xxxvill METEOROLOGY OF BEN NEVIS. 



corresponding curves of temperature indicate the close relations of the two 

 classes of phenomena. The hours of the clay and the months of the year when 

 solar radiation is strongest, and the ascending air-currents consequently also 

 strongest, are the times when the sky is most clouded. On the other hand, the 

 times when terrestrial radiation is strongest, and descending air-currents now 

 play the chief part, cloud is less and skies are clearer. 



Diurnal Variation of the Rainfall. — The hourly observation of rain and 

 other forms of precipitation commenced on June 24, 1884. These observa- 

 tions are made with considerable difficulty, but the staff of observers have 

 spared no labour to make them as complete and accurate as the circumstances 

 under which they are made admit of. The mean monthly amounts for each 

 hour for the 3| years are given in Table V. It is premature, owing to the brief 

 period over which the observations extend, to attempt to draw any satisfactory 

 conclusions from the results. The means for the seasons and the year seem, 

 however, to point to an excess above the daily means from 10 a.m. to 9 p.m., 

 and a deficiency during the other hours. 



Diurnal Variation of Sunshine. — A sunshine recorder was added to the 

 instruments in the end of February 1884. The mean hourly amounts for the 

 months, seasons, and year are given in Table VII. For the four years the mean 

 number of hours annually was 669, or about 15 per cent, of the possible sun- 

 shine. The month of most sunshine was June 1887, the number of hours 

 being 206 ; then follow August 1885 and May 1884, with 116 and 106 hours 

 respectively; the months of least sunshine were January and October 1886, 

 the hours being respectively 15 and 16. As regards the three complete years 

 for which there are observations, there were 680 hours of sunshine in 1885, 576 

 hours in 1886, and 898 for 1887, being 16, 14, and 20 per cent, respectively of 

 the possible sunshine. 



The most notable feature of the curves showing the partition of the sunshine 

 among the hours of the day is the greater amount before noon than after it, 

 in spring, summer, and autumn. In these nine months there are 306 hours 

 sunshine before noon, and 285 hours after it ; whereas in the three winter 

 months the numbers are 35 and 45 hours. The mean monthly amounts of the 

 three consecutive hours of most sunshine are 13*1 hours, from 11 A.M. to 2 p.m. 

 in winter; 21 7 hours, from 9 a.m. to noon in spring; 19*2 hours, from 7 a.m. 

 to 10 a.m. in summer; and 15*8 hours, from 10 a.m. to 1 p.m. in autumn. The 

 diminution in the hours of sunshine which occurs later on in the day is doubt- 

 less occasioned by the air currents which ascend from the heated sides of the 

 mountain during the warmer hours of the day, and the larger amount of cloud 

 arising from the condensation of the aqueous vapour which necessarily follows. 



In July 1885 there were 162 hours of sunshine, but in July 1886 only 51 

 hours. In these two months, which represent fairly well the extremes of weather 



