THE METEOROLOGY OF BEN NEVIS
Several large and very important volumes of the Royal Society of Edinburgh
are devoted to statistics connected with the meteorology of Ben Nevis.
Most of the abstracts have been arranged by Dr. Buchan; while Messrs.
Buchanan, Omond, and Rankine have taken a fair share of the work.
This Observatory, as Mr. Buchanan remarks, is unique, for it is
established in the clouds; and the observations made in it furnish a
record of the meteorology of the clouds. It is 4406 feet above the level
of the sea; and as there is a corresponding Observatory at Fort William,
at the base of the mountain, it is peculiarly well fitted for important
observations and weather forecasting. The mountain, too, is on the west
sea-coast of Scotland, exposed immediately to the winds from the Atlantic,
catching them at first hand. It is lamentable to think that, when the
importance of the observations made at the two Observatories was becoming
world known, funds could not be got to carry them on. Ben Nevis is the
highest mountain in the British Islands, best fitted for meteorological
observations; yet these have been stopped for want of money.
Dr. Buchan's valuable papers were published before any one dreamed of the
stoppage of the work, which had such an important bearing on men engaged
in business or taken up with open-air sport. From these I shall sift out a
few facts that even "mute, inglorious" meteorologists may be interested in
knowing.
For a considerable time the importance of the study of the changes of the
weather has come gradually to be recognised, and an additional impetus was
given to the prosecution of this branch of meteorology when it was seen
that the subject had intimate relations to the practical question of
weather forecasts, including storm warnings. Weather maps, showing the
state of the weather over an extensive part of the surface of the globe,
began to be constructed; but these were only indicators from places at the
level of the sea.
The singular advantages of a high-level observatory occurred to Mr. Milne
Home in 1877; and Ben Nevis was considered to be in every respect the most
suitable in this country. The Meteorological Council of the Royal Society
of London offered in 1880, unsolicited, L100 annually to the Scottish
Meteorological Society, to aid in the support of an Observatory, the only
stipulation being that the Council be supplied with copies of the
observations.
From June to October, in 1881, Mr. Wragge made daily observations at the
top of the Ben; and simultaneous observations were made, by Mrs. Wragge,
at Fort William. A second series, on a much more extended scale, was made
in the following summer.
Funds were secured to build an Observatory; and, in November 1883, the
regular work commenced, consisting of hourly observations by night as well
as by day. Until a short time ago, these were carried on uninterruptedly.
Telegraphic communications of each day's observations were sent to the
morning newspapers; and now we are disappointed at not seeing them for
comparison.
The whole of the observations of temperature and humidity were of
necessity eye-observations. For self-registering thermometers were
comparatively useless when the wind was sometimes blowing at the rate of
100 miles an hour. Saturation was so complete in the atmosphere that
everything exposed to it was dripping wet. Every object exposed to the
outside frosts of winter soon became thickly incrusted with ice.
Snowdrifts blocked up exposed instruments. Accordingly, the observers had
to use their own eyes, often at great risks.
The instruments in the Ben Nevis Observatory, and in the Observing Station
at Fort William, were of the best description. Both stations were in
positions where the effects of solar and terrestrial radiation were
minimised. No other pair of meteorological stations anywhere in the world
are so favourably situated as these two stations, for supplying the
necessary observations for investigating the vertical changes of the
atmosphere. It is to be earnestly hoped, therefore, that funds will be
secured to resume the valuable work.
The rate of the decrease of temperature with height there is 1 deg. Fahr. for
every 275 feet of ascent, on the mean of the year. The rate is most rapid
in April and May, when it is 1 deg. for each 247 feet; and least rapid in
November and December, when it is 1 deg. for 307 feet. This rate agrees
closely with the results of the most carefully conducted balloon ascents.
The departures from the normal differences of temperature, but more
especially the inversions of temperature, and the extraordinarily rapid
rates of diminution with height, are intimately connected with the
cyclones and anti-cyclones of North-Western Europe; and form data, as
valuable as they are unique, in forecasting storms.
The most striking feature of the climate of Ben Nevis is the repeated
occurrence of excessive droughts. For instance, in the summer and early
autumn of 1885, low humidities and dew-points frequently occurred.
Corresponding notes were observed at sea-level. During nights when
temperature falls through the effects of terrestrial radiation, those
parts of the country suffer most from frosts over which very dry states of
the air pass or rest; whereas, those districts, over which a more humid
atmosphere hangs, will escape. On the night of August 31 of that year, the
potato crop on Speyside was totally destroyed by the frost; whereas at
Dalnaspidal, in the district immediately adjoining, potatoes were
scarcely--if at all--blackened.
The mean annual pressure at Ben Nevis was 25.3 inches, and at Fort William
29.8, the difference being 4-1/2 inches for the 4400 feet.
For the whole year, the difference between the mean coldest hour, 5 A.M.,
and the warmest hour, 2 P.M., is 2 deg. For the five months, from October to
February, the mean daily range of temperature varied only from O.6 to 1.5.
This is the time of the year when storms are most frequent; and this small
range in the diurnal march of the temperature is an important feature in
the climatology of Ben Nevis; for it presents, in nearly their simple
form, the great changes of temperature accompanying storms and other
weather changes, which it is so essential to know in forecasting weather.
The daily maximum velocity of the wind occurs during the night, the daily
differences being greatest in summer and least in winter. A blazing sun in
the summer daily pours its rays on the atmosphere, and a thick envelope of
cloud has apparently but little influence on the effect of the sun's rays.
Thunder-storms are essentially autumn and winter phenomena, being rare in
summer.
According to Mr. Buchanan, the weather on Ben Nevis is characterised by
great prevalence of fog or mist. In continuously clear weather it
practically never rains on the mountain at all. In continuously foggy
weather, on the other hand, the average daily rainfall is 1 inch. There is
a large and continuous excess of pressure in clear weather over that of
foggy weather. The mean temperature of the year is 3-1/2 degrees higher
in clear than in foggy weather. In June the excess is 10 degrees. The
nocturnal heating in the winter is very clearly observed. This has been
noticed before in balloons as well as on mountains. The fog and mist in
winter are much denser than in summer. Whether wet or dry, the fog which
characterises the climate of the mountain is nothing but _cloud_ under
another name.