662 CLASSIFICATION OF THE CLOUDS. 



see small clouds here and there in the atmosphere, which appear to be the 

 result of small gatherings, or concentrated parts of the evening mist, which, 

 rising in the morning, grow into small masses of cloud, and the atmosphere 

 becomes clear. As the sun rises, these clouds become larger, by adjacent 

 ones coalescing, and at length a large Cloud is formed, assuming a cumu- 

 lated irregular hemispherical shape; this usually subsides in,the evening 

 as it is formed in the morning, breaking into small masses, then fragments, 

 and evaporating, when it is succeeded by Stratus, to the formation of which 

 it may have contributed. In fine weather these Clouds form soon after 

 sunrise, increase during the day, and subside with more regularity, and 

 have a more hemispherical form, than in changeable weather. When well- 

 formed Cumuli prevail for three or four days, the weather is settled. 

 Cumuli reflect a strong silvery light when opposed to the sun, like Alpine 

 mountains covered with snow. 



Cumulus sometimes takes a cylindrical shape, forming itself into long 

 horizontal rolls, between which gleams of light are seen, but which are 

 often so closely packed as to hide the blue sky ; these are called Roll- 

 cumulus. This variety of Cumulus is an addition to Howard's nomen- 

 clature. It is inserted owing to the frequency of this appearance at sea, 

 but it is necessary to observe that the effect is simply one of perspective. 



Cumulo-stratus designates Cirro-stratus blended with Cumulus, and 

 either appearing intermixed with the heaps of the latter, or super-adding 

 a wide structure to its base. Cumulo-stratus is most frequent during a 

 mean or changeable state of the barometer, when the wind blows from the 

 West, with occasional deviations from the North and South. Its mean 

 altitude is about 6,000 feet. 



This Cloud may be always regarded as a preliminary to the production 

 of rain ; and it frequently forms in the following manner : — the Cumulus, 

 which in common passes along in the current of the wind, seems retarded 

 in its progress, increases its density, spreads out laterally, and at length 

 overhangs the base, in dark and irregular protuberances. The change to 

 Cumulo-stratus often takes place at once in all the Cumuli which are near 

 to each other; and their bases uniting, the superstructure rises up with 

 mountain-like or rocky summits The change from Cumulus to Cumulo- 

 stratus is often preceded by Cirro-stratus. 



Cumulo-stratus varies in appearance ; those Clouds in which hail 

 i.howers and thunder storms form, look extremely black before the change 

 to rain, and have a menacing aspect, as they are seen coming slowly up 

 with the wind. Cumulo-stratus sometimes evaporates or changes into 

 Cumulus ; but, in general, it ends in Nimbus and a fall of rain or snow. 

 Sometimes only one part forms Nimbus, the other remaining Cumulo- 

 stratus. 



Nimbus is a flat rain-cloud, whose average altitude is from 4,000 to 

 5,000 feet. Cxtmido-nivihus is a rain-cloud, of a rocky appearance, whoso 

 base is usually elevated about 4,500 ft., while its summit may rise to 

 9,000 feet. 



Nimbus always precedes a fall of snow, rain, or hail ; and has received 

 its name from a notion of the ancients, who distinguished between the 

 Imber, or shower, and the Ni7nbus, or cloud, from which the rain comes. 



