1895-96.] 
Mr John Aitken on Clouds. 
93 
Notes on Clouds. By John Aitken, Esq., F.R.S. 
(Read May 4, 1896.) 
There are two points connected with clouds on which I wish to 
make a few remarks. The first is on the classification of clouds, 
and the second on the manner in which certain forms of clouds are 
produced. It may be as w r ell to remark at the outset that the 
observations are those of an “ outsider,” being in a department of 
meteorology to which the writer has given but little attention, and 
they have been written with the view of calling the attention of 
specialists, and getting their opinion on the subject. 
It appears to me that in classifying clouds they ought first of all 
to be divided into tw T o great classes. In the one class should be 
placed all clouds in the process of formation , and in the other those 
in the process of decay. The two classes might be called Clouds in 
Formation and Clouds in Decay. We may take Cumulus clouds 
as an example of the former, and Nimbus of the latter. My 
observations made in the clouds themselves have shown that there 
is a difference in the structure of these two classes of clouds. In 
clouds in formation the water particles are much smaller and far 
more numerous than in clouds in decay ; and while the particles in 
clouds in decay are large enough to be seen with the unaided eye 
when they fall on a properly lighted micrometer, they are so small 
in clouds in formation that, if the condensation is taking place 
rapidly, the particles cannot be seen without the aid of a lens of 
considerable magnifying power. In the former case the number of 
particles falling per square millimetre is small, while in the latter 
they are so numerous that it is impossible to count them. 
It appears that one good end might be served by adopting this 
classification. It would direct the attention of observers more to 
looking on the processes going on in decay for an explanation of 
many of the forms observed in clouds. In most books on clouds, 
when describing the different shapes of clouds, it is almost always 
assumed that they are in the process of formation , and the whole 
explanation of the shapes taken by the clouds is founded on this 
