United States of America

United States of America

The United States is the fourth-largest country in the world, with an area of over 9.8 million sq km/3.7 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 to 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 - a part of the North American 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 Russia 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 Sea 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.

The Northeastern States (New England)

Including (with towns and cities in parentheses) Maine (Portland), New Hampshire (Concord), Vermont (Montpelier), Massachusetts (Boston), Rhode Island (Providence), Connecticut (Hartford), Delaware (Dover), New Jersey (Newark), Maryland (Baltimore), the eastern parts of the larger states of New York (New York City) and Pennsylvania (Philadelphia), and the District of Columbia (Washington DC).

This region can experience changeable weather around the year with moderate amounts of precipitation in all months. Towards the north the winters are wet and usually snowy, but south of New York summer tends to be slightly wetter. Summer heat waves can produce temperatures over 38°C/100°F for a few days and such hot spells are usually made the more uncomfortable because the humidity on or near the coast is high. In the great cities of this densely populated area heat waves are even more uncomfortable for the temperatures in the city streets are often a few degrees higher than those recorded at meteorological stations, usually in large parks or rural districts.

Very cold spells can affect the whole region from time to time in winter or even in spring, with very severe snowfalls likely in the north. The region includes the northern Appalachian Mountains, whose heights rise to between 1,200-2,000 m/4,000-6,600 ft. At these higher levels winters can be prolonged and severe and there are many opportunities for winter sports. In summer the mountains provide resorts where relief can be obtained from the heat and humidity of the extensive coastal plains which contain the largest cities. This region has a more extreme or continental climate than the British Isles; summers are warmer and winters colder. It is also more extreme in other respects; day-to-day changes in temperature can be much greater and individual falls of rain and snow are often heavier than in most parts of Britain.

Although this is one of the less sunny parts of the United States it receives more sunshine round the year than most of northwest Europe. Daily sunshine hours on the coast, and at lower levels inland, average from four to five in winter and as much as nine or ten in summer. Locally, sunshine may be reduced on the coast by fog both in summer and winter; inland, or in the larger cities, winter fog may reduce the sunshine. Some valleys in the Appalachians are particularly foggy due to a combination of industrial pollution and valley mists in winter.

Characteristic weather for this region is represented by the tables for Baltimore, Boston, New York City, Philadelphia, Portland, Providence and Washington.

The Southern Atlantic States

Including (with towns and cities in parentheses) Florida (Miami, Orlando), Georgia (Atlanta), North Carolina, South Carolina (Charleston), Virginia (Richmond, Norfolk), West Virginia (Charleston).

Characteristic weather for this region is represented by the tables for Charleston, Miami and Norfolk.

There is a gradual increase in the warmth of winter southwards along the Atlantic coast, so that Florida has an almost tropical climate with only very rare and short cold spells when frost and snow occur. The northern part of Virginia and much of West Virginia in the Appalachian Mountains have winter conditions more typical of the northeastern region. On the other hand there is much less difference between the north and south of this region in terms of summer temperatures. The contrast is rather in the length of the summer season and the warmth of spring and autumn. Florida has a very oceanic climate, being much influenced by the surrounding warm Atlantic waters, so that summer temperatures do not reach the heights sometimes recorded as far north as New York. On the other hand Florida and the coastal lowlands of Georgia and the Carolinas have mild winters and frost and snow are much less frequent than in Washington DC, or North Virginia. In northern Florida and southern Georgia snow only falls every ten or fifteen years, but in southern Virginia it falls in at least two years out of three.

The proportion of the annual rainfall coming in the summer months increases southwards and a significant amount of this is associated with thunderstorms. Florida has more thunderstorms than any other state in the USA - over a hundred a year in parts of the state. This region is also affected by hurricanes, or less severe tropical storms, at least once or twice a year and they account for some of the heavier falls of rain in the months July to October. This is the sunniest part of the eastern United States, with sunshine hours averaging from about six in winter to as much as nine or ten in summer. Florida is particularly sunny in winter which, combined with its much warmer temperature at this time, makes it a popular winter resort. The summer months in Florida are slightly less sunny than in areas farther north because of the regular afternoon build-up of cloud leading to thunderstorms.

The Northern Interior (the Midwest)

Including (with towns and cities in parentheses) all those states between the western Appalachians and the foothills of the Rocky Mountains approximately to the north of 37°N: western Pennsylvania (Pittsburgh), North Dakota (Bismarck), South Dakota (Sioux Falls), Minnesota (Duluth, Minneapolis, St Paul), Wisconsin (Milwaukee), Michigan (Detroit), Nebraska (Omaha), Iowa (Des Moines), Illinois (Chicago), Ohio (Cincinnati, Columbus), Kansas (Kansas City, Dodge City), Missouri (St Louis), Indiana (Indianapolis), Kentucky (Louisville).

