The peculiarity of the rain-fall of the basin of the Mediterranean depends on its prox imity to the burning sands of Africa, a predominance of northerly winds, and the position ofgAlte Pyrenees and Spanish sierras to the w., on which the s.w. winds dis charge their rains before arriving on the n. shores of the Mediterranean. In the valley of the Rhone four times more rain falls in autumn than in summer; and s. of the Alps six times more rain falls with the n.e. than with the s.w. winds, being the reverse of what takes place in England. In Italy the quantity diminishes as we approach the south. On the coasts of the Mediterranean it rarely rains in summer, but frequently in winter. -In the valley of • the Rhone the annual fall ranges from 20 in. at its mouth to 63 in. at St. Rambert, the average being 30 inches. This is also the average of the valley of the Po; but on ascending to the Alps it rises, as at Tolmezzo, to 96 inches.
The rain-fall in the w. of the American continent is distributed similarly to that' of Europe—the amount being dependent on the physical configuration of the surface over which the westerly winds blow. The yearly amount increases as we proceed northward; thus at San Francisco it is 21 in.; at Fort Readhig, 29 in. ; at Fort Oxford, 72 in.; at Fort Vancouver, 47 in.; at Astoria, 86 in.; at Stcilacoom (Washington territory), 54 in. ; and at Sitka, in Alaska, 82 inches.
But in the United States the manner of the distribution of the rain is very different from that of Europe. The United States are dependent for their rain not on the Pacific ocean, but on the gulf of Mexibo. There can be little doubt that, but for the high range of the Rocky mountains in Central America, the greater part:of the states would be an arid waste. These mountains are so high as to present an effectual barrier to the passage of the trade-winds which blow over the gulf of Mexico; they are, on this account, turned northward, and spread themselves over the states, especially over the low basin of the Mississippi. These winds being characterized great heat, and loaded with much moisture from the warm waters of the gulf of Mexico, tend to disturb the statical equilibrium of the atmosphere. When they have blown for some time, vast accumula
tions of heat and moisture take place, the equilibrium is destroyed, a great storm arises in consequence, sweeping eastward over the states, and in many cases crossing the Atlantic and descending with violence on western Europe. In the states, the southerly winds preceding the storm give place to the dry n.w. winds, which rapidly clear the sky and bring brilliant bracing weather in their train. It appears, in short, that the s. winds from the gulf of Mexico spread the moisture over the states, and the n.w. wind disen gages this moisture from them by getting below them, by their greater density, and thrusting them into the higher regions of the atmosphere. If this be the case, as the phenomena seem to warrant, then the heaviest rain-falls will be in the valleys, and the least on the higher grounds—a mode of distribution quite different from what prevails in Europe. And such is really the case, for the greatest amount of rain falls in Florida, the low flats of the Mississippi, then along its valley, and lastly in Iowa, or in that remarkable depression at the head of the river; and the least quantities ou the Alleghanies, especially on their higher parts, and on the high grounds of the Missouri district. The following figures,,giving the average annual amount in inches, show this in a clear light: Pensacola, 57; Fort Brooke, 55; and Fort Pierce, 63—in Florida: Monroeville, 66; and Mobile, 64—in Alabama: Natchez, 58; Jackson, 53—in Missis sippi: Rapides, 68: New Orleans, 52—in Louisiana: Savannah, 48—in Georgia: Nash ville, 55—in Tennessee: Dubuque, 33—in Iowa. At Athens, in Georgia, s. of the Alleghanies, the amount is 36 in. ; at Alexandria, in Virginia, also 36 in.; and at Jeffer son, in Missouri, 38 incites. In the northern states the quantity diminishes at most places to between 27 and 45 in., and the mode of its distribution becomes assimilated to that of Europe.
When rain-drops fall through a stratum of air below 32°, they become frozen, and form hail (q.v.). When the vesicles arc formed in air under 32°, snow is the result.