Country Guide

The United States is the fourth-largest country in the world, with an area of over 7.8 million sq km/3 million sq mi. It is bordered on the north by Canada and on the south by Mexico. Alaska and the Hawaiian Islands are both states of the Union, but because of their geographical detachment from the United States, they are described under separate headings.

The area described here is situated between 25° and 49° N and lies entirely outside the tropics. It includes areas with a very great range of weather and climatic conditions around the year. On occasions parts of the USA experience extremes of heat and cold characteristic of hot tropical deserts or cold Arctic continental regions. Another feature of the weather and climate of the United States is the variation of weather over quite short periods at all seasons of the year.

The reason for this variation of weather is the country's position in the belt of disturbed westerly winds so that, for much of the year, most regions of the USA are affected by cyclonic storms, or depressions, with their associated warm and cold fronts. Most of the southwest to east or northeast winds bring cloud, precipitation, and disturbed, changeable weather. The central and northeastern parts of the USA are particularly liable to sudden changes of temperature during such periods of disturbed weather.

The central part of the USA, the Great Plains, which extend from the Rockies in the west to the Appalachian Mountains in the east, is mainly flat and mostly below 600 m/2,000 ft in height. This area is wide open to the influence of two very contrasting types of air-masses. Cold polar and Arctic air can sweep southwards from the Canadian Arctic regions, and warm, humid tropical air can move north from the Caribbean and the Gulf of Mexico.

Each imports its own properties of temperature and humidity. When one air-mass replaces another, particularly during winter and spring, the temperature may change by as much as 22°C/40°F and 28°C/50°F within a few hours. Such sudden changes may also occur in the northeast of the country as far south as Virginia; farther south on the Atlantic and Gulf coasts the temperature changes are less dramatic but still produce a significant weather change.

On the Pacific coast and west of the main chain of the Rocky Mountains the influence of the Pacific Ocean makes for a more equable climate with a much smaller range of temperatures from winter to summer or from day to day. The maritime influences are to a large extent excluded from the centre of the country by the great mass of mountains and plateaux country which comprises the Rockies - the Western Cordillera.

The large size of the North American continent also makes for seasonal extremes of temperature: winter cold and summer heat. Only the Pacific shores and, to a lesser extent, the coast of the Gulf of Mexico and the Atlantic south of Virginia, benefit from the sea's moderating effect of keeping temperatures more equable around the year.

Compared with countries of western Europe in the same latitude, the United States has greater extremes of temperature, and daily or weekly changes are more noticeable. Much of the Midwest has a more extreme or continental climate than central or eastern Europe. Only Canada or the Russian Federation east of the Urals are more extreme in terms of their annual range of temperature.

Some parts of the USA are liable to experience two particularly violent and destructive weather phenomena: hurricanes and tornadoes. Hurricanes affect the southeastern states bordering the Gulf of Mexico and the Atlantic once or twice in most years. These tropical storms, which bring very strong winds and torrential rainfall, move northeastwards from the Caribbean region before dying out in mid-Atlantic. They are described in more detail for the Caribbean islands.

A tornado is a very much more local and destructive storm of wind, often described as a 'whirlwind' or, in the USA, as a 'twister'. Tornadoes can cause almost complete destruction of buildings on a narrow path not more than a few hundred yards wide. They mainly occur in spring and summer on days when there are violent thunderstorms associated with rapid changes of temperature along, or near, a cold front.

Much of the western third of the United States consists of a series of high mountain chains and interior plateaux and basins which are collectively termed the Rockies or the Western Cordillera. Weather and climate are here very variable from place to place depending on altitude and the degree of exposure or shelter.

There are many lofty mountain ranges with peaks above 4,250 m/14,000 ft, extensive high plateaux between 1,200 m/4,000 ft and 2,000 m/7,000 ft and some small areas, such as Death Valley and the Salton Sink in southern California, which are below sea level. This makes for a great variety of climatic conditions with some very wet and snowy mountain regions and some semi-arid or even desert lowlands with great extremes of temperature.

By contrast, in the central plains of the USA and, to a lesser extent, on the Atlantic coast, changes of weather and climate are much more gradual, and almost imperceptible, over great distances.

For a more detailed account of the weather and climate of this large country it is convenient to divide the USA into the following climatic regions, broadly coinciding with particular groups of states: the northeastern states, the southern Atlantic states, the Midwest or northern interior, the southern interior and the Gulf states, the states of the Rocky Mountains regions, the states of the Pacific northwest, California, and Alaska.

Climatic tables for the more important and representative places are included with the description of the appropriate region.

Click on the links below for a detailed look at the United States regions:

USA
Northeastern States (New England)
southern Atlantic States
Northern Interior (Midwest)
Southern Interior and Gulf States
States of the Rocky Mountains Region
States of the Pacific Northwest
California
Alaska
Hawaii