BEN NEVIS AND FORT- WILLI AM OBSERVATORIES. 697 



Fohn-like character ; nor that warm spells caused by an excess of south-westerly winds 

 should not be in evidence, as these are the winds that bring cloud and storm to the 

 region where the Observatories are situated. But it is noteworthy that the cold periods 

 in April and May, which are so well marked all over Europe, and felt perhaps in other 

 quarters of the world as well, should be found both at Ben Nevis and Fort-William. 

 Their occurrence is due to all but world-wide fluctuations in the pressure and tempera- 

 ture of the atmosphere, and the mountains of Scotland do not protect these Observatories 

 from their advent. The High-Level Observatory is not high enough to be above the 

 causes of these two cold spells, though the different frequency of wind direction from 

 that at low-level places in Scotland shows that Ben Nevis is sufficiently high to be at 

 a level of different atmospheric circulation.* 



In Table III. the differences of temperature between Ben Nevis and Fort- William 

 range from 18°'8 on 13th April to 12°"1 on 22nd December, and in Table IV. from 

 17° 5 for the five days ending 30th April to 14°"2 for the five days ending 26th 

 December. A glance at Table III. will show that the dates of these greatest and least 

 differences are not fortuitous ; the difference is persistently large in spring and small in 

 the depth of winter No day in April has the difference below 16 c, 0, and on no day 

 in December is it above that figure, while in the table of five-day means the April 

 differences all lie between 16°"8 and 17 c '5, and the December differences between 14°2 

 and 1 5°'l. The causes of the larger differences in spring and early summer appear to 

 be, firstly, that there is less moisture in the air than at other seasons, and consequently 

 less condensation of vapour into cloud at or below the level of the summit, with its 

 accompanying liberation of heat ; and secondly, that the summit is still covered with 

 snow at that season, so that the direct solar radiation is largely absorbed in melting 

 that snow, while at Fort- William it is free to warm the air. The small differences in 

 winter are to a great extent due to the occasional occurrence of anti-cyclonic conditions, 

 with relatively high temperatures at the summit of Ben Nevis, while cold stagnant air 

 keeps the temperature at Fort- William low. But the seasonal range of differences is 

 small compared with what prevails at many continental high- and low-level stations, 

 where anti-cyclones are commoner in winter, and where the low-level station is in a 

 confined valley, and not, like Fort- William, on the shores of a sea loch. 



* Trans. Roy. Soc. Edin., vol. xlii. p. 499 : " Winds of Ben Nevis," R. T. Omond and A. Rankin. 



