A wide range of seasonal and regional variations, from arctic in Alaska to subtropical in Florida, is the most distinctive feature of the continent-size U.S. mainland's climate. In January, temperatures in the coldest parts of the country average -12° C/10° F, but can plunge to -40° C/-40° F or even lower. In July, the thermometer can top 38° C/100° F in much of the nation.

The U.S. mainland
Weather conditions of the 48 contiguous mainland states are primarily influenced by huge air masses moving into the country from several directions, the mountain chains running north to south, and ocean currents flowing along the Atlantic and Pacific coasts.

Cold Canadian air
Polar air moving south from Canada lowers temperatures and humidity in all seasons throughout the Interior Plains. Moist maritime air from the Atlantic and Pacific oceans influences the coastal climates in various ways, lowering temperatures and humidity when the flow is from the north and raising both when from the south. The high mountain ranges of the Cordillera block moist, warm ocean air from reaching areas inland of the Pacific coast. There is no similar barrier to cold air coming down from Canada, so that the interior is markedly drier and, in winter, colder than the region's coastal areas. The San Francisco Bay area on the central California coast has a Mediterranean-type climate, mild year-round weather with a difference of about 13° C/24° F between high and low temperatures. Farther north, the climate is classically maritime in the coastal portions of the states of Oregon and Washington, the wettest areas of the U.S. Summers are cool and winters mild with heavy rainfall.

Gulf Stream
The Gulf Stream flowing north from the Caribbean Sea on the long journey to Europe gives the Atlantic and Gulf Coastal Plain a more moderate and humid climate than most of the rest of the country. Rainfall is usually plentiful, although there are occasional dry years.

Driest regions
The most arid regions are the intermountain plateaus and basins of the Cordillera and the southwestern deserts covering much of the states of Arizona and New Mexico as well as inland southern California.

Weather extremes
Weather extremes are the most pronounced in the Interior Plains. The well-watered eastern half is prone to floods every few years when rainfall is especially heavy. Farther west, the region becomes increasingly arid and has experienced historic droughts. During the 1930s, it became known as the nation's "Dust Bowl."

Regional storms
Both the Interior Plains and the Atlantic and Gulf Coastal Plain are subject to violent storms. Hurricanes originate in the Gulf of Mexico, Caribbean Sea, and subtropical Atlantic Ocean and can sweep ashore anywhere from the southern tip of Texas to New England. There are usually one to several big storms every year, and every few years a catastrophic storm that causes devastating damage. The hurricane "season" runs from late summer into November.

In the Interior Plains, essentially the same type of circular windstorm is known as a tornado. It can be equally destructive but is contained to a much smaller area with a much shorter duration than a hurricane, which can go its destructive way for a week or longer over a great expanse of territory. A tornado's life span is usually measured in minutes and it leaves a narrow path of devastation.

Alaska and Hawaii
The climates of the two non-contiguous states show distinctive characteristics of their own. Although Alaska is much farther north, its southern and western coastal areas enjoy a maritime climate similar to coastal Washington and Oregon thanks to the year-round moderating effect of air masses moving in from the Pacific Ocean. Inland Alaska is a different matter. Without the benefit of the warming sea air, its summers are brief and cool and its winters long and extremely cold.

Tropical Hawaii, in sharp contrast, enjoys one of the world's balmiest climates. It is always summer. The major weather variation is in rainfall which is much heavier on the leeward side of the islands, facing away from the prevailing winds, than on the windward sides, which are semiarid.