Precipitation Patterns in North America

Both annual precipitation and seasonal distribution of precipitation depend on: (1) The moisture content of the air and vertical motions associated with surface heating and cooling, (2) major pressure systems, and (3) frontal and orographic lifting. This lifting has its greatest effect when the prevailing moist wind currents blow across major mountain systems.

In North America, the greatest precipitation is on the Northern Pacific coastal plains and the western slopes of the mountains, due to the influx of moist air from the Pacific Ocean. Maximum fall is on the Pacific Northwest coast of the United States, with amounts decreasing both north and south of this region. The inland valleys receive less precipitation than the coastal plains and coastal mountains. Along the western slopes of the next major ranges, such as the Sierra-Cascades, further lifting of the moist air again causes an increase in the total precipitation. A third, and final, lifting of these westerlies occurs on the western slopes of the Rocky Mountains, which extract most of the remaining precipitable moisture.

In each of these cases of orographic lifting, there is a decrease in precipitation activity as the air flows across the crests. Previous precipitation has left the air less moist. The lifting force has ceased, and often there is subsidence on the leeward side, which further reduces the degree of saturation. Such a leeward area is said to lie in a rain shadow, a term derived from its similarity to the shadows cast by the western mountains as the sun goes down. This explains why the inland valleys receive less precipitation than the coastal plains and mountains. The Great Basin area in the United States lies in such a rain shadow, and ranges from semi-desert to desert.

East of the Rocky Mountains, air of Pacific origin has become relatively dry, and its importance as a source of precipitation is replaced by moist air from the Gulf of Mexico. The influence of Gulf air extends northward well into Canada. Annual precipitation increases to the east and south under the more frequent intrusions of moist air from the Gulf and the Atlantic. The greatest annual precipitation is along the Gulf coast and the southern end of the Appalachians.

In most areas of the continent, there is considerable variation in annual rainfall. Wet and dry years may occur irregularly in poorly defined patterns, or as wet and dry fluctuations of variable duration. Within any one climatic region, a characteristic variation can usually be identified. Common ones are: Normally moist but with occasional critically dry years; typically dry with only infrequent relief; or longer period fluctuations of alternating wet years and dry years. The seasonal distribution of precipitation varies widely over the continent and is often as important in fire weather as the total annual amount.