Part 5

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.

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.

RAIN.

The atmosphere at a given temperature is capable of retaining only a given quantity of aqueous vapour, invisibly diffused through it, at which temperature it is said to be _saturated_. Should the temperature from any cause be lowered, the aqueous vapour at once becomes visible in the form of either cloud, dew, rain, snow, or hail. It has already been shown that, although marshes and rivers, inland seas and lakes, yield by evaporation watery vapours to the air, the ocean is the great source of rain, whence it is lifted in vast quantities by the sun’s radiant heat, to be subsequently condensed by passing into cooler regions, or by contact with cold mountain peaks, falling to earth as a fertilizing shower or a devastating flood.

Sir John Herschel accounts for the formation of raindrops by saying:—“In whatever part of a cloud the original ascensional movement of the vapour ceases, the elementary globules of which it consists being abandoned to the action of gravity, begin to fall. The larger globules fall fastest, and if (as must happen) they overtake the slower ones, they incorporate, and the diameter being thereby increased, the descent grows more rapid, and the encounters more frequent, till at length the globule emerges from the lower surface of the cloud at the ‘vapour plane’ as a drop of rain, the size of the drops depending on the thickness of the cloud stratum and its density.”

Rain is very unequally distributed, there being portions of the torrid zone where it _never_ falls, one locality in Norway where it falls three days out of four, and another on the western side of Patagonia, at the base of the Andes, where it falls every day. The quantities recorded as having fallen at one time in some localities are simply appalling. A fall of one inch is considered a very heavy rain in Great Britain, and this fact will enable the reader partially to realize the following stupendous recorded falls:—Loch Awe, Scotland, 7 inches in 30 hours; Joyeuse, France, 31 inches in 22 hours; Gibraltar, 33 inches in 26 hours; hills above Bombay, 24 inches in one night; and on the Khasia Hills, where the annual rainfall is 600 inches, 30 inches have been known to fall on each of five successive days. Mr. G. J. Symons, the able editor of the “Meteorological Magazine,” and indefatigable superintendent of 2,000 Rain Gauges throughout the United Kingdom, has compiled a table, showing the equivalents of rain in inches, its weight per acre, and bulk in gallons, the following portion of which, while very useful to the farmer, will enable the curious reader to make some interesting calculations, based on the figures quoted above:—

TABLE SHOWING EQUIVALENT OF INCHES OF RAIN IN GALLONS, AND WEIGHT PER ACRE.

Inches of Rain Tons per Acre Gallons per Acre 0·1 10 2262 0·2 20 4525 0·3 30 6787 0·4 40 9049 0·5 50 11312 0·6 61 13574 0·7 71 15836 0·8 81 18098 0·9 91 20361 1·in. 101 22623

The instruments called Rain Gauges or Pluviometers are, as their name implies, constructed to measure the amount of rain falling in any given locality, and those in most general use have this principle in common: that the graduated glass always bears a definite relation to the area of the receiving surface. A very extraordinary and hitherto unexplained fact in connection with the fall of rain, and which justifies the opinion that its formation is not limited to the region of visible cloud, is that a series of rain gauges placed at different elevations above the soil are found to collect very different quantities of rain, the amount being _greater_ at the _lower_ level. Thus, twelve months’ observations by Dr. Heberden determined that the amount of rain on the top of Westminster Abbey was only twelve inches, that on a house close by but much lower eighteen inches, and on the ground during the same interval of time twenty-two inches. Accordingly, ten inches is the height at which meteorologists have agreed the edge of the rain gauge should be placed from the ground. The spot chosen should be perfectly level, and at least as far distant from any building or tree as the building or tree is high, and, if the gauge cannot be equally exposed to all points, a south-west aspect is preferable. It is also important that the rain gauge should be well supported, in order to avoid its being blown over by the wind; and, should frost follow a fall of rain, the instrument should be conveyed to a warm room to thaw before measuring the collected contents. The graduated glass furnished with each instrument should stand quite level when measuring the rain, and the reading be taken midway between the two apparent surfaces of the water.

The best form of rain gauge is that in use in the Meteorological Office.

Howard’s Rain Gauge consists of a vertical glass receiver, or bottle, through the neck of which the long terminal tube of a circular funnel, five inches in diameter, is inserted. A metal collar or tube fits over the outside of the neck of the receiver, and aids in keeping the funnel level, while the tube extends to within half an inch of the bottom, thus ensuring the retention of every drop of rain which falls within the area of the funnel. The glass vessel furnished with the instrument is graduated to 100ths of an inch. A modification of this instrument is made with a glass tube at the side graduated to inches, 10ths, and 100ths, showing the amount of rainfall by direct observation, thus dispensing with the use of a supplementary graduated measure.

In Glashier’s Rain Gauge special provision is made, in two ways, to prevent possible loss by evaporation, even in the warmest months of the year. 1. The receiving vessel is partly sunk beneath the soil, thus keeping the contents cool. 2. The receiving surface of the funnel, accurately turned to a diameter of eight inches, terminates at its lower extremity in a curved tube, which, by always retaining the last few drops of rain, prevents evaporation. The graduated vessel, in this instance also, is divided to 100ths of an inch, having due regard to the larger area, 8 in. of the funnel. For use in tropical climates, where, as has been shown, the rainfall is excessive, a modification of this instrument is supplied by the instrument makers, having an extra large receiver and tap for drawing off the collected rain.

