3. The Stratus, from its spreading out horizontally in a continuous layer, and increasing from below. (Figure 10.)
These three primary forms are subject to four modifications:—
The first is the Cirro-cumulus, consisting of small roundish and well-defined masses, in close horizontal arrangement. (Figure 3.)
The second is the Cirro-Stratus, and the masses
which compose it are small and rounded, but thinned off towards a part, or towards the whole of their circumference. They are sometimes separate, and sometimes in groups. (Figures 4, 5, 6.)
The third is the Cumulo-Stratus, which is made up of the cirro-stratus blended with the cumulus. (Figure 8.)
The fourth is the Cumulo-Cirro-Stratus, or Nimbus. This is the true rain-cloud, or system of clouds from which rain is falling. (Figure 9.)
The term modification applies to the structure or manner in which a given mass of cloud is made up, and not to its precise form or size, which in most clouds varies every instant. Mr. Howard remarks, that it may be at first difficult to distinguish one modification from another, or to trace the narrow limits which sometimes separate the different modifications; but a moderate acquaintance with the subject will soon enable any one to point out the various forms, and to a great extent to judge of the state of the weather by them. In order, therefore, to assist the reader in gaining a certain amount of knowledge on this interesting subject, it may be useful to state more fully the various phenomena of the different forms of clouds already enumerated.