Note: Do not rely on this information. It is very old.
Rain, the moisture of the air condensed into drops too large to float in the air as cloud or mist. It generally originates in clouds, the upper part of which, being more free from dust motes than the lower, forms larger drops. Whatever tends to lower the temperature of the air below the dew-point is a cause of rain. Thus when the wind blows over a wide ocean, the rainfall is large; when the wind blows from warmer into colder regions, the rainfall is large; and when the wind blows over a mountain range, the rainfall is high on the windward, and low on the leeward side of the range.