METEOROLOGY OF BEN NEVIS IN CLEAR AND IN FOGGY WEATHER. 475 



Temperature. — The mean temperature of the year is 3 "57° F. higher in clear weather 

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

 when it reaches 10*11° F. In the first three months of the year the difference is in the 

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

 excess being 2*92° F. in February. The mean temperature of the months October to 

 March is almost the same for both kinds of weather, being 28*70° F. for clear and 28*51° F. 

 for foggy weather. Amongst the hourly values the greatest excess of temperature in 

 clear over that in foggy weather is 13*4° F. at fifteen hours in June, and the greatest 

 difference in the opposite sense is 4*3° F. at one hour in March. The range of 

 mean hourly temperature is much greater in clear than in foggy weather in every 

 month. 



In this respect again the difference between clear weather and foggy weather on 

 Ben Nevis is the same in kind as that between the dry season and the rainy season in 

 the Tropics. 



In fogg} 7 weather the mean daily range of temperature is very small in winter, and 

 by no means large in summer. The maximum mean daily range is 2*5° F. in May, 

 when the mean monthly temperature is 31*42° F. The maximum mean monthly 

 temperature, 39*58° F., falls in August, and then the daily range is only 1*7° F. In 

 clear weather the maximum mean monthly temperature, 46*46° F., falls in July, and 

 the maximum mean daily range, 8*2° F., falls in the same month. In clear weather 

 the effect of adding heat is to raise the temperature of the air and to increase its 

 volume. In foggy weather a large portion of it is rendered latent by the evaporation 

 of a portion of the water in a very fine state of subdivision disseminated through it. 

 It may be doubted whether the direct heat of the sun has any part in producing the 

 diurnal range of temperature during continuous foggy weather. At the upper surface 

 of the fog-cloud we would expect the sun's rays to be largely reflected from the 

 dazzlingly white surface which such a cloud alwaj^s presents when viewed from above 

 in bright sunshine, and that the remainder would be absorbed in evaporating the upper 

 layer of fog, leaving nothing to be transmitted downwards. There is, as a rule, 

 a resultant current of air upwards during the day ; and when the cloud, as is often 

 the case, does not spread to a great distance from the mountain side, the lower ground 

 is enjoying sunshine, and the upward circulation is very active. The fact that the 

 month of May has the greatest amount of clear weather and also the greatest daily range 

 of temperature in foggy weather seems to support this view. In winter the daily range 

 of temperature is under 1° F. 



Mean Hourly March of Temperature in each Month. — This is conveniently treated 

 with respect to the hourly change of temperature during the day and without respect , 

 to the actual temperatures at these hours. 



If we review the curves which have been drawn for each month, and are to be 

 found in the original paper, we see that they fall into two distinct types. The one 



