Luke Howard in his Askesian Lectures, 1802, divides clouds into three primary modifications: cumulus, stratus, and cirrus, with intermediate forms resulting from combinations of the primaries, viz., cirro-cumulus, cirro-stratus, cumulo-stratus, and cumulo-cirro-stratus or nimbus. This nomenclature is now universally adopted.
CIRRUS, or mare’s tail cloud, appears as parallel, flexuous, or diverging streaks or fibres, partly straight. It is the lightest and the highest of all clouds, being seldom less than three miles, and often ten miles, above the earth, and shows the greatest variety of form. On account of its great height it is assumed to consist of minute snowflakes or crystals of ice, the refractions and reflections from which produce the halos, coronæ, and mock suns and moons which occur chiefly in this cloud and its derivatives. It retains its varied outlines longer than any other cloud; at sunrise it is the first to welcome the sun’s rays, and at sunset the last to part with them. It is the most useful of all clouds for weather warnings.
1. _Serene, settled weather may be expected_ when groups and threads of cirri are seen during a gentle wind after severe weather.
2. _A change to wet may be expected_ when, after continued fair weather, filaments, or bands of cirri (_apparently_ stationary), with converging ends, travel across the sky.
3. _Rain or snow, and windy, variable weather may be expected_ when cirri with fine tails vary much in a few hours.
4. _Continued wet weather may be undoubtedly expected_ when horizontal sheets of cirri fall quickly and pass into the cirro-stratus.
5. _A storm of wind and rain may be expected within forty-eight hours_ when fine threads of cirri seem brushed backward from the south-west.
CUMULUS.—This modification of cloud is most frequently seen on bright summer days, and is appropriately called “the day cloud” and “the summer cloud.” It is formed only in the daytime, in summer calms, and results from the rise of vapours from rivers, lakes, and marshes into the colder regions of the air, the lower portions of which are readily saturable. They are characterized by a horizontal base, from which they rise in dense conical and hemispherical masses rivalling mountains in their magnitude.