I qo 



NA TURE 



{Dec.Z, 188/ 



The identify of cloud forms all ever the world has 

 recently been demonstrated both before tl e Royal Society 

 and the Royal Meteorological Society of London by the 



Fig. I. — Cirrus wisp over cumulus. Folkestone. 



Fig. 2. — Stratus. London 



Fig. 3. — Cumulo-nimbus, cirrifying above. Borneo. 



exhibition of about fifty photographs of clouds taken by 

 the writer in various longitudes, and in latitudes ranging 

 from 72° N. to 56° S. Some of these are reproduced in 



the illustrations of this article, and the conclusion of 

 identity is irresistible. The cirrifying cloud over an 

 irregular cumulus, in Fig. 3, might be seen over any 

 summer thunderstorm in England, though 

 this example is from tropical Borneo ; while 

 the fleecy cirro-cumulus in Fig. 4, which was 

 taken near the Falkland Islands, about 51° S.. 

 differs in no respect from the similar cloud 

 we so often see at home. Fig. .6 is a strato- 

 cumulus from near Teneriffe, in the heart of 

 the North-east Trade ; but the writer has 

 seen an absolutely identical sky from the 

 summit of North Cape, far within the Arctic 

 Circle. 



The different structures of clouds can 

 certainly be reduced essentially to five or 

 six types. A great deal must of course 

 depend on the definition we adopt of a kind 

 or species of cloud. We believe that one 

 German meteorologist in Rhineland says 

 that he has discovered 30,000 different kinds 

 of cloud, and that he has not yet finished his 

 classification. This is absurd ; for though 

 no two clouds are ever exactly the same, 

 any more than any two faces, still certain 

 broad types of cloud structure can readily 

 be recognized. 



The first primary type of structure is the 

 cirriform or hairy. The thin fibres of white 

 silvery cloud which constitute a cirrus may 

 assume an almost infinite variety of forms. 

 The commonest is the simple wisp of white 

 threads such as is shown in Fig. i, floating 

 at a high level over a heavy mass of cumulus 

 cloud. Sometimes the cirrus lies in long 

 straight stripes, which Ley has shown have 

 a great value in forecasting weather ; or 

 at other times assumes the " penniform " 

 or plume-like appearance which, accord- 

 ing to Vines, precedes a hurricane in the 

 Antilles. 



Cirrus as a rule is formed at very high 

 levels — 20,000 to 25,000 feet— and the con- 

 stituent particles are undoubtedly frozen, but 

 we occasionally find a fibrous structure at 

 low levels, where the constituent particles 

 are certainly in a fluid form. Both the cirrus 

 and cumulus in Fig. i are composed of icy 

 particles, for the picture was taken on a cold 

 winter day in England when snow showers 

 were flying about. But in Fig. 3 we see a 

 fibrous combed-out structure at quite a low 

 level in Borneo, where the temperature both 

 of the air and the rain makes it certain that 

 the whole cloud mass was made up of liquid 

 particles. 



The true cumuloform structure of cloud 

 can never be mistaken. The rising mass of 

 condensed vapour assumes a rocky, lumpy 

 appearance, which is well delineated in the 

 lower portion of Fig. i. The varieties of 

 form are infinite. Sometimes beautiful little 

 isolated cloudlets, each with its own flat 

 base, float all over the sky, while at other 

 times we only see mountainous masses rising 

 above a gloomy cloud bank on the horizon, 

 as in Fig. 3. 



Essentially different from the above is the 



stratiform structure which is depicted in 



Fig. 2. Here we have a thin layer of flat 



cloud, at low level, more or less broken, but 



showing no trace of either a fibrous, rolled, or lumpy 



stiucture. When the sky is broken, this form of cloud is 



unmistakable, but when overcast it is impossible to dis- 



