METEOROLOGY 



45 



of formation is concerned cloud sheets may be due to a layer of 

 damp air becoming sufficiently cooled by expansion, owing to a 

 fall of pressure, to form cloud, while cloud heaps are due to air 

 rising and on expansion becoming cooled so as to form a cloud 

 (Meteorological Glossary). 



Clouds are identified as being connected with different heights 

 at which they exist. It will be sufficient to describe the principal 

 cloud forms, though it may be noted that the International Meteoro- 

 logical Committee have drawn out a list of definitions and descrip- 



FIG. 7. Campbell- Stokes' Sunshine Recorder. 



tions of cloud forms which includes all the intermediate forms of 



clouds. 



Howard's original classification described three forms : 

 Cirrus (Mares' tails), at a high altitude, feathery and white, 



consists of crystals of ice. 



Cumulus, rounded in appearance at the top, base usually flat 

 Stratus, formed in layers, foglike in appearance. To these may 



be added Nimbus, of no definite shape, the source usually of rain and 



snow. 



SUNSHINE. The Campbell-Stokes' sunshine recorder focusses 



the rays of the sun by means of a glass sphere so that they fall on 



