918 REPORT—1885. 
In establishing this national observatory, the object sought to be attained was 
a knowledge of atmospheric changes and disturbances such as would lead to a 
juster conception of the great movements of the atmosphere, and of the weather 
preceding, accompanying, and following these movements. From the knowledge 
thus gained, and from the Ben Nevis daily observations themselves, the value of 
forecasts of the weather of the British Islands will, it is believed, be muck 
enhanced. 
The importance attached to the Ben Nevis Observatory as contributing aid in 
framing weather forecasts for the British Islands was soon apparent in discussing 
the observations in some of their relations to the weather changes of North- 
Western Europe. The occurrence of remarkable differences from the normals was 
disclosed in the vertical distribution of the temperature, pressure, and humidity of 
the atmosphere between the top of Ben Nevis and the sea level at Fort William. 
As the discussion proceeded it became more and more apparent that these diffe- 
rences gradually grouped themselves into separate classes according to the different 
types of weather which either prevailed at the time or occurred soon after. 
A series of large inquiries thus open up which will lead to important scientific 
and practical results. These inquiries are, it is unnecessary to say, among the 
most difficult in physics, and for their satisfactory investigation the directors have 
thought it essential in the first place to investigate as completely as possible the 
meteorology of the top of the mountain and of Fort William at its base. This 
being done, the bearing of the Ben Nevis observations on the weather of the 
British Islands can be more satisfactorily investigated, particularly the bearing of 
the observations on the coming weather—an investigation now being prosecuted. 
With regard to the climatology of Ben Nevis, reference is made to the Report 
submitted to this Meeting (see page 90), and to the previous Reports. A large 
number of other observations haye been made and entered in ‘the log’ of the 
observatory which yet remain undiscussed. These embrace optical observations, 
the state of the sky, and other eye-observations, which cannot be conveniently 
entered in the daily weather sheet, but which are of the greatest importance in 
the meteorology of Ben Nevis, particularly in investigating weather changes. 
The greatest interest attaches to departures from the normals occurring so 
repeatedly, of which the thermometric and hygrometric are the most striking. 
Thus the state of complete saturation of the atmosphere which may be regarded 
as the characteristic of the weather of the Ben frequently gives way, often quite 
suddenly, and the air becomes dry, frequently intensely dry, the sky singularly 
clear, and the temperature rises rapidly, resulting in the relative humidity often 
falling to 30, or on rarer occasions below 20. 
These recurring periods of intense dryness and heat do not occur at any of the 
stations of the Scottish Meteorological Society at lower levels; and from Mr. 
Wragge’s observations made in 1882 at the eight stations established that year on 
the slopes of Ben Nevis from sea level to the top, it was shown that the weather 
conditions of this type are confined to the highest part of the mountains. Heavy 
rains appear always to accompany this weather, occurring in some neighbouring 
region, These are the essential characteristics of the foehn; and its frequent 
occurrence on Ben Nevis which stands isolated above all the hills around it, leads 
to the conclusion that it is also of not infrequent occurrence in all parts of Europe, 
at heights in the atmosphere of about 4,400 feet and upwards. 
If this be so, consequences of the greatest importance follow. Instances have 
occurred when the temperature on Ben Nevis has been from 5° to 8° higher than 
in the vicinity near sea level, and as the normal temperature of Ben Nevis is 1673 
lower than at Fort William, it follows that the temperature sometimes rises fully 
20° above the normal difference. Now as regards the aerial stratum from sea 
level to 4,406 feet high, this manner of distribution of temperature with height 
will not be productive of any violent movements. But it is quite otherwise for 
heights greater than Ben Nevis, where it is evident temperature will fall with 
height at a rate very greatly exceeding the normal. It is probable that it is from 
the conditions of unstable equilibrium here indicated that whirlwinds and the 
more dangerous gusts which occur during storms have their origin. 
