions obtained — namely, some- 

 where to the south or south-east of the British Isles, 

 usually over the south of France and over the Spanish 

 peninsula, there was a distinct high-pressure area, or anti- 

 cyclone ; and that to the west or north of Scotland there 

 was a low-pressure area, or cyclone. Between these two 

 positions the barometric gradient was chiefly for south- 

 westerly to westerly winds, and was usually pretty steep. 

 The charts also showed that, so long as the anticyclone 

 maintained its position to the south-east, so long did 

 cyclones sweep in from the Atlantic with the above 

 gradient wind, and pass our islands in a north-easterly or 

 easterly direction. Thunder and lightning were noted 

 in Ireland on several of the nights that St. Elmo's Fire 

 was seen on Ben Nevis. Only on one occasion was 

 any thunder and lightning observed on Ben Nevis, 

 about the times of St. Elmo's Fire appearing, and 

 then the phenomenon was seen two hours before the 

 thunderstorm came on. 



It might be inferred, from what has been said, that as 

 St. Elmo's Fire appeared at the change of weather, when 

 the centre of the storm had passed, it would be a good 

 prognostic of improving weather. Such is not the case, 

 however, for almost invariably another cyclone is a 

 proaching, and another spell of bad weather is experiencei 

 soon after the St. Elmo's storm has passed. 



A. R 



se, J 



1 



Note.— Since the paper, of which the foregoing is an 

 abstract, was written, several additional cases of St. 

 Elmo's Fire have been observed on Ben Nevis. On one 

 occasion we were fortunate enough in securing a photo- 

 graph of the phenomenon, which shows St. Elmo's Fire 

 as three small spots of white on the top of the 

 kitchen chimney, the chimney being but very faintly 

 seen. 