Characteristic weather for this region is represented by the tables for Charleston, Chicago, Columbus, Des Moines, Detroit, Dodge City, Duluth, Indianapolis, Kansas City, Louisville, Minneapolis, Omaha, Pittsburgh and St Louis.

There are extensive plains in the valleys of the Ohio, Missouri, and northern Mississippi rivers. Most of the region is below 600 m/2,000 ft and much of it is below 300 m/1,000 ft. It has the most continental climate of any part of the United States. Winters are cold, summers warm with quite frequent heat waves and drought. There is a gradual increase in summer warmth southwards, but a more noticeable increase in the severity and length of winter northwards. Winter precipitation is light, particularly in the west of this region, and much of it falls as snow. In the north, along the Canadian border and around the Great Lakes, winter conditions can occasionally be very severe with blizzards, as very cold air sweeps south from the Canadian Arctic.

There is a gradual decrease in the amount of annual precipitation westwards, and the western plains suffer most frequently from drought. The eastern states of Michigan, Indiana, Illinois, and Kentucky have a heavier annual precipitation with much wetter winters than those farther west. An unpleasant feature of the weather in the north of this region is the frequent occurrence of freezing rain in winter as rain falls from a warm air-mass onto ground previously frozen hard. This is a serious danger to road traffic and may occur on as many as five to ten days a year.

Almost the whole of the region has at least one winter month with an average temperature below freezing but, since the weather is frequently changeable, unseasonably mild conditions may occur for a few days even in midwinter. Clear skies and abundant sunshine are a feature of the weather for much of the time, even in winter. Daily sunshine hours average from four to five in winter and as much as ten or eleven in summer.

A feature of the western part of this region at the foot of the Rockies is the occasional warm dry wind, the chinook, which raises temperature and quickly melts snow in winter and spring. This is a föhn-type wind, warmed as the air descends to the east of the mountains.

The Southern Interior and Gulf States

Including (with towns and cities in parentheses) Oklahoma (Oklahoma City), Arkansas (Little Rock), Tennessee (Nashville), Texas (Dallas, Houston), Louisiana (New Orleans), Mississippi (Jackson), Alabama (Birmingham).

This large region includes the states roughly south of 37°N between the Rockies and the Appalachians and those with a coastline on the Gulf of Mexico. The general sequence of weather and climate around the year is rather similar to that in the Midwest; but, being in a more southerly latitude and more open to the flow of warm tropical air from the Atlantic and the Gulf of Mexico, the winters are both warmer and shorter than those farther north. It is rare for a winter month here to have an average temperature below freezing point, but occasional very cold spells may last for a few days when Arctic air penetrates this region from the north. Occasional snow and frost can occur as far south as the shores of the Gulf of Mexico, and in western Texas such cold spells are more frequent and more severe. Summers are a little warmer than farther north, but the increasing length of the summer period and the warmth of spring and autumn are more noticeable.

The eastern part of this region is much wetter than the west. Annual precipitation is almost everywhere between 1,000 mm/40 in and 1,250 mm/50 in in the east, but it falls as low as 350 mm/15 in and 500m/20 in in the west. Summer is the wettest season and thunderstorms are very frequent in the east of this region. Parts of the states of Tennessee and Alabama include the southern Appalachian Mountains; here winter precipitation is heavier and the weather and climate are more like those of the eastern Atlantic states. Most of the region has a sunny climate, particularly the western parts of Texas and Oklahoma. Sunshine hours a day average from five to six in winter to ten or eleven in summer. The summer heat is rarely unpleasant, except along the coast of the Gulf of Mexico, where the combination of heat and humidity can be trying. Compare the afternoon relative humidity at New Orleans with that at Dallas or Oklahoma City in the tables.

Weather characteristic of this region is also shown by the climactic tables for Atlanta, Birmingham, Houston, Little Rock and Nashville.

This region is the most affected by weather hazards: hurricanes and tornadoes, mentioned in the general account of the United States.

The States of the Rocky Mountains Region

Including the mountainous country comprising all or large parts of Montana (Great Falls), Idaho, Wyoming (Cheyenne), Nevada (Reno, Las Vegas), Utah (Salt Lake City), Colorado (Denver), Arizona (Phoenix), New Mexico (Albuquerque, Santa Fe), western Texas (El Paso).

It is possible to make a broad distinction between the three northern states of Idaho, Montana, and Wyoming, and the rest of the mountainous country of the great Western Cordillera. In general these northern states are cooler in both winter and summer, have a much longer cold season, and are generally wetter than those farther south. Within this whole region, however, there are so many local variations of temperature and precipitation, because of the range of altitude, that one can find cold spots in the southern parts of the region and some dry areas in the north. The tables for this region give a good indication of the range of altitude and its effect on temperature in each month. For example, there is no great difference between precipitation and temperatures for Cheyenne in Wyoming and Santa Fé in New Mexico, both of which are above 1,800 m/6,000 ft. On the other hand, temperatures are very much higher in all months at Phoenix (Arizona) at 330 m/1,083 ft than at Santa Fé.

