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 5011312
0·6 6113574
0·7 7115836
0·8 8118098
0·9 9120361
1·in.10122623

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.

48.
Howard’s Rain Gauge.
Scale about 1/5.

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.

49.
Symons’s Rain Gauge.
Scale about 1/7.

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.

50.
Symons’s Storm Rain Gauge.
Scale about 1/12.

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.

51.
Beckley’s Pluviograph. Scale about 1/7.

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.

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.