Fig. 719.—Storm clouds.
Rain, Snow, and Dew.
Rain is produced by the condensation of vapour. “Vesicular vapours, or minute globules of water filled with air,” compose the clouds, and at last these vesicles form drops, and get heavy enough to come to the ground. Perhaps they are not sufficiently heavy to do so, and then they are absorbed or resolved into vapour again before they can get so far, because the lower strata of air are not yet saturated, and can therefore contain more moisture.
On the other hand, we may experience rain from a cloudless sky. This is no very uncommon case, and occurs in consequence of the disturbance of the upper strata when warm and cold currents come into collision and condense the vapours.
Rain is very unequally distributed. We shall find that the region of calms, which we mentioned in a former page, is also the zone of the greatest amount of rain. The heated air rises and falls back again, there being little or no wind to carry it away. The rainy season, therefore, sets in when a place enters the zone of calms. Equatorial districts have two rainy seasons, as they enter twice a year into the region of calms, but most places have only a wet and dry season, while north and south of the calm region we find rainless districts, or zones tempered by the trade winds, which are dry winds.
But if we suppose—as indeed is the case in South America—that these dry winds happen to come in contact with a cool mountain, the moisture of the air is precipitated in rain. In Australia, on the contrary, we have portions of land actually burnt up for want of rain, because the mountain chain breaks the clouds, so to speak, on a limited corner of the island, while the interior is parched. The winds also coming over India from the Bay of Bengal discharge clouds and rain in the Himalayan slopes. So we perceive that the situation of mountain chains have much to do with the rain-fall, and of necessity, therefore, with the vegetation and fertility of the land. This is another noticeable link in the great chain of Nature.
Fig. 720.—Meteorological Observatory, Pic du Midi.
Perhaps it may now be understood why westerly and south-westerly winds bring rain upon our islands, and why the counties such as Westmoreland and Cumberland and those in Wales receive more rain than any other part of the United Kingdom. Seathwaite, so well known to tourists in the lake district, has the proud position of the wettest place in these islands. We find that when the westerly wind sets in it has come across the warm Atlantic water and become laden with moisture, which, when chilled by the mountains, is precipitated as rain.
The amount of rain that falls in the United Kingdom is carefully measured by rain-gauges, some of which are extremely simple. The water is caught in a funnel-mouthed tube, and measured in a measuring glass every four-and-twenty hours. Thereby we can tell the annual rainfall in any given district, whether it be twenty inches or a hundred. One inch of rain actually means one hundred tons of water falling upon one acre of land. Therefore, if the annual report of rainfall (including all moisture) be twenty inches, we have an aggregate of 2,000 tons of water upon every acre of surface within the district. Twenty inches is a very low estimate. Some places have an annual rainfall of forty or fifty inches. In Cumberland we find 165 inches has been recorded! If we then multiply these last figures we get the enormous quantity of 16,500 tons of water upon every acre of land in the district in one year. It is reported from India that in the Khasia Hills the average is 610 inches, which must be the maximum rainfall in the world. At other places, in the north-west provinces, the fall is only seven inches. Sometimes in tropical rains we find fifteen inches of rain in a day, and that has been exceeded.