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.
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.