Dew is a deposition of moisture from the air, resulting from the condensation of the aqueous vapour of the atmosphere on substances which have become cooled by the radiation of their heat. This is, in fact, the substance of Dr. Wells’s famous Theory of Dew, enunciated in 1814, and which, according to Dr. Tyndall, “has stood the test of all subsequent criticism, and is now universally accepted,” and by which all the phenomena of dew may be explained.

Dr. Wells’s experiments were interesting and conclusive. He exposed definite weights (10 grains) of wool to the air on clear nights, one on a four-legged stool, the other under it, the upper portion gained 14 grains in weight, the lower only 4 grains. On an evening when one portion of wool, protected by a curved pasteboard roof, gained only 2 grains, a similar portion on the top of the miniature roof gained 16 grains. A little reflection will suggest the explanation: radiation from the wool was arrested by the pasteboard cover, while the portion fully exposed to the sky lost all its heat, and thus condensation ensued. Dr. Wells speaks with such candour, and so pointedly, on this fact and its consequences, that his words may be advantageously quoted: “I had often, in the pride of half-knowledge, smiled at the means frequently employed by gardeners to protect tender plants from cold, as it appeared to me impossible that a thin mat, or any such flimsy substance, could prevent them from attaining the temperature of the atmosphere, by which alone I thought them liable to be injured. But when I had learned that bodies on the surface of the earth become during a still and serene night colder than the atmosphere, by radiating their heat to the heavens, I perceived immediately a just reason for the practice I had before deemed useless.”

Familiar instances of the formation of dew will have been noted by many “watchers;” e. g., breathing on a cold pane of glass, a tumbler of cold water becoming dew-covered on being brought into a warm room, the outside of a tankard of iced claret cup, &c. When, radiation is so free and rapid that the temperature is below the freezing point, the dew freezes as it forms, producing hoar-frost.

In our climate the air is never completely dry, nor completely saturated with moisture, and the amount of aqueous vapour held in suspension is very variable. This fact has important bearings on many branches of industry, as also on the hygienic qualities of the atmosphere. The consideration that a certain amount of moisture in the air is necessary to the continuance of health will suggest the importance of maintaining that due proportion in the atmosphere of sick rooms, where the artificial heat so injudiciously used, often disturbs the healthful hygrometric condition of the air. Mr. Glaisher is of opinion that the medical profession should enforce, as far as lies in their power, the use of this simple and effective instrument, which gives indications so important to the comfort of the patient.

The amount of moisture in the air is estimated by the use of instruments called Hygrometers, which may be thus classified:—

1. Hygrometers of Absorption.—Made with hair, oatbeard, catgut, seaweed, grass, chloride of calcium.

2. Hygrometers of Condensation.—Regnault’s, Daniell’s, Leslie’s, Dyne’s.

3. Hygrometers of Evaporation.—Mason’s Psychrometer, or Wet and Dry Bulb Thermometers.

By an ingenious application of the affinity of the oatbeard for moisture, Damp Detectors are constructed for tourists, commercial travellers, &c., to test moisture and avoid the consequences of sleeping in damp beds. They are strongly gilt, and resemble in size and shape a lady’s watch.

In Saussaure’s Hygrometer the frame is of brass, and the scale of the same metal silvered. It has an attached thermometer, and the indications are the result of the contraction and expansion of a prepared human hair, consequent upon its absorbing or yielding moisture. The scale is divided on the arc of a circle, and an index needle, working on an enlarged arc, multiples the indications.

Regnault’s Hygrometer (Fig. 39) consists of a thin and highly polished silver tube or bottle, into the neck of which is inserted a delicate thermometer. The bottle has a lateral tubular opening, to which is attached a flexible tube with an ivory mouthpiece.

Ether is poured into the silver tube in sufficient quantity to cover the bulb of the thermometer. The ether is then agitated by breathing through the flexible tube until, by the rapid evaporation thus produced, a condensation of moisture takes place, readily observable on the bright polished silver surface, and the temperature indicated by the thermometer at that moment is the dew-point.

Daniell’s Hygrometer, or Dew-point Thermometer (Fig. 40), consists of a glass tube, bent twice at right angles, each extremity terminating in a bulb about 1-1/2 inch in diameter, supported on a brass stand, to which a thermometer is attached to indicate the temperature of the surrounding air. The lower bulb is of blackened glass, to facilitate the observation of the dew-point; it is about three parts filled with pure ether, and contains a very delicate thermometer. The upper bulb at the extremity of the short stem is transparent, but covered with thin muslin, upon which, when an observation is made, pure ether is slowly dropped. The evaporation rapidly lowers the temperature, until a moment arrives at which dew condenses on the black bulb. A quick eye is necessary to note this and the temperature shown by the thermometer simultaneously, the latter showing the degree at which the atmosphere is saturated with moisture at the time of observation. To avoid error, it is usual to note the temperature at which the dew disappears, and take the mean of the two temperatures.

Dyne’s Hygrometer, for showing the dew-point by direct observation, by means of iced water and black glass, enables the observer to dispense with the use of ether, and shows the dew-point with great distinctness.

The hygrometer in most general use is the wet and dry bulb thermometer, and for which Mr. Glaisher has calculated an elaborate set of tables, a brief abstract of which sufficient for general purposes is subjoined.

The total quantity of aqueous vapour which at any temperature can be diffused in the air being represented by 100, the percentage of vapour actually present will be found in the table opposite the temperature of the dry thermometer, and under the difference between the dry bulb and wet bulb temperatures. The degree of humidity for intermediate temperatures and differences to those given in the table can be easily estimated. Thus dry bulb 51°, wet bulb 46°, give 69 for the degree of humidity.

The instrument, as shown at page 48, consists of two thermometers attached to a support, which may be either slate or wood. The bulb of one of the thermometers has some thin muslin tied over it, and is kept moist by the capillary action of a thread dipping into a cistern of water placed underneath. It will be obvious that the amount of evaporation will be in proportion to the dryness of the air, and that the differences of temperature indicated by the two thermometers will be greatest when the atmosphere is dry, and least when the air is damp.

HYGROMETER PRECAUTIONS.

It is important that the instrument should be protected not only from the sun’s direct rays, from rain and snow, but also from wind, the currents of which would, by increasing evaporation, cause the wet bulb thermometer to indicate a temperature not strictly due to the hygrometric condition of the atmosphere. For this purpose Thermometer Screens are employed. Illustrations of two forms are shown at Figs. 42 and 43; they should be placed facing the north at a distance of four feet from the ground. Fig. 42 shows the form adopted by the Board of Trade, for marine service, while Fig. 43 shows Mr. Stevenson’s double-louvred screen with perforated bottom, which ensures free ingress and egress of air, the exclusion of snow and rain, and the direct rays of the sun. Professor Wild recommends overlapping segments of sheet zinc for the construction of these screens, as possessing the advantage over wood of becoming sooner in thermic equilibrium with the surrounding air, and thus preventing radiation. Stevenson’s Screen should be erected on legs four feet high, and should stand over grass on open ground. It should not be under the shadow of trees, nor within twenty feet of any wall.

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.

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.

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.

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:—

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.

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:—

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.

52.53.
Stutter’s Self-recording Rain Gauge. Scale about 1/7.