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Clouds are continually varying in their form and appearance, but may be classed under the four principal heads of the cirrus, the cumulus, the stratus, and the nimbus.

The cirrus is a light, fleecy cloud resembling a lock of hair or a feather.

The cumulus or summer cloud is generally massive and of a round form; sometimes of small size, and sometimes covering nearly the whole sky, and occasionally appearing in the horizon like mountains capped with snow.

The stratus is a horizontal, misty cloud sometimes observed on fine summer evenings comparatively near the ground, and often crossing the middle regions of mountainous or hilly districts.

The nimbus or rain cloud has a uniform gray tint; it is fringed at the edges when these are displayed, but usually covers the whole sky. The region of clouds is a zone extending in the atmosphere from about one to four miles above the Earth. The most elevated clouds, which are light and fleecy, are those comprehended under the name of cirrus, and the lowest are those which are called stratus.

The cirro-cumulus, cirro-stratus, and cumulo-stratus are only modifications and combinations of the above-named principal classes.