These values are not very different, on the whole, from those which have been arrived at elsewhere, and in making a comparison it must be borne in mind that there is always a little want of precision in cloud nomenclature. As a whole, the Exeter maxima are greater than the foreign ones, and this is very markedly so in the case of cirrus, for which the American highest record is 14,930 metres, the Swedish record is 13,376, while the Exeter value is 27,413 metres, or about 17 miles. But this extreme measurement, and several others unusually large, were made in one morning, a day of very hot damp weather, when cloud formed at seven different levels: cumulus at a height of 1·9 miles, alto-cumulus at 3·9 miles, cirro-cumulus at 4·7 miles, cirro-stratus (No. 1) at 8 miles, cirro-stratus (No. 2) at 9·6 miles, cirrus at 11·5 miles, and cirrus excelsus at 17 miles. By about half-past one in the afternoon the sky was completely overcast with dull grey clouds, which cleared off at half-past four, and at half-past five in the evening the cirrus had fallen to 7·9 miles, and the cirro-cumulus to 4·3 miles. If this one day’s observations had been omitted, the Exeter maximum would only have been little more than 1000 metres above the record from across the Atlantic, but 1000 metres is a height worth noting.
While the Exeter maxima are all rather greater, we find the minima for cirrus, cirro-stratus, and cirro-cumulus are rather less than at the foreign stations; that is to say, that clouds are formed over Devonshire both at lower and at higher levels than seems to be the case in Massachusetts or Sweden. It seems probable that this is due to a greater humidity on our western coasts, such as we should suppose would be the case from their position and the prevailing winds and ocean currents. If so, we should expect the great convection clouds to be larger. Thus, at Exeter, out of only fifteen examples of cumulo-nimbus, the top varied from 2004 metres to 6409, with an average base level of 1045. At Upsala the maximum was 5970 and the minimum 1400, with an average base level of 1400. The mean thickness of the Swedish clouds was only 1400 metres, while that of the Devonshire specimens was more than 2000 metres.
Again and again, during the progress of these measurements, it was found that the greatest altitudes and the richest development of the higher varieties occurred towards the end of a spell of fine calm weather, when convection had had free play day after day. A slight fall of the barometer, only the hundredth part of an inch, would usually, under those circumstances, bring about abundant formation of high clouds, frequently of the undatus kind. All the cumulus clouds, by which we mean to include alto-cumulus and cirro-cumulus, are most frequent when the levels of condensation are rising, while the stratiform clouds are an indication of no vertical movement or of active descent. Pure cirrus is indicative rather of movement in a horizontal direction, and may occur when the condensation levels are stationary, or when they are rapidly changing either way.
In broken weather the natural movements of the atmosphere and of its vapour are masked and disturbed by the strong eddies brought by the cyclonic systems. It not unfrequently happens that the region of disturbance does not reach up to the level of the highest cirrus, or, what is more probable, the cyclonic system leans so far forward that we may have in its rear the upper clouds floating quietly far above the comparatively shallow region of disturbance, while in front the upper part of the storm system projects above undisturbed air.
The frequent appearance of cloud almost at the same time at more than one level is at first rather difficult to understand, but it will be noticed that when this occurs the barometer almost invariably falls. Now, if we suppose that the air is nearly saturated at more than one level, and that the whole is then bodily relieved of some of the superincumbent mass, so that the barometer falls, the mass of air will at once swell up, being cooled from top to bottom simultaneously, and wherever it is damp enough cloud will be formed.
The converse is equally true. If we have cloud at several levels, and the whole is compressed by the addition of more air above, which is the case when the barometer rises, that compression will be accompanied by the generation of heat and the consequent disintegration and disappearance of the clouds.
CHAPTER X
CLOUD NOMENCLATURE
Since a considerable number of new terms have been suggested in the foregoing pages, it may be convenient to collect them and tabulate them, so as to show their relation to those already recognized by the International system.
In the atlas put forward by the committee, sixteen varieties are recognized by distinct names, and these are drawn up in tabular form with appropriate abbreviations for use in making records.