RAIN. At a given temperature air is capable of containing no more than a certain quantity of aqueous vapor invisibly dissolved through it, and when this amount is pres ent, it is said to be saturated. Air may at any time be brought to a state of saturation by reducing its temperature; and if it be cooled below this point, the whole of the vapor can now no longer be held in suspension; but a part of it, passing from the gaseous to the liquid state, will be deposited in dew, or float about in the form of clouds. If the temperature continues to fall, the vesicles of vapor that compose the cloud will increase in number, and begin to descend by their own weight. The largest of these falling fast est, will unite with the smaller ones they encounter in their descent, and thus drops of rain will be formed whose size will depend on the thickness and density of the cloud. The point to which the temperature of the air must be reduced in order to cause a por tion of its vapor to form cloud or dew, is called the dew-point.
Hence, the law of aqueous precipitation may be stated; Whatever lowers the temper ature of the air at any place beloW the dew-point, is a cause of rain. Various causes may conspire to effect this object, but it is chiefly brought about by the ascent of the air into the higher regions of the atmosphere, by which, being subjected to less pressure, it expands, and in doing so, its temperature falls. Ascending currents are caused by the heating of the earth's surface, for then the superincumbent air is also heated and conse quently ascends by its levity. Air-currents are forced up into the higher parts of the atmosphere by colder, drier, and therefore heavier wind-currents getting beneath them, and thus wcdgeways thrusting them upward; and the same result is accomplished by ranges of mountains opposing their masses to the onward horizontal course of the winds, so that the air, being forced up their slopes, is cooled, and its vapor liberated in showers of rain or snow. Again, the temperature of the air is lowered, and the amount of the rainfall increased, by those winds which convey the air to higher latitudes. This occurs chiefly in temperate regions, or in those tracts traversed by the return trade-winds, which . in the north temperate blow from the s.w., and in the south temperate zone, from
the north-west. The meeting and mixing of winds of different temperatures is also known to produce rain, but not nearly to the extent at one time believed. It is also increased or diminished according as the prevailing winds arrive immediately from the sea, and are therefore moist, or have previously passed over large tracts of lands, and particularly .mountain ranges, and are therefore dry. Since the rain-fall is evidently much modified by the temperature of the earth's surface over which the rain-producing winds blow, it follows that sandy deserts, by allowing solar and nocturnal radiation to take immediate effect in raising or depressing the temperature, and forests, by delaying, if not, in many cases, counteracting these effects of radiation, have each a peculiar influence on the rain-fall.
Rain is the most capricious of all the meteorological phenomena, both as regards its frequency and the amount which falls in a given time. It rarely or never falls in cer tain places, which are, on this account, designated the rainless regions of the globe—the coast of Peru, in South America; the great valley of the rivers Columbia and Colorado, in North America; Sahara, in Africa; and the desert of Gobi, in Asia, are examples; whilst, on the other hand, in such regions as Patagonia, it rains almost every Again, the quantities which have been recorded at some places to have fallen at one time, are truly enormous. In Great Britain, if an inch fall in a day, it is considered a very heavy rain. In many parts of the Highlands of Scotland, 3 in. not unfrequently fall in one day. On Dec. 5, 1863, there fell at Portree, in Skye, 12 in. in 13 hours; and on the same day. 5.2 in. fell at Drishaig, near loch Awe, where also, two days afterward, 7.12 in. fell in 30 hours. At Seathwaite, in Borrowdale, 6.62 in. fell on Nov. 27, 1345. But it is in continental, and especially tropical countries, where the heaviest single showers have recorded. The following are a few of the most remarkable: At Joyeuse, in France, 31.17 in. fell in 22 hours; at Geneva, 30 in. in 24 hours; at Gibral tar, 33 in. in 26 hours; on the hills above Bombay, 24 in. in one night; and on the Khasia hills, 30 in. on each of five successive days.