METEOROLOGY OF BEN NEVIS IN CLEAR AND IN FOGGY WEATHER. 477 



the date of maximum. In foggy weather the hour to hour irregularities of mean 

 temperature are, as might be expected, much less striking, with, however, the exception 

 of the month October, when we have a very remarkable oscillation of temperature from 

 hour to hour during the whole morning. If we were to calculate the mean temperature 

 of the morning from the odd hours, 1, 3, 5 .... 11 , we should find it quite half a 

 degree lower than if we used the even hours, 2, 4 .... 12. One reason for the 

 greater uniformity of the temperature curves in foggy weather than in clear weather is 

 that there are no effects of alternating cloud and sunshine. In clear weather we have 

 these effects, and to some extent they must be held responsible for the irregularities 

 apparent. This, however, applies only to the daylight hours, and we see that the 

 irregularities in clear weather are by no means confined to these hours. 



In dealing with the barometric pressure we have found something very similar to 

 what we have just noticed in regard to temperature, namely, that the curves for foggy 

 weather are much smoother and more uniform than those for clear weather, and the 

 clear weather barometric curves present nearly as irregular and serrated an outline as 

 those of temperature. 



The Table on page 14 gives a summary for the year of the movements of 

 temperature. The mean dew-point for each month is given for clear weather, 

 and the excess over it of the mean monthly temperature in foggy weather is given. 

 It is only in June and July that the mean temperature of the air in foggy weather 

 is under the mean dew-point in clear weather. As the air is always completely 

 saturated in foggy weather on Ben Nevis, the temperature of the air is also its 

 dew-point. 



In this paper, no distinction is made between one kind of fog and another, and they 

 are not distinguished in the monthly sheets of the Observatory. There are, however, 

 several different kinds of fog, and these are clearly distinguished by the observers living 

 on the mountain. There is the very wet fog, which is called mist in the log, and there 

 is the comparatively dry fog, which is logged fog. Then both the fog and mist in 

 winter seem to be much denser than in summer. These belong to the elements of 

 meteorology which cannot be expressed in numbers. They are as important as those 

 which can be so expressed ; and they can be brought into the discussion of the 

 meteorology of the mountain with their due weight and importance only by men who 

 have spent a considerable time there as observers. Whether wet or dry, the fog which 

 characterises the climate of the mountain is nothing but cloud under another name. 

 The lower surface of the clouds which form on the hills rising out of the Western Ocean 

 is found generally at a height of about three thousand feet above the sea. On the 

 west coast of Scotland the air is very damp, and the clouds abundant, consequently the 

 observatory on the summit of Ben Nevis is usually situated in the heart of the common 

 clouds of the country. It may, therefore, be claimed that it is in reality an observatory 

 established in the clouds, and that the observations made in it furnish a record of 

 the meteorology of the clouds. In this respect the Observatory of Ben Nevis is unique. 



