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.