United States

In addition to confining Pacific temperatures to the coastal margin, the Pacific Coast Ranges are high enough to make a local rain shadow in their lee, although the main barrier is the great rampart formed by the Sierra Nevada and Cascade ranges. Rainy on their western slopes and barren on the east, this mountain crest forms one of the sharpest climatic divides in the United States.

The rain shadow continues east to the Rockies, leaving the entire Intermontane Region either arid or semiarid, except where isolated ranges manage to capture leftover moisture at high altitudes. East of the Rockies the westerly drift brings mainly dry air, and as a result, the Great Plains are semiarid. Still farther east, humidity increases owing to the frequent incursion from the south of warm, moist, and unstable air from the Gulf of Mexico, which produces more precipitation in the United States than the Pacific and Atlantic oceans combined.

Although the landforms of the Interior Lowlands have been termed dull, there is nothing dull about their weather conditions. Air from the Gulf of Mexico can flow northward across the Great Plains, uninterrupted by topographical barriers, but continental Canadian air flows south by the same route, and, since these two air masses differ in every important respect, the collisions often produce disturbances of monumental violence. Plainsmen and Midwesterners are accustomed to sudden displays of furious weather—tornadoes, blizzards, hailstorms, precipitous drops and rises in temperature, and a host of other spectacular meteorological displays, sometimes dangerous but seldom boring.

The change of seasons

Most of the United States is marked by sharp differences between winter and summer. In winter, when temperature contrasts between land and water are greatest, huge masses of frigid, dry Canadian air periodically spread far south over the midcontinent, bringing cold, sparkling weather to the interior and generating great cyclonic storms where their leading edges confront the shrunken mass of warm Gulf air to the south. Although such cyclonic activity occurs throughout the year, it is most frequent and intense during the winter, parading eastward out of the Great Plains to bring the Eastern states practically all their winter precipitation. Winter temperatures differ widely, depending largely on latitude. Thus, New Orleans, Louisiana, at 30° N latitude, and International Falls, Minnesota, at 49° N, have respective January temperature averages of 55 °F (13 °C) and 3 °F (−16 °C). In the north, therefore, precipitation often comes as snow, often driven by furious winds; farther south, cold rain alternates with sleet and occasional snow. Southern Florida is the only dependably warm part of the East, though “polar outbursts” have been known to bring temperatures below 0 °F (−18 °C) as far south as Tallahassee. The main uniformity of Eastern weather in wintertime is the expectation of frequent change.

Winter climate on the West Coast is very different. A great spiraling mass of relatively warm, moist air spreads south from the Aleutian Islands of Alaska, its semipermanent front producing gloomy overcast and drizzles that hang over the Pacific Northwest all winter long, occasionally reaching southern California, which receives nearly all of its rain at this time of year. This Pacific air brings mild temperatures along the length of the coast; the average January day in Seattle, Washington, ranges between 33 and 44 °F (1 and 7 °C) and in Los Angeles between 45 and 64 °F (7 and 18 °C). In southern California, however, rains are separated by long spells of fair weather, and the whole region is a winter haven for those seeking refuge from less agreeable weather in other parts of the country. The Intermontane Region is similar to the Pacific Coast, but with much less rainfall and a considerably wider range of temperatures.

During the summer there is a reversal of the air masses, and east of the Rockies the change resembles the summer monsoon of Southeast Asia. As the midcontinent heats up, the cold Canadian air mass weakens and retreats, pushed north by an aggressive mass of warm, moist air from the Gulf. The great winter temperature differential between North and South disappears as the hot, soggy blanket spreads from the Gulf coast to the Canadian border. Heat and humidity are naturally most oppressive in the South, but there is little comfort in the more northern latitudes. In Houston, Texas, the temperature on a typical July day reaches 93 °F (34 °C), with relative humidity averaging near 75 percent, but Minneapolis, Minnesota, more than 1,000 miles (1,600 km) north, is only slightly cooler and less humid.