Much of this region has a low precipitation, particularly in the south where large areas of Arizona, New Mexico, Utah, and Colorado are desert or semi-desert with annual precipitation below 300 mm/12 in or even 200 mm/8 in. This is a consequence of the rain shadow of the western mountains in California, which extract much of the moisture from air which comes in from the Pacific. It is also a result of the frequent and persistent anticyclonic weather which prevails in this region.

The table for El Paso, in Texas, has been included with those for this region, for this part of western Texas is mountainous.

The highest-recorded and lowest-recorded temperatures in the tables show that some extremely high and also extremely low temperatures have been recorded at different places in this mountain region: very low temperatures in the north and very high temperatures in the south. The southern part has the sunniest climate in the United States; both Phoenix and Las Vegas have about eight hours sunshine a day in winter and between twelve and thirteen hours in the summer months. The high summer temperatures in this area are made more bearable by the low humidity and the climate of this whole region is generally healthy. Under extreme conditions, however, both heat stress and cold stress can be experienced.

See also the climactic table for Salt Lake City.

The States of the Pacific Northwest

Including (with towns and cities in parentheses) Washington (Seattle, Spokane), Oregon (Portland), western Idaho (Boise).

This climatic region has weather and climate very similar to that of northwest Europe, and Britain in particular. Some parts of the state of Idaho in the Rocky Mountain region have similarities with it. This region includes a number of high mountains, part of the western cordillera, which rise to over 4,250 m/14,000 ft and are snow-covered throughout the year. Thus the higher parts of these two states have some similarity with the weather and climate of the northern part of the Rocky Mountains.

The coastal districts have the smallest annual range of temperatures anywhere in the United States; winters are mild and summers only moderately warm. It is a cloudy region and the least sunny part of the USA, with a large number of rainy days. Some of the mountain areas are very wet with as much as 2,500-3,000 mm/100-120 in of precipitation a year. By contrast, in the sheltered valleys and in some of the extensive high plateaux districts, annual precipitation is as low as 300m/12 in. This is also the one region of the country where winter is the wettest season, although some rain and changeable weather can occur in all months. There is no real summer drought such as occurs farther south in California.

The tables for Seattle and Portland are representative of the coastal districts, while those for Spokane and Boise are typical of areas farther inland at moderate height. Sea fog in summer can affect some of the coastal regions and reduce sunshine and lower temperature.

The region owes its wetness and mildness to the influence of the Pacific Ocean and the frequent passage of cyclonic depressions which originate on the North Pacific polar front. The air-masses involved in these depressions do not have the extreme conditions of temperature which give so much of the interior of the United States a continental type of climate with frequent alternations of warm and very cold weather as well as a great contrast between summer heat and winter cold.

The average number of hours of sunshine a day ranges from two to three in winter and nine to ten in summer on the coast. Inland and at higher levels the winters are sunnier with as much as five to six hours a day.

California

California enjoys a very distinctive climate of the Mediterranean type and this climatic region is almost coincident with the state boundary. The northern coast of California has a climate similar to the coastal districts of the northwest but there is a gradual increase in summer temperature southwards and a decrease of rainfall until the summers become completely dry in central and southern California. In the southeast of the state precipitation decreases until conditions become similar to those of the desert regions of neighbouring Arizona and northern Mexico. Most of California enjoys mild and moderately wet winters and warm to hot and very dry summers. There are some large mountain regions within the state: the Coast Ranges and the Sierra Nevada, which rise to over 4,400 m/14,500 ft. These mountains have a heavy precipitation and, at higher levels, much of this is snow so there are many opportunities for winter sports within a state which is often associated with sun, sea and warmth.

The tables for San Francisco, Los Angeles and San Diego are representative of the coastal region. San Francisco is unusual in having cool to mild summers. This is a very local feature caused by the frequent sea fog which sweeps into the bay through the Golden Gate gap in the Coast Ranges. Elsewhere this sea fog rarely affects the land but the cool waters of the California current help to maintain much lower summer temperatures on the coast than inland. In the great Central Valley of California and in the desert areas in the southeast, summer temperatures are much higher. Frost and snow are very rare occurrences on the coast but occur more frequently inland in winter. The table for Death Valley shows the extremely high temperatures here in summer. This place has not only experienced the highest temperatures in the United States but some of the highest recorded anywhere in the world.

