XX11 



METEOROLOGY OF BEN NEVIS. 



mean annual temperature of Ben Nevis and Fort- William is 15° 9, the least 

 monthly mean difference 14°2 occurring in November, and the greatest 17° '8 

 in May. The greatest difference between the temperature of the top and 

 bottom of the mountain occurs from March to June, when the top is still 

 under a deep covering of snow, the air is driest, the great annual increase of 

 temperature is in progress, and the temperature of the Atlantic relatively to 

 that of the air is lowest. On the other hand, the difference is least from 

 November to January, when the mountain is most frequently and most densely 

 enveloped in cloud and mist, the atmosphere oftenest completely saturated, and 

 westerly winds from the Atlantic very greatly predominate. 



These results show that the rate of the decrease of temperature with height 

 is one degree Fahrenheit for every 275 feet of ascent on the mean of the year, 

 the rate being most rapid in April and May, when it is one degree for each 

 247 feet, and least rapid in November and December, when it is one degree for 

 307 feet. This rate agrees closely with the results of the most carefully con- 

 ducted balloon ascents, and of those of the very few other pairs of stations in 

 other countries from which tolerably satisfactory data can be obtained. 



A comparison of the observations of the two stations for the period shows 

 a variation with the hour of the day more decidedly marked than the variation 

 with season. The following are the differences between their mean daily 

 maxima and the differences between their mean minima : — 





Jan. 



Feb. 



Mar. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



Year. 



Difference of Mean Maxima, 

 Difference of Mean Minima, 



15° -9 

 14° -4 



17°'4 

 14°-1 



19-6 

 15° -4 



22° -4 

 14°-3 



22-1 

 14°-7 



21 c -0 

 13°-1 



20° -2 

 13° -4 



20°-5 

 13° -0 



19° -2 

 12° -8 



18°-4 

 13° -8 



15° -5 

 13°-0 



15°6 



13°-1 



19° -0 

 13°-8 



Hence in winter the decrease of temperature with height is nearly as great 

 during the night as during the day, but from April to September the rate of fall 

 during the day is about a half more than that of the night. In April the rate 

 of decrease of temperature with height is one degree for 196 feet during the 

 warmest time of the day, whereas during the coldest time in the early morning- 

 it is one degree for 301 feet. 



The rates of decrease observed from day to day, however, differ widely 

 from these means and from each other. The greatest fall of temperature from 

 Fort-William to the Observatory noted to the end of 1887 was 28 0, 1 at 2 p.m. of 

 June 8, 1885. On the other hand, however, the temperature at the top has 

 frequently stood higher than the temperature of Fort-William. The greatest 

 deviation in this direction occurred at 8 a.m. of November 18, 1885, when, while 

 the temperature at Fort- William was 22°-2, that of the top was 35° \1, or 12° 9 

 higher. The departures from the normal differences of temperature, but more 

 especially the inversions of temperature, and the extraordinarily rapid rates of 



