CLOUDS AND CLOUD-DRIFT 29 



6 miles ! At this elevation the temperature is below zero, 

 and strato-cirrus, when spread out as described above, must be 

 very fine ^now as distinguished from the very minute particles 

 of ice which form cirrus and cirro-stratus. This fine snow 

 then falls slowly by its own weight, and, melting, it often 

 produces those quiet after-rains which follow the heavy rains 

 and squalls of the cumulus. 



From what has been said about the spreading out of this 

 cloud, it might be supposed that it had no average drift ; but 

 if well-formed cumuli at considerable distances be watched, it 

 will be found that while their average drift is from the south- 

 east over the western half of Jamaica, the drift of the strato- 

 cirrus issuing from them is generally north-east. 



(5) Cumulus. This cloud consists of large rounded masses 

 resting on flat bases ; it is often called the thunder-cloud. 

 Its texture is apparently very solid; and it is formed by 

 ascending columns of heated vapour. When rain falls from 

 the base of the cloud it is called 



(6) Cumulo-nimbus, and this rain takes up the drift of 

 the whole cumulus. The velocity of the wind accompanying 

 the rain is thus the same as the velocity of the whole cloud ; 

 and these squalls are sometimes very severe, especially on the 

 mountains. 



(7) Alto-cumulus. A thin sheet of small separate clouds, 

 arranged more or less regularly into groups or lines. The* 

 clouds are soft, rounded masses, like fleeces of wool, and the 

 whole cloud often resembles a flock of sheej). Its average 

 drift is from the south-east. 



(8) Alto-stratus. *'A thin sheet of a grey or bluish 

 colour, showing a brilliant patch in the neighbourhood of 

 the sun and moon, and which, without causing halos, may 

 give rise to coronae." Thick and thin are merely relative 

 terms; and this cloud is thin when compared with strato- 

 cirrus. It has a soft, watery look. It is not often seen 

 in Jamaica. 



(9) Strato-cumulus. *' Large globular masses or rolls of 

 dark cloud, frequently covering the whole sky, especially in 

 winter, and occasionally giving it a wavy appearance. The 

 layer of strato-cumulus is not as a rule very thick, and 

 patches of blue sky are often visible through the intervening 



