The New Student's Reference Work/Rain

Rain is caused by whatever lowers the temperature of the air and causes a part of the watery vapor, with which it is saturated, to assume the liquid state. Various causes may conspire to effect this object, but the most important of these originate in winds and other movements of the atmosphere. The more important principles regulating the relation of the winds to the rainfall are these: (1) When the winds have traversed a considerable extent of the ocean before reaching land, the rainfall is large; (2) when the winds, on arriving at the land, advance into higher latitudes or into colder regions, the rainfall is greatly increased because the air is brought below the point of saturation more rapidly; (3) if the winds, on arriving at the land, at once advance into warmer regions, the rainfall is small; (4) if the winds travel across a range of mountains, the rainfall is largely increased on the side facing the winds but reduced on the other side. There are certain places where rain rarely or never falls, as the coast of Peru in South America, the great valley of the River Colorado in North America, the desert of Sahara in Africa and the desert of Gobi in Asia; while, on the other hand, in such regions as Patagonia it rains nearly every day. In all places within the tropics where the trade-winds are blowing regularly and constantly rain is of very rare occurrence, the reason being that, as these winds come from a higher latitude, their temperature is increasing and consequently they take up moisture rather than part with it; and the return trade-winds, which blow above them in an opposite direction, having previously parted with their moisture, are dry and cloudless. When, however, these winds are forced up mountain ranges, as is the case on the east of Hindustan, they bring rain, which falls chiefly during the night, when the earth is coolest. In respect to rainfall Europe may be divided into two distinct regions — western Europe and the countries bordering on the Mediterranean. As the southwest winds, which are the return trades, descend to the earth and blow over the surface of Europe, it follows that the western parts, especially where mountain ranges stretch north and south, are rainy districts. Hence the rainiest regions of Europe are Norway, Ireland, the west of England and France, and Spain and Portugal. On the coast of the Mediterranean it rarely rains in summer, but frequently in winter. In Italy the quantity diminishes as we approach the south; and south of the Alps six times more rain falls with the northeast than with the southwest winds, the exact reverse of the case in England.

The manner of the distribution of rain in the United States is very different from what it is in Europe. The Rocky Mountains in North America are so high as to present an effectual barrier to the passage of the trade-winds which blow over the Gulf of Mexico, by which means they are turned northward and spread themselves over the country, especially the low basin of the Mississippi. When they have blown for some time, vast accumulations of heat and moisture take place, and sometimes great storms arise in consequence, sweeping eastward across the land and in many cases crossing the Atlantic and descending with violence on western Europe. It appears, in short, that the south winds from the Gulf of Mexico spread the moisture over the country, and the northwest winds separate this moisture from them by getting below them and forcing them into the higher regions of the atmosphere. As a consequence of this the heaviest rainfalls are in the valleys and the lightest on higher grounds. Thus the greatest amount of rain falls in Florida, the low flats of the Mississippi and along its valley, and the least quantity on the Alleghanies, especially on their higher parts, and on the high grounds of the Missouri River. In the northern states the quantity diminishes considerably, and the mode of its distribution is more similar to that of Europe.