Rainfall patterns in the United States
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[edit] Western
Major elements are prevailing westerly winds and ocean currents moving toward the equator. At high latitudes the current is warmer than land, westerly winds take up moisture and cool when moving over land. When the land is warmer than the ocean, clouds do not produce precipitation, but accrete additional moisture, generating rain eventually at higher elevations. When the land is cooler than the ocean, then westerlies cool as they move inland and rain occurs in the lowlands.
[edit] Eastern and central
Polar air masses (above 50° latitude) are distinct from lower latitude air masses. The westerlies are warm air masses that move poleward from 30°N. In the eastern US the westerlies are often laden with moisture from Gulf of Mexico and Atlantic. When polar and westerly air masses meet, precipitation occurs.
[edit] Inland continental areas
Areas not proximate to large bodies of water warm more rapidly than coastal or lakeside regions. Hot air masses can rise from the center of the continent forming a low pressure area. This low pressure zone draws water laden clouds from the coasts. When this area warms, adiabatic change occurs and ultimately precipitation. The outcome is more summer rain, and less winter precipitation.
[edit] Informal definitions
"A lot" of rainfall is a generally accepted term for 70-90 inches of rainfall yearly.
