3. _Thunder may be expected_ when cumuli of hemispherical form are characterized by an extreme silvery whiteness. 4. _Rain may be expected_ when cumuli increase in number towards evening, sinking at the same time into the lower portions of the air. [Illustration: 46. Stratus.] STRATUS.—As its name implies, this is a horizontal sheet of cloud formed near the earth at night (whence it has been called “the night cloud”) by the condensation of moist air from rivers, lakes, and marshes, or damp ground which has lost its day-heat by radiation, especially in calm clear evenings, after warm days. It appears as a white mist near, and sometimes touching, the earth. It attains its maximum density about midnight, but is dissipated by the rays of the morning sun. Its formation, watched from a height over a large city, is highly interesting, and is attributed by Sir John Herschel to the soot suspended over such localities, each particle of which acts as “an insulated radiant, collects dew on itself, and sinks down rapidly as a heavy body.” Still more interesting is it to observe from a similar elevation the dissipation of this cloud when the sun has attained such an altitude that its rays fall on the upper surface of the stratus cloud, which then heaves like the billows of the ocean, while the whole mass seems to rise spontaneously from the earth, and speedily vanishes “into air, into thin air.” 1. _The finest and most serene weather may be expected_ when stratus clouds present the appearances just described. CIRRO-CUMULUS, or “mackerel sky,” is a well-known form of cloud occurring in small roundish masses, looking like flocks of sheep at rest, and often at great heights. It is seldom seen in winter. 1. _Increased heat may be expected_ when cirro-cumuli appear. 2. _A storm or thunder may be expected_ when cirro-cumuli occur mingled with cumulo-stratus in very dense, round, and close masses. 3. _Warm wet weather, and a thaw, may be expected_ when cirro-cumuli occur in winter. CIRRO-STRATUS “appears to result from the subsidence of the fibres of cirrus to a horizontal position, at the same time approaching laterally. The form and relative position when seen in the distance frequently give the idea of shoals of fish.” It is called “the vane cloud” and “mackerel-backed sky.” 1. _Rain, snow, and storm may be expected_ when _cirro-stratus_ is seen alone or mingled with cirro-cumulus, especially if the cirro-cumulus passes away. 2. _Fair weather may be expected_ when from a mixture of cirro-stratus and cirro-cumulus the former disappears, leaving the latter in possession of the sky. 3. _Thunder and heat_ are generally attended by waved cirro-stratus.