CLASSIFICATION OF THE CLOUDS. 661 



The la^st variety of Cirro-stratus is a large shallow veil of Cloud, which 

 extensively overspreads the sky, particularly in the evening and during the 

 night, and through which the sun and moon appear dimly. It is in this 

 Cloud that those peculiar refractions of light, of the sun and moon, called 

 halos, mock suns, &c., usually appear, and which are tolerably certain 

 prognostics of rain or snow. There are minor varieties which may fre- 

 quently be observed. 



Cirro-stratus usually terminates in forming an intimate union with some 

 other Cloud, to produce rain ; but at times it evaporates or changes into 

 some other modification. 



Cirro-cumulus and Cumulo-cirrus are scarcely distinguishable, both 

 appearing as thin layers of fleecy structure, but the former float at a mean 

 altitude of 20,000 ft., while the altitude of the latter only averages 

 12,000 feet. 



Cirro-cumulus (Cirrus and Cumulus) is an assemblage of nuheculce, or 

 small roundish clouds, either detached from, or in contact with, each other, 

 and frequently reaching, to appearance, into the azure sky, commonly 

 attended by an increased temperature, and found to accord with a rising 

 barometer. The most striking feature is observed in summer, before or 

 about the time of thunder-storms. The component nubeculae are then very 

 dense, round in form, and in closer apposition than usual. This kind of 

 Cloud is so commonly a forerunner of storms, that it has been assumed by 

 some as a tempestuous prognostic. In rainy and variable weather another 

 variety of this Cloud appears, contrasted very strikingly with that above 

 mentioned, being of a light fleecy texture, without any regular form in its 

 nubeculae. Sometimes the latter are so small as scarcely to be discernible, 

 but the sky seems speckled with innumerable little white transparent spots. 

 The Cirro-cumulus of fair summer weather is of a medium nature, not so 

 danse as the stormy variety, nor so light as the variable one. Its nubeculae 

 vary in size and proximity. In fine dry weather, with light gales of North 

 and easterly winds, small detachments rapidly form and subside again, 

 generally in a horizontal axTangement. 



When Cirro-cumulus prevails, we may anticipate an increase of tem- 

 perature in summer; and in winter the breaking up of a frost, or warmer 

 and wet weather. In the summer time, extensive beds of this Cloud, 

 viewed by moonlight, have a very beautiful appearance, which has been 

 compared to a flock of sheep at rest. Cirro-cumulus subsides either slowly, 

 as if by evaporation, or changes into some other modification. 



Stratus comprehends fogs and all those creeping mists which in 

 summer evenings fill the valleys, but disappear in the mornings. The best 

 time for observing its formation is on a fine evening, after a hot summer's 

 day; we shall then observe that, as the Cumuli of the day decrease, a 

 white mist forms near the ground; this Cloud, as the Cumuli evaporate, 

 by degrees arrives at its density. In autumn it remains longer in the 

 morning. In winter it often puts on a still denser appearance, and remains 

 during the day, and even for many days successively. 



Cumulus is a flat-based rocky-topped Cloud, whose base is usually 

 elevated about 4,500 feet. The progressive formation of Cumulus is seen 

 in fine settled weather. If we then observe the sky soon after sunrise, we 



