58 REPORT— 1900. 



In comparing the mean values a similar remark holds good, the greater 

 values at Exeter being due to a small number of extreme observations. 



The minimum altitudes recorded at Exeter compare fairly well with 

 the others, some of the ditfereuces being most probably due to nomenclature. 



Several series of observations have been made in a single day with the 

 object of determining the rise or fall of clouds. It is clear from these 

 that on an average day the cloud planes rise steadily until the early 

 afternoon, between 2 and 3 p.m., when the maximum for the day is 

 usually reached. This is followed by a fall, which gets more and more 

 rapid towards sunset. In calm weather, or weather with only a moderate 

 breeze and no great barometric disturbance, this diurnal rise and fall is 

 very clearly marked ; but in broken weather, with strong winds, showers, 

 or barometi'ic changes, it may be completely masked. 



Cumulus is the result of an upward movement, but cirro-cumulus and 

 alto-cumulus may sometimes be the result of a descending movement, in 

 which case the lumpy form is never persistent, but passes into a stratiform 

 cloud very quickly. 



True cirrus of the whispy form is described by some meteorologists as 

 due to a rapid ascending current, by others to an equally rapid descent. 

 The measurements made indicate that this form of cloud may exist with 

 an upward or a downward movement, or with no recognisable movement 

 at all. 



The greatest altitudes have been found with thunderstorm conditions, 

 the lowest (excepting fog) with cyclonic. 



The measurements compared in the foregoing tables have all been 

 made between April and October inclusive. In the winter months the 

 ground has generally been too wet for use, and the figures from the 

 foreign stations are for the summer months only. It seems difficult at 

 first to see why the altitudes should, on the whole, be greater at Exeter, 

 the greater humidity of the air leading rather to the expectation of more 

 easy cloud production, and therefore lower altitudes. But the fact of 

 thunderstorm conditions being attended, as they seem always to be 

 attended, by great cloud altitudes suggests another explanation. This is 

 that vapour in a cloud-producing quantity exists to a greater height 

 above Devonshire. It will be noticed that the greater altitudes are true 

 only of the higher clouds, and that the mean level of the base plane of 

 cumulus and cumulo-nimbus is actually lower at Exeter than at either 

 of the other stations. 



The photographs collected some years ago by the Committee have 

 been placed in the care of the Royal Meteorological Society, with the 

 exception of prints from the negatives belonging to the Secretary, who 

 will add them as opportunity offers. 



During the past year the Secretary has made a number of experiments 

 with the Ives and Joly processes for photography in natural colours, but 

 has found that, although either process can be made to record the colour 

 of a cloud, the tints of a sunset, or even the colours of the rainbow, the 

 reproduction of the colours is so far from being an automatic process that 

 neither method promises to be of very great meteorological value except in 

 the hands of experts. 



