796 MR J. Y. BUCHANAN ON THE 



opposite sense is 4 "3° R 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 foggy 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 3 1*42° F. The maximum mean monthly 

 temperature, 39 "58° F., falls in August, and then the daily range is only 17° 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 alwa}^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 we see that they 

 fall into two distinct types. The one type is predominant in the winter months and 

 the other in the summer months. For this purpose the winter months are October to 

 March, and the summer months Aj3ril to September. The principal feature which 

 distinguishes the winter months from the summer months in clear weather is the 

 occurrence in the former of a pronounced nocturnal heating effect. This shows itself 

 particularly in the curves for November, December, January, and February ; in October 

 and March the passage between the summer and winter types is apparent. 



Confining our attention more especially to the four months about midwinter we 

 find the abnormal feature referred to very strongly marked in November. The 

 mean temperature of the twenty-four hours is 3274°, and it occurs four times, namely, 

 at midnight, at 1.40 a.m., 9 a.m., and 3.50 p.m., the curve showing two maxima at 