Luke Howard, in his “Climate of London,” says: “It must be a subject of great satisfaction and confidence to the husbandman to know at the beginning of a summer, by the certain evidence of meteorological results on record, that the season, in the ordinary course of things, may be expected to be a dry and warm one, or to find, in a certain period of it, that the average quantity of rain to be expected for the month has fallen. On the other hand, when there is reason, from the same source of information, to expect much rain, the man who has courage to begin his operations under an unfavourable sky, but with good ground to conclude, from the state of his instruments and his collateral knowledge, that a fair interval is approaching, may often be profiting by his observations, while his cautious neighbour, who ‘waited for the weather to settle,’ may find that he has let the opportunity go by.” This superiority, however, is attainable by a very moderate share of application to the subject, and by the keeping of a plain diary of the barometer and rain gauge, with the hygrometer and vane under his daily notice.

Symons’s Rain Gauge resembles Howard’s, but has the advantage of having the glass receiver enclosed in a black or white japanned metal or copper jacket with openings permitting an approximate observation of the collected rain. The metal jacket is also furnished with strong iron spikes, which are firmly pressed into the soil, as shown at Fig. 49, thus ensuring perfect steadiness by its power to resist the wind. The graduated measure contains half an inch of rain (for a 5 inch circle) divided into 100ths.

Mr. Symons has devised another rain gauge of so ingenious and interesting a character that it needs only to become generally known among amateur meteorologists to be in universal demand. By its means an observer at a distant window may read off the rain as it falls. It is shown at Fig. 50, where the usual 5-inch funnel surmounts a long glass tube attached to a black board bearing a very open scale marking tenths of an inch in _white_ lines; a white float inside the tube constitutes the index, which rises as the rain increases in quantity. If, as sometimes happens during a thunderstorm, the rainfall is excessive, a second tube on the left permits the measurement of a second inch of rain. It will be obvious that if the _time_ at which the rain begins to fall be noted the _rate_ at which it falls, as well as the quantity, is indicated at sight by this instrument.

Crossley’s Registering Rain Gauge has a receiving surface of 100 square inches. The rain falling within this area passes through a tube to a vibrating bucket, which sets in motion a train of wheels, and these move the indices on three dials, recording the amount of rain in inches, 10ths, and 100ths. Printed directions are furnished with each instrument, and the simplicity of the mechanism ensures due accuracy. A test measure, holding exactly five cubic inches of water, sent with each gauge, affords the means of checking its readings from time to time.

Beckley’s Pluviograph possesses the exceptional merit of recording with equal precision all rainfalls, from a slight summer shower to a heavy storm of rain. It may be placed in a hole in the ground, with the receiving surface raised the standard height of ten inches above its level.

Fig. 51 illustrates the construction of the instrument.

The funnel has a receiving surface of 100 square inches, protected by a lip 1-1/4 inch deep, to retain the splashes. The rain flows into a copper receiving vessel on the right, which, floating in a cistern of mercury, sinks and draws down with it a pencil, which records the event on a white porcelain cylinder moved by a clock. When the receiving vessel is full the syphon comes into action, rapidly drawing off _the whole_ of the water, the vessel rising almost at a bound, the action being recorded by a vertical line on the porcelain cylinder. Two or more cylinders are supplied with each instrument; and, as the pencil marks are readily removed by a little soap and water, a clean one may be always kept at hand for exchange once in every twenty-four hours.

The Rev. E. Stutter’s Self-recording Rain Gauge is ingenious, and for a self-recording instrument is very moderate in price, while it efficiently shows the rainfall for every hour in the twenty-four (Figs. 52, 53).

An eight-day clock with its upright spindle revolves a small funnel with a sloping tube, the end of which passes successively over the mouth of the twelve or twenty-four compartments in the rim of the instrument; beneath each compartment is placed a tube, as shown in the sectional figure. All rain received by the outer funnel drips into the smaller revolving funnel, and flows down the sloping tube, the end of which is timed to take an hour in passing over each compartment, so that the rain, for example, which falls between twelve and one o’clock will be found in the tube marked 1. Each tube can contain half an inch of rain, and any overflow falls into a vessel beneath, and can be measured; the tube which has overflown shows the hour.

V.—MOTION.

Wind is air in motion. The motion of the air is caused by inequality of temperature. The earth becomes warmed by the sun, and radiates the heat thus acquired back upon the air, which, expanding and becoming lighter, ascends to higher regions, while colder and denser currents rush in to occupy the vacated space. Two points are to be noted in connection with this rush of air which we call wind, viz., its _direction_ and _velocity_ or _force_. Both are estimated scientifically by instruments called Anemometers,[13] while mariners and the dwellers on our coasts have a nomenclature of their own by which to indicate variation in the _force_ of the wind, founded on the amount of sail a vessel can carry with safety at the time. In the matter of _direction_ winds are classed as constant, periodical, and variable.

[Footnote 13: _Anemos_, the wind; _metron_, measure.]