The winter precipitation of California is caused by the same sequence of cyclonic depressions as bring rain to the sates of the northwest. In summer such disturbances are pushed farther north by the almost permanent presence of the North Pacific subtropical anticyclone which brings the calm, settled and sunny weather. This anticyclone is also responsible, however, for the most unpleasant and dangerous weather phenomenon which particularly affects the great urban area of Los Angeles' urban smog. This is a combination of fog and pollution from automobiles and industry. The pollution is trapped beneath a layer of warm air which overlies the coast; the light winds are unable to disperse it beyond the encircling hills and mountains.

Apart from this particular hazard most of California has a very agreeable and healthy climate throughout the year: sunny and dry with only short periods of relatively cold weather in winter. The visitor should obviously avoid going to such 'hot spots' as Death Valley without taking sensible precautions, or ignoring the fact that very heavy snowstorms can occur in the mountains of California. California is one of the sunniest states in the country. Sunshine hours a day average from seven to eight in winter to as many as twelve to fourteen in summer in the driest regions inland. On the coast they are rather less: from six to seven in winter and nine to ten in summer. The reduction in summer sunshine on the coast is because of sea fog.

Alaska

Alaska is one of the states of the USA but is described separately here because of its geographical separation from the rest of the continental United States. Twice as large as Texas, it is the largest state of the Union. It comprises the northwestern lands of the North American continent, between 60° and 72°N, and two separate and distinct appendages. There is a narrow mountainous coastal strip with numerous offshore islands extending south to 55°N to give Alaska a long land border into the North Pacific between 50° and 55°N towards the coast of Siberia.

Much of Alaska is mountainous as it includes the northern ranges of the Rocky Mountains, with some of the highest mountains in North America. Large and impressive glaciers descend from these mountains almost to sea level. Inland there are extensive lowlands including the valleys of the Yukon and Porcupine rivers.

The interior and north coast of Alaska have a cold Arctic or sub-Arctic climate similar to that described for northern Canada. The mountains have permanent snow and ice and the lowlands suffer from permafrost. The rivers remain frozen from September until late May. The table for Fairbanks is representative of much of interior Alaska. The short summer can be surprisingly warm for the latitude and this is helped by the long hours of daylight and, in fine weather, the prolonged sunshine. Winters are long and very severe. Wind chill is a serious hazard when low temperatures are accompanied by strong winds. The low annual precipitation is largely snow, but summer is the wettest season and some rain occurs then.

The table for Barrow on the shores of the Arctic Ocean shows that summer here is colder and shorter. The sea is frozen for most of the year or partially blocked by drift ice in summer.

On the Pacific coast the weather and climate are rather different. This is a region of much heavier precipitation with more changeable and disturbed weather throughout the year. Summer temperatures are cool and may be less warm than inland. Winters are cool but mild compared with the very low temperatures inland. Weather and climate here are very much influenced by the frequent frontal depressions which develop in the North Pacific between Japan and the Aleutian Islands. Cloud and fog are frequent at all seasons. The table for Anchorage, in a deep-sheltered bay on the west coast, shows warmer winter temperatures than inland. Anchorage, however, is much colder than the offshore islands and the Aleutians, which benefit from the relatively warm sea temperatures of the Pacific. The coastal region and the islands have weather and climate very similar to that experienced on the coasts of Norway. The climatic table for Atka is representative of the weather and climate of the Aleutian Islands.

Hawaii

Including (with towns and cities in parentheses) Hawaii (Pepe'ekeo), Maui, Oahu (Honolulu), Kauai, Molokai, Lanai, Kahoolawe, Nilhau.

These islands are a state of the USA; they are situated between 18° and 22°N in the central Pacific, almost midway between North America and Japan. In area the islands are rather smaller than Wales or the state of Massachusetts; about 16,400 sq km/6,400 sq mi. There are eight main islands; all are hilly and mountainous and consist of both extinct and active volcanoes. On the islands of Hawaii and Maui these peaks exceed 3,000 m/10,000 ft in height.

The islands have a tropical oceanic climate with temperatures much moderated both by altitude and by regular sea breezes at lower levels. As the tables show, there is no great difference in average daily temperatures around the year and, although warm or even hot, the combination of temperature and humidity is rarely unpleasant. There are some remarkable differences in annual rainfall between the southwest coasts, which are relatively dry (see the table for Honolulu), and the northeastern coasts exposed to the trade winds (see the table for Pepe'ekeo), which receive much heavier rainfall in all months. In the drier parts of the islands the wettest season is the time of low sun between October and March, which is rather unusual in the tropics.

Some mountain slopes on the island of Hawaii are amongst the wettest regions in the world, with an annual rainfall exceeding 10,000 mm/400 in. The difference in the amount of cloud between the wetter and drier areas causes the average daily sunshine hours to vary between seven and ten hours throughout the year at Honolulu to a mere four to five hours at the wetter places. The islands are occasionally affected by tropical cyclones between May and November, which otherwise is the drier time of year. Such severe storms, however, are less frequent here than in the Caribbean or the South China Sea and west Pacific.

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