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.
Cumulo-stratus.—This form of cloud results from the mingling of the cumulus and cirro-stratus; it appears sometimes as a thick bank of cloud with overhanging masses. The cloud known as “distinct” cumulo-stratus appears as a cumulus surrounded by small fleecy clouds.
1. Thunder may be expected when “distinct” cumulo-stratus appear.
2. Sudden atmospheric changes may be expected when cumulo-stratus appear.
47.
Nimbus.
Nimbus, or cumulo-cirro-stratus.—The name of this cloud at once suggests that it is produced by a combination of the three primary forms of cloud. The nimbus is popularly known as “the rain cloud.” It is really a system of clouds, having its origin chiefly in the tendency of the cumulo-stratus to spread, overcast the sky, and settle down to a dense horizontal black or grey sheet, above which spreads the cirrus, and from below which rain begins to fall.