te ON METEOROLOGICAL OBSERVATIONS ON BEN NEVIS. 181 
and one cannot sufficiently admire the heroic endurance with which the 
observers have made the hourly observations by night and by day, during 
‘summer and during winter. 
_ he direction of the winds indicates a well-marked diurnal variation. 
‘From 3 to 8 a.m. northerly winds of about 24 miles an hour, and from 
“11 a.m. to 2 p.m. southerly winds of about 3 miles an hour prevail. 
" From three years’ observations, ending May 1887, it appears that the 
“mean temperatures of the different winds are, §., 32°6; S.W., 82°5; 
OW., 31°4; N.W. and S.E., 30°25; E., 27°8; N., 27°°6, and N.H., 26°°5. 
‘The warmest point in the windrose oscillates from S.W. in winter, passing 
through S. to 8.H. in summer. The annual temperature range of easterly 
winds is 20°°7, but westerly only 15°'6. 
Observations of the rainband were begun im June 1885. The ob- 
served higher values are accompanied, or soon followed, by a heavy rain- 
fall, which tends to become less heavy in the next twelve hours. The 
lower values, on the other hand, though they may be neither accompanied 
nor followed in the next three hours by any rain, are followed by a con- 
siderable rainfall before the twelve hours are run. With the same rain- 
band value precipitation is less with a higher and greater with a lower 
temperature. Ifthe temperature immediately falls the rainfall is greatly 
increased, but if it rises it is less than it would have been if the tempera- 
ture remained constant. The highest values, with accompanying very 
heavy rains, are part and parcel of the cyclones which come to us from 
the Atlantic laden with moisture and warmth. The rainband is not 
affected during heavy rains, the result of moisture-laden air ascending 
from lower levels ; and during the states of the air attending the rapid 
deposition of snow crystals no rain falls, though at the time the rainband 
values are high. 
As respects forecasting the weather, the most important observations 
are those showing a decreasing rainband from hour to hour. A compari- 
son of these observations with the daily weather-charts and subsequent 
observations show that the decreasing rainband indicates that the moist 
air aloft is slipping away or sinking below the level of the summit, and 
that the air taking its place is comparatively dry. Now this state of 
things appears to be the earliest indication we at present have that an 
anticyclone is beginning to form and settle over this part of Europe. 
St. Elmo’s Fire is not an infrequent occurrence on Ben Nevis. The 
Observed cases have occurred during the night and during the winter 
months from September to February. A careful discussion of the cases 
shows that the weather which precedes, accompanies, and follows has 
Sm peculiar characteristics not only on Ben Nevis but also over the 
West of Europe generally-—indeed, so well marked is the type of weather 
and so notorious is it for its stormy character, that it is familiarly known 
at the observatory as ‘ St. Elmo’s weather.’ It is farther observed that in 
almost every case another cyclone, with its spell of bad weather, follows 
the particular cyclone in which St. Elmo’s Fire is observed. 
The winter thunderstorms occur under the identical weather condi- 
tions under which St. Elmo’s Fire occurs. They invariably occur on the 
south-east side of the ecyclone’s centre, with the easterly passage of which 
they appear to be intimately connected. The thunderstorms and cases of 
sheet-lightning of Ben Nevis are essentially autumn and winter occurrences, 
70 per cent. of the whole having occurred from September to February. 
They are rare in summer, only eight having occurred from May to August, 
