IJ£c. ^, ii>87j 



NA TURE 



,^3i 



tinguish pure stratus from the flat under surface of 

 some kinds of cumulus or nimbus. 



The term nimbus is applied to any cloud which is pre- 

 cipitating rain. In practice we find two 

 rather distinct types — a strato nimbus or 

 flat cloud, and a cumulo-nimbus or rocky 

 rain-cloud. The former is characteristic of 

 the rainfall in front of an extra-tropical 

 cyclone, the latter of the precipitation from 

 squalls and thunderstorms all over the world. 

 Our illustration (Fig. 3) represents a distant 

 view of the clouds over a thunderstorm in 

 Borneo. Below we see the rocky summits 

 of a mass of cumulo-nimbus, while apparently 

 above, but really at about the same level, we 

 find the characteristic fibrous structure that 

 is called "goat's hair" by some, or "false 

 cirrus" by others. 



Another typical structure is that which 

 has been called in all times by all nations 

 fleecy, woolly, or some cognate name. In 

 this, clouds assume the appearance of a 

 fleece of wool. Each little mass of condensed 

 vapour has a peculiar fibrous structure, quite 

 different from true cirrus. The density and 

 level of formation vary a good deal. When 

 the cloud is thin at up to about 25,000 feet, 

 most meteorologists call it cirro-cumulus ; 

 but when denser, and down at about 18,000 

 feet, the name of cumulo-cirrus has been 

 proposed to distinguish this low variety. 

 Fig. 4 is an excellent specimen of cirro- 

 cumulus, from a photograph ta'cen near the 

 Falkland Islands 



There is a form of cloud intermediate 

 between pure cirrus and pure stratus which 

 is so common and so characteristic of bad 

 weather that it has universally been classified 

 as cirro-stratus. We apply the term to a 

 sky which is covered with a thin layer of 

 cirrus fibres, more or less mixed up with a 

 formless haze or veil of scattered ice-particles. 

 Sometimes the cirrus threads are thin and 

 white as the finest gossamer, and float 25,000 

 to 27,000 feet above the earth, but at other 

 times the structure is coarser, and the level 

 of formation not more than 1 8,ooD feet. The 

 first kind is called cirro-stratus, the second 

 strato-cirrus. Fig. 5 is an example of a 

 rather heavy cirro-stratus^ taken near Dover. 

 It will be observed that there are two distinct 

 lines of structure about which the cloud 

 masses are grouped, and that the lines in- 

 tersect one another at a certain angle, so 

 that the whole has a certain reticulated 

 appearance. This is most characteristic of 

 cirro-stratus. 



Cirro-stratus with its hairy structure, and 

 cirro-cumulus with its fleecy appearance, 

 might at first sight appear to be radically 

 different from one another ; but they are not 

 so really. It is by no means uncommon 

 to see a patch of fibrous cirro-stratus sud- 

 denly become fleecy for a few minutes, 

 and then return again to its former state. 

 We cannot give the reason for this, as 

 the origin of both structures is at present 

 imknown. 



Theri is a form of cloud, intermediate 

 between stratus and cumulus, to which the 

 word strato-cumulus is appropriately applied. 

 In this the cloud layer is too lumpy to be called pure 

 stratus, and not rocky enough to be called cumulus. 

 Fig. 6 is an excellent specimen of this type, taken near 



Teneriffe ; and here we see the lumpy masses of cloud 

 getting apparently thinner and thinner as they approach 

 the horizon, till they look at last like a series of stripes or 



Fig. 4 — Cirro-cumulus, or fleecy structure. Falkland Islands. 



Fig. 5 — Cirro-stratus. Folkestone. 



Fig. 6.— Strato-cumulu;. Near Teneriffe. 



rolls parallel to ths horizon. This of course is the result 



of perspective. , •. j 



The ten varieties of cloud which we have now described 



