| Months. | Inches. | Days. | Hours. | Months. | Inches. | Days. | Hours. |
| January. | 1·86 | 19 | 88 | July. | 2·27 | 17 | 68 |
| February. | 0·37 | 9 | 25 | August. | 2·45 | 12 | 72 |
| March. | 3·40 | 22 | 130 | September. | 1·70 | 12 | 55 |
| April. | 2·34 | 14 | 80 | October. | 3·23 | 21 | 94 |
| May. | 3·04 | 16 | 90 | November. | 1·12 | 10 | 53 |
| June. | 2·45 | 20 | 83 | December. | 1·44 | 17 | 66 |
“During the year 1862, the rainfall amounted to 25·67 inches. Rain fell on 179 days, that is, on nearly every other day. The hours of rain were estimated at 904; therefore, if the rain had fallen continuously, it would have lasted nearly 38 days and nights.”[10] The value of similar estimates of the rainfall by numerous observers would be very great to meteorology.
116. The principle of measurement in all these gauges is the relation existing between the areas of the collecting and receiving surfaces; that is, between the area of the funnel into which the rain falls, and the area of the cylinder which receives it. In Howard’s and Glaisher’s gauges, this cylinder is virtually the measuring glass itself; in the others, above described, the measuring scales show the same depth of water as in the cylinder of the gauge.
The cylinder being of less diameter than the funnel, and receiving all the rain collected by the funnel, it follows that its contents will have an increased depth. Now equal cylindrical volumes, having different diameters, are to each other in length inversely as the squares of the diameters. Hence, if the funnel be 9 inches and the cylinder 3 inches in diameter, a fall of 1 inch of rain will be represented in the gauge by 9 inches; for 3² : 9² : : 1 : x = 9. In this case, therefore, a length of nine inches of the measuring glass, tube, or scale, would represent an inch of rainfall, and be divided into tenths and hundredths of the artificial inch.
117. Position for Rain-Gauge, &c.—Rain gauges should be placed on the ground, in any position exposed to a free fall of rain, snow, or hail, where neither walls, buildings, nor trees shelter or cause eddies of wind. They should be supported by a frame, or other means, to prevent them being blown down by the wind, but so that they can be readily emptied.
During snow or frost, the gauge must be watched, and its contents melted by placing it in a warm room, either when the amount is to be measured, or the funnel is filled up with snow. A tin vessel of equal area to the funnel may at such times be useful as a substitute.
Rain gauges are constructed of metal, usually copper, which, besides being readily workable, is little affected by atmospheric influences. If made of iron or zinc, they should be well japanned; if of copper, this is not so essential. The capacity of a gauge should be sufficient to contain at least the probable maximum fall of rain in a day at the locality. Those required for rainy districts must be of large size.
118. Causes of Rain.—When the invisible vapour which is diffused in the atmosphere becomes sufficiently cooled, it appears visible as mist or cloud, and a further reduction of temperature causes its precipitation as rain, hail, or snow. The cooling of the higher regions of the atmosphere is doubtless the chief cause of this condensation; but the property which aqueous vapour possesses of radiating heat may also contribute to the result. Moreover, the law which regulates the amount of vapour which air at any particular temperature can sustain in a transparent state, determines that when two bodies of air at different temperatures, saturated with vapour, intermix, some moisture must be rendered visible; and hence, it is not only possible, but highly probable, that rain may result from the conflict of different winds. Let us imagine two cubic yards of air, both saturated with moisture, but having the respective temperatures of 50 and 70 degrees, to come into contact. There will be a tendency to equalize the temperature to a mean, which is 60°; and during this process, some of the vapour will be condensed.
| For | in the air at | 50° | there is | 110·7 | grains of | vapour[11] | |
| and | " | 70 | " | 216·0 | " | ||
| Total amount of vapour | 326·7 | " | |||||
| But two cubic yards of air at 60° can only sustain | 313·2 | " | |||||
| Hence there will be deposited | 13·5 | " | rain. | ||||
It may be conceded, therefore, that when a warm and moist current of air encounters a body of cold air which may not be extremely dry, the mixture is unable to retain the whole of the vapour in an invisible state; so that the excess becomes visible as mist or fog, and, when the temperature has become sufficiently lowered, rain. The British Isles are more or less enveloped in fog, or mist, at the commencement of easterly winds, which, with a sudden change of wind, is exhibited even in summer; while the south-westerly winds, warm, and arriving from the ocean, deposit large quantities of rain by the cooling effect of the land, colder by reason of its latitude. When rain occurs with a northerly wind, it is probably due to the deposition from an upper south-westerly current, often apparently proved by the movements of the upper clouds.