1894.] On Rapid Variations of Atmospheric Temperature. 109 



diminished the regularity of the march of temperature is found to 

 diminish also, but the great variability of the temperature of the 

 air is best shown by the curve drawn by a recording thermometer of 

 sufficient sensibility combined with a clock movement of suitable 

 velocity. Such an instrument draws a sinuous line which is generally 

 smooth during the night and serrated during the day. The shape and 

 the crowdedness of the teeth on the serrated daylight portion of the line 

 have a close connection with, and are to a certain extent an indication 

 of, the character of the existing weather. In general the indented 

 character of the daylight curve is an indication of the disturbing in- 

 fluence of the sun on the equilibrium of the atmosphere which continues 

 just as long as he is above the horizon ; after sunset the atmosphere 

 quickly reverts to a state of greater stability. It is obvious, there- 

 fore, that the indented character of the daylight curve indicates not 

 only changes of temperature in the air but also motions and changes 

 o f motion in it. These motions are generally vertical and too subtle 

 and local to be observed with an anemometer. In the course of 

 frequent observations in the open air and under varying circum- 

 stances, I have many times had occasion to remark these rapid 

 oscillations of temperature and at the same time to deplore the 

 difficulty of accurately measuring them. It is principally with the 

 view of directing attention to this instrumental difficulty that the 

 following observations are put together. At the same time, though few 

 in number, they have to do with a very remarkable species of weather, 

 known by its Alpine name of Fohn. It has been most observed 

 in the valleys stretching in a northerly direction from the main sum- 

 mit line of the chain of the Alps and takes the form of an abnor- 

 mally warm wind blowing from the mountains towards the plain. 

 It has largely occupied the attention of continental meteorologists, 

 and more particularly it has been the subject of exhaustive investi- 

 gations by Hann, who has shown by very strong evidence that its 

 high temperature must be due to its compression in descending from 

 a great altitude. In the descriptions of the Fohn, attention is 

 almost exclusively directed to the high average temperature of the 

 air, and no mention is made of their extraordinary variations, 

 although every observer must have noticed them. They are so great 

 as to be recognised at once by the sensations and at the same time so 

 rapid as to elude almost every other method of estimation or measure- 

 ment. It has also, I believe, not been before remarked that the true 

 Fohn occurs in our own country and with its characteristics quite as 

 well marked as in Switzerland. It is sometimes supposed that a great 

 absolute height of mountain chain is required for its production ; but 

 this is not so. A relative height of 1,000 to 1,200 metres is quite 

 sufficient for its production ; and this is equally available on the west 

 coast of Scotland and on the northern slopes of the Alps. 



