Part 12

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.

Hygrometers should be exposed in the shade free from air-currents. The covering of the wet bulb must be very thin. The supply of water must be carefully regulated. The bulb must be constantly moist, yet not too wet. The supply of water must be ample in dry weather. In damp weather water must not drip from the wet bulb. Water reservoir should be as far as possible from the dry bulb. Dry bulb must never receive moisture from any source. Use distilled, rain, or softest water procurable, for wet bulb. When lime deposits from use of hard water change muslin and worsted. Replenish reservoir after, or long before, taking an observation. Well wash muslin and worsted before using. Also wash occasionally while in use. Change muslin twice a month or according to condition. Dust and blacks must not be allowed to accumulate on muslin.

⎧ When wet bulb is frozen, wet with ice-cold water by brush. ⎪ The water will first freeze, then cool to air-temperature. ⎨ After which wet bulb falls a trifle lower than dry one. ⎩ Then temperature of evaporation may be noted.

⎧ In thick fog wet bulb reads above dry bulb. ⎪ In cold calm weather, wet bulb reads above dry bulb. ⎨ This is owing to the air being perfectly saturated. ⎪ Covered bulb cannot therefore show temperature as well as uncovered. ⎩ In such cases both readings are assumed to be identical.

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.

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.

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.”