METEOROLOGY OF BEN NEVIS. xxvii 



observations at the top of the mountain with the observed sea-level pressure at 

 Fort- William, indicates that, in those instances, either the normal vertical dis- 

 tribution of temperature, or its abnormal vertical distribution, as the case may 

 be, is uniformly distributed from sea-level to a height of at least 4406 feet. 



But a very considerable number of cases occur in which pressure at the 

 top, reduced to sea-level, does not closely agree with the sea-level pressure 

 observed at Fort- William, thus indicating that the change of temperature does 

 not proceed at a uniform rate from Fort-William to the top of the mountain. 

 Two of the more prominent exceptional classes of such observations may be 

 here referred to. In anticyclonic weather it frequently happens that tempera- 

 ture at the top is relatively much higher than at Fort-William, the higher tem- 

 perature being restricted to the higher parts of the mountain. A fine instance 

 of this was observed by Mr Wragge on August 8, 1882. During his ascent that 

 morning, temperature steadily fell till a height of about 2375 feet was reached, 

 when temperature had fallen to 46° ; it then suddenly and rapidly rose to 53° *2 

 at the Red Burn Crossing, 2700 feet high, and to 55°*9 at the top ; the tem- 

 perature at Fort-William at the same instant being 57°*2. The air on the Ben 

 passed, at the same time, from a state of saturation to one of intense dryness. 

 The other exceptional class of instances occurred with those east winds which 

 are not felt at the Observatory at the top, but are confined to lower levels. 

 These being relatively cold winds, a uniform rate of change of temperature with 

 height does not obtain. An easterly wind prevailing from sea-level to the top, 

 and an easterly wind limited to lower heights, are two very different types of 

 weather, and have very different bearing on coming weather. It is not neces- 

 sary to insist here on the very great importance, in forecasting weather, of an 

 accurate knowledge as to whether the changes of temperature that so markedly 

 occur in unsettled weather, are confined to a height extending but little above 

 the earth's surface, or reach to at least a height of between 4000 and 5000 feet. 



Rainfall. — The following are the monthly amounts of the rainfall at the 

 Observatory and Fort- William during the months when it has been observed 

 simultaneously at both stations. 



From the monthly means obtained the annual rainfall on the top of 

 Ben Nevis is 129*47 inches, the largest monthly fall being 19'53 inches in 

 December, and the least 5*66 inches in April. For the same periods, the mean 

 annual rainfall at Fort- William was 77*33 inches, which is 52*14 inches less 

 than that of the Observatory. The rainfall of the Ben Nevis Observatory is 

 the largest anywhere yet observed in Scotland ; and, as regards the British 

 Islands, it is only exceeded at The Stye, Seathwaite, and a few other points in 

 the same part of the Lake District. 



The largest rainfall on any day during the four years was 5*34 inches on 

 December 12, 1885 ; and on January 1, 1887, there fell 4*45 inches. On the 



