METEOEOLOGY OF BEN NEVIS IN CLEAR AND IN FOGGY WEATHER. 801 



Rainfall. — In continuous clear weather it practically never rains on the mountain 

 at all. In continuous foggy weather, on the other hand, the average daily rainfall is 

 almost exactly 1 inch. The maximum amount of rain in any one day during the 

 thirteen years fell on the 2nd October 1890, and it amounted to 7 "287 in. The greatest 

 rainfall in any one hour occurred between fifteen and sixteen hours on 14th January 

 1890, when 0'85 in. fell. 



Barometric Pressure. — The difference between clear and foggy weather is shown 

 very clearly. There is a large and continuous excess of pressure in clear weather over 

 that in foggy weather. At every hour of the year, without a single exception, the 

 mean monthly pressure is several tenths of an inch higher in clear weather than in 

 foggy weather. The mean excess for the whole year is 0*456 in. (11'6 millim.). At 

 page 791 the results of the barometric observations are summarised in two tables in 

 terms of the inch and the millimetre respectively. 



The Tension of Aqueous Vapour is an important element of the meteorology of this 

 mountain. In foggy weather the vapour tension is that of saturation at the tempera- 

 ture of the air, and the variations are slight. In clear weather the variations are 

 considerable, but then the air never reaches the point of saturation, consequently 

 it never loses moisture by precipitation. At a height of 4000 feet it has also no 

 opportunity of increasing its supply of moisture, consequently any change in the 

 relative amount of moisture is due to change of air as a whole. Attention is directed 

 to the fact that, in the barometric pressure and the tension of aqueous vapour, we have 

 the data for an exact analysis by volume of the air, in so far as it consists of aqueous 

 vapour and permanent gas. At page 794 the results of the observations of tension of 

 aqueous vapour are summarised in inches and millimetres, and the percentage by volume 

 of the aqueous vapour in the non-saturated air is included. 



Temperature. — The mean temperature of the year is 3*57° Fahr. higher in clear 

 weather than in foggy weather. Amongst the monthly values this excess is greatest in 

 June, when it reaches 10 '11° Fahr. In the first three months of the year the difference 

 is in the opposite sense. The temperature is then higher in foggy weather than in 

 clear weather, the excess being 2 "92° Fahr. in February. The range of mean hourly 

 temperature is much greater in clear than in foggy weather in every month. 



The Ben Nevis observations show very clearly the nocturnal heating in the winter 

 nouths, which has been observed before both on mountains and in balloons. This 

 )ccurs in both clear and fogg}-' weather, though it is more pronounced in the clear 

 veather. 



No distinction is made between one kind of fog and another, and they are not 

 listinguished in the monthly sheets of the observatory. There are, however, several 

 afferent kinds of fog, and these are clearly distinguished by the observers living on the 

 lountain. There is the very wet fog, which is called mist in the log, and there is 

 be comparatively dry fog, which is logged fog. Then both the fog and mist in winter 

 3em to be much denser than in summer. These belong to the elements of meteorology 



