546 MESSRS OMOND AND RANKIN ON THE 



Here, again, we see that the Ben Nevis excess is four points to the right of the sea-level 

 excess. This fact seems to point to the cause of the heavy rainfall experienced with 

 north-westerly winds on the summit, and with south-westerly and westerly winds at 

 sea-level. The south-westerly and westerly sea-level winds sweep in from the Atlantic, 

 and in rising over the high hills on the west coast they meet with a north-westerly wind, 

 which is a much colder direction on the summit than S.W., and in consequence a rapid 

 condensation of vapour, resulting in a heavy precipitation, takes place. Thus the rainfall 

 is greatly increased by the action of the N.W. cold condensing current at and above the 

 level of Ben Nevis, over what it would be from the mere effect of altitude on the ascend- 

 ing warm currents from below. The table indicates that in this kind of weather there 

 is an excess of pressure (at sea-level) to the south, and a defect to the N. or N.W.; and 

 that the wind excess on Ben Nevis is very nearly an outflowing wind from the centre 

 of low pressure. This explains the heavy rainfall usually experienced in or near the rear 

 of shallow depressions ; for when we have a warm, moist current below, and a current 

 from a much colder direction above, and when the warm current tends to ascend and 

 the cold current to descend, we have the most favourable conditions for producing a 

 heavy rainfall. In almost all storms that pass our western and north-western coasts we 

 have these conditions, and the result is that our west coast is about the wettest spot in 

 Europe. In the driest year we have a distribution of sea-level pressure exactly opposite 

 to that in the wet year. Here we see that with a low pressure to the south of Ben 

 Nevis, the excess winds are much the same above and below, — that there is no indi- 

 cation of an outflowing wind. It is rather remarkable that in both the driest and 

 wettest years we have N. and N.E. somewhat in excess on the summit, but in the driest 

 year these directions are much more in excess at sea-level than above, while the moisture- 

 bringing sea-level winds are much in defect, showing that the dryness is due in a great 

 measure to the defect in the sea-level oceanic winds, and that the wetness even on Ben 

 Nevis is due more to the sea-level winds than to the high-level winds ; that is to say, 

 though the N.W. wind on Ben Nevis is the wettest wind, yet all its wetness comes from 

 below, — it is a moist current only in virtue of its being a condensing medium for the 

 warm and moist ascending currents from below. Rainfall is a very important element 

 of climate, and it is very interesting to note the considerable differences which exist at 

 moderate altitudes, as shown by these tables, in the direction of the wind, and conse- 

 quently in the temperature of the air, when it is most abundantly produced. 



Diurnal Variation in the Direction of Wind. 



There is no marked diurnal variation in the direction of the wind on Ben Nevis, 

 though the mean wind-velocity shows a distinct daily period. The latter, as at all true 

 hill stations, rises to a maximum in the very early morning, and falls to its minimum in 

 the afternoon, the reverse of what occurs at sea-level. But the direction of the wind 



