CLOUDS.
The important office performed by clouds in the economy of nature entitles them to extended consideration. A cloud may be defined as “water-dust,” since aqueous vapour diffused through the air is invisible until the temperature is sufficiently lowered to produce condensation; no satisfactory explanation, however, has yet been given of the mode of suspension of this water-dust, nor why it remains suspended in opposition to gravitation. It is tolerably certain that electricity is not without its influence, though the apparently stationary character of some clouds is deceptive, for while there may be no apparent motion in the mass the particles constituting the mass are undergoing continuous renewal, which justifies the assertion of Espy that every cloud is either a forming or dissolving cloud. Aeronauts in ascending from the earth pass through many successive alternations of cloud-strata and clear air which owe their existence to the varying temperature and degrees of humidity of the atmospheric currents so superposed.
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.
44.
Cirrus.
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.
45.
Cumulus.
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.
Their formation is due to the convection of heat from the earth’s surface, which renders the lower atmospheric strata capable of holding a larger amount of aqueous vapour and simultaneously establishes an upward current, which reaching the colder regions of the air brings about the condensation of the aqueous vapour into the elegant and ever-beautiful forms admired alike “by saint, by savage, and by sage.” These begin as mere specks, which enlarge until the sky is nearly covered in the afternoon, and towards sunset they generally disappear, their tops becoming cirri when the air is dry.
1. Fine, calm, warm weather may be expected when cumuli are of moderate size and of pleasing form and colour.
2. Cold, tempestuous, rainy weather may be expected when cumuli cover the sky, rolling over each other in dense, dark, and abrupt masses.
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.
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.
1. A cessation of rain may be expected when the grey lower portion of nimbus begins to break up.
2. A thunderstorm may be expected when the nimbus character of the cloud is very perfect.
3. Very copious showers may be expected when the cirri projected from the top of the rain-cloud are very numerous.
Amount of Clouds.—Any record of the proportion of sky covered by cloud should be made on a scale of 0 to 10. A clear sky is registered 0, and a sky wholly obscured as 10, any intermediate condition being represented by 5—7, or other figures deemed appropriate by the observer. The kind of cloud should be noted, as also the direction in which it is driven by the wind, whether in the upper or lower strata of the air. This operation may be assisted by an ingenious arrangement, exhibited by Mr. Goddard in 1862, and called a “cloud reflector,” obtainable at any optician’s. Observations at the Greenwich Observatory establish the facts that the least amount of cloud exists during the night, especially in May and June, and the greatest amount at midday, and in winter; also that from November to February three-fourths of the heavens are obscured by sun-repelling clouds.
Height of Clouds.—Great diversity of opinion exists on this point. It is asserted, on the one hand, that the region of clouds does not extend beyond five miles above sea-level, but Glaisher has attained a height of 36,960 feet, and from thence saw clouds floating at a great height above him; and it is considered probable that cirri are often ten miles above the earth.
Velocity of Clouds.—This is of two kinds: 1st. Velocity of Propagation; and 2nd. Velocity of Motion. The first occurs when at a given altitude the dew-point is suddenly attained, when the sky on one occasion was covered from the eastern to the western horizon at the rate of 300 miles per hour. The second is dependent on the force of atmospheric currents, which is much greater in the upper regions of the air than in those nearer the earth. Accurate observations of the shadows of clouds, borne across the fields on a summer’s day, warrant the assertion that an apparently slow motion of clouds is equal to eighty miles an hour, while a velocity of 120 miles is attained without impressing the observer with the idea of rapidity.
On the subject of clouds Admiral Fitzroy says:—
| May be Expected | |
| ----- | |
| Fine weather | When clouds are “soft-looking or delicate.” |
| Wind | When clouds are hard-edged or oily-looking. |
| Less wind | In proportion as the clouds look softer. |
| More wind | The harder, more “greasy,” rolled, tufted, or ragged the clouds look. |
| Rain | When small-inky-looking clouds appear. |
| Wind and rain | When light scud clouds are seen driving across heavy masses. |
| Wind only | When light scud clouds are seen alone. |
| Change of wind | When high upper clouds cross the sun, moon, or stars in a direction different from that of the lower clouds, or the wind then felt below. |
| Wind | With tawny or copper-coloured clouds. |
The following “Weather Warnings” may be gathered from the Colour of the Sky:—
Whether clear or cloudy, a rosy sky at sunset presages fine weather; a sickly greenish hue, wind and rain; a red sky in the morning, bad weather, or much wind or rain; a grey sky in the morning, fine weather; a high dawn (i. e., when the first indications of daylight are seen above a bank of clouds), wind; a low dawn (i. e., when the day breaks on or near the horizon), fair weather. Light, delicate, quiet tints or colours, with soft, indefinite forms of clouds, indicate and accompany fine weather; but gaudy or unusual hues, with hard, definitely outlined clouds, foretell rain and probably strong wind. Also a bright yellow sky at sunset presages wind; a pale yellow, wet; orange or copper-coloured, wind and rain: and thus, by the prevalence of red, yellow, green, grey, or other tints, the coming weather may be told very nearly; indeed, if aided by instruments, almost exactly.
After fine, clear weather the first signs in a sky of a coming change are usually light streaks, curls, wisps, or mottled patches of white distant cloud, which increase and are followed by an overcasting of murky vapour that grows into cloudiness. This appearance, more or less oily or watery as wind or rain will prevail, is an infallible sign.
Usually, the higher and more distant such clouds seem to be, the more gradual, but general, the coming change of weather will prove.
Misty clouds, forming or hanging on heights, show wind and rain coming, if they remain, increase, or descend; if they rise or disperse, the weather will improve or become fine.
| May be Expected | |
| ----- | |
| Fine weather | When the sky is grey in the morning. |
| Wind | With a high dawn. |
| Fair weather | With a low dawn. |
| Wind | When the sky at sunset is of a bright yellow. |
| Rain | When the sky at sunset is of a pale yellow. |
| Wind and rain | When the sky is orange or copper colour. |
| Fine weather | When the sky has light, delicate, quiet tints and soft, indefinite forms of clouds. |
| Rain and wind | When the sky has gaudy, unusual hues, with hard, definite outlined clouds. |
| Fair weather | When sea-birds fly out early and far to seaward. |
| Stormy weather | When sea-birds hang about the land, or fly inland. |
| Fair weather | When dew is deposited. Its formation never begins under an overcast sky, or when there is much wind. |
| Rain | On what is called a good hearing day. |
| Rain | When remarkable clearness of atmosphere, especially near the horizon, exists, distant objects, objects, such as hills, being unusually visible or well defined. |