METEOROLOGY OF BEN NEVIS. XXXIX 



experienced at the Observatory as respects sunshine and cloud, it is interesting 

 to note that their diurnal curves of atmospheric pressure were all but identical 

 as regards the amplitude of the daily range and the times of occurrence of the 

 two maxima and minima. A blazing sun daily pours its rays on the atmo- 

 sphere, and a thick envelope of cloud or its absence has apparently but little 

 influence on the thermodynamic effect of the solar rays as expressed in the 

 oscillation of the diurnal curve of pressure. 



Miscellaneous. 



But in addition to the usual routine work of a Meteorological Observatory of 

 the first order, other observations have been carried on, mostly of a novel character, 

 for which the meteorology of Ben Nevis affords exceptional facilities. Most of 

 these observations have been at least partially discussed, and the results published 

 by the Royal Society of Edinburgh and the Scottish Meteorological Society, of 

 which the following is a brief resume : — 



Formation of Snow Crystals from Fog. — This is one of the most striking 

 phenomena to be observed at high-level stations. As Ben Nevis is situated in the 

 path of the Atlantic cyclones, with their vapour-laden wind systems, the formation 

 of snow crystals on the Observatory and all surrounding objects exposed to the 

 drifting fogs when temperature is low proceeds often at an astonishingly rapid rate. 

 The forms and arrangements of the crystals vary with the surface to which they 

 adhere, but all belong to the feathery or fir-cone type, the branches lying at an 

 angle of 30°, with the main axis pointing to windward. On a flat board they 

 gather first and most abundantly near its edges, forming a beautiful border round 

 it, while the centre remains clear, thus suggesting interesting conclusions regarding 

 the relative velocities of the wind at different points of the board. On the other 

 hand, a round post shows an almost uniformly disposed mass of crystals all over its 

 windward half, and so accurately do they point to windward, that it is possible to 

 trace changes in the direction of the wind from the successive layers of crystals at 

 different angles. The rate of growth of the crystals varies with the density of the 

 fog and the speed of the wind. For the winds and fogs of most usual occurrence a 

 growth of about half an inch per hour is the average, but under exceptionally favour- 

 able conditions 2 inches per hour has been reached. With a damp feeling in the 

 air, the crystals are icy and hard; but with a temperature considerably under 32° 

 the crystals are looser in texture, and are easily brushed away. On one occasion, 

 during the winter of 1884-85, a post 4 inches square grew into a slab of crystalline 

 snow of about 5 feet broad and 1 foot thick in less than a week, strong south- 

 easterly winds with low temperatures prevailing during the whole time. 



The effect of this growth seriously affects all instruments placed outside the 

 Observatory. The cups of Robinson's anemometer are no longer hemispheres, but 



