The term "United States", when used in the geographical sense, means the continental United States, Alaska, Hawaii, Puerto Rico, Guam, and the U.S. Virgin Islands.The United States shares land borders with Canada and Mexico and maritime (water) borders with Russia, Cuba, and The Bahamas in addition to Canada and Mexico.

There are several U.S. territories in the Pacific and Caribbean.

Area

From 1989 through 1996, the total area of the US was listed as 9,372,610 km² (3,618,780 sq mi) (land + inland water only). The listed total area changed to 9,629,091 km² (3,717,813 sq mi) in 1997 (Great Lakes area and coastal waters added), to 9,631,418 km² (3,718,711 sq mi) in 2004, to 9,631,420 km² (3,718,710 sq mi) in 2006, and to 9,826,630 km² (3,794,080 sq mi) in 2007 (territorial waters added). Currently, the CIA World Factbook gives 9,826,675 km² (3,794,100 sq mi), the United Nations Statistics Division gives 9,629,091 km² (3,717,813 sq mi), and the Encyclopædia Britannica gives 9,522,055 km² (3,676,486 sq mi)(Great Lakes area included but not coastal waters).^[4] All sources considers only 50 states and Federal District, but excluding overseas territories.

By total area (water as well as land), the United States is either slightly larger or smaller than the People's Republic of China, making it the world's third or fourth largest country. China and the United States are smaller than Russia and Canada in total area, but are larger than Brazil. By land area only (exclusive of waters), the United States is the world's third largest country, after Russia and China, with Canada in fourth. Whether the US or China is the third largest country by total area depends on two factors: (1) The validity of China's claim on Aksai Chin and Trans-Karakoram Tract. Both these territories are also claimed by India, so are not counted; and (2) How US calculates its own surface area. Since the initial publishing of the World Factbook, the CIA has updated the total area of United States a number of times.

General Characteristics

The United States shares land borders with Canada (to the north) and Mexico (to the south), and a territorial water border with Russia in the northwest, and two territorial water borders in the southeast between Florida and Cuba, and Florida and the Bahamas. The contiguous forty-eight states are otherwise bounded by the Pacific Ocean on the west, the Atlantic Ocean on the east, and the Gulf of Mexico to the southeast. Alaska borders the Pacific Ocean to the south, the Bering Strait to the west, and the Arctic Ocean to the north, while Hawaii lies far to the southwest of the mainland in the Pacific Ocean.

Forty-eight of the states are in the single region between Canada and Mexico; this group is referred to, with varying precision and formality, as the continental or contiguous United States, and as the Lower 48. Alaska, which is not included in the term contiguous United States, is at the northwestern end of North America, separated from the Lower 48 by Canada.

The capital city, Washington, District of Columbia, is a federal district located on land donated by the state of Maryland. (Virginia had also donated land, but it was returned in 1849.) The United States also has overseas territories with varying levels of independence and organization.

Physiographic Divisions

The eastern United States has a varied topography. A broad, flat coastal plain lines the Atlantic and Gulf shores from the Texas-Mexico border to New York City, and includes the Florida peninsula. Areas further inland feature rolling hills and temperate forests. The Appalachian Mountains form a line of low mountains separating the eastern seaboard from the Great Lakes and the Mississippi Basin.

The five Great Lakes are located in the north-central portion of the country, four of them forming part of the border with Canada, only Lake Michigan situated entirely within United States. The southeast United States contain subtropical forests and, near the gulf coast, mangrove wetlands, especially in Florida. West of the Appalachians lies the Mississippi River basin and two large eastern tributaries, the Ohio River and the Tennessee River. The Ohio and Tennessee Valleys and the Midwest consist largely of rolling hills and productive farmland, stretching south to the Gulf Coast.

The Great Plains lie west of the Mississippi River and east of the Rocky Mountains. A large portion of the country's agricultural products are grown in the Great Plains. Before their general conversion to farmland, the Great Plains were noted for their extensive grasslands, from tallgrass prairie in the eastern plains to shortgrass steppe in the western High Plains. Elevation rises gradually from less than a few hundred feet near the Mississippi River to more than a mile high in the High Plains. The generally low relief of the plains is broken in several places, most notably in the Ozark and Ouachita Mountains, which form the U.S. Interior Highlands, the only major mountainous region between the Rocky Mountains and the Appalachian Mountains.

The Great Plains come to an abrupt end at the Rocky Mountains. The Rocky Mountains form a large portion of the Western U.S., entering from Canada and stretching nearly to Mexico. The Rocky Mountain region is the highest region of the United States by average elevation. The Rocky Mountains generally contain fairly mild slopes and wider peaks compared to some of the other great mountain ranges, with a few exceptions (such as the Teton Mountains in Wyoming and the Sawatch Range in Colorado). The highest peaks of the Rockies are found in Colorado, the tallest peak being Mount Elbert at 14,440 ft (4,400 m). The Rocky Mountains contain some of the most spectacular, and well known scenery in the world. In addition, instead of being one generally continuous and solid mountain range, it is broken up into a number of smaller, intermittent mountain ranges, forming a large series of basins and valleys.

West of the Rocky Mountains lies the Intermontane Plateaus (also known as the Intermountain West), a large, arid desert lying between the Rockies and the Cascades and Sierra Nevada ranges. The large southern portion, known as the Great Basin, consists of salt flats, drainage basins, and many small north-south mountain ranges. The Southwest is predominantly a low-lying desert region. A portion known as the Colorado Plateau, centered around the Four Corners region, is considered to have some of the most spectacular scenery in the world. It is accentuated in such national parks as Grand Canyon, Arches, Mesa Verde and Bryce Canyon, among others.

These areas contain some spectacular scenery as well, as evidenced by such national parks as Yosemite and Mount Rainier. West of the Cascades and Sierra Nevada is a series of valleys, such as the Central Valley in California and the Willamette Valley in Oregon. Along the coast is a series of low mountain ranges known as the Pacific Coast Ranges. Much of the Pacific Northwest coast is inhabited by some of the densest vegetation outside of the Tropics, and also the tallest trees in the world (the Redwoods).

Alaska contains some of the most dramatic and untapped scenery in the country. Tall, prominent mountain ranges rise up sharply from broad, flat tundra plains. On the islands off the south and southwest coast are many volcanoes. Hawaii, far to the south of Alaska in the Pacific Ocean, is a chain of tropical, volcanic islands, popular as a tourist destination for many from East Asia and the mainland United States.

Climate

Due to its large size and wide range of geographic features, the United States contains examples of nearly every global climate. The climate is temperate in most areas, subtropical in the Southern United States, tropical in Hawaii and southern Florida, polar in Alaska, semiarid in the Great Plains west of the 100th meridian, Mediterranean in coastal California and arid in the Great Basin. Its comparatively favorable agricultural climate contributed (in part) to the country's rise as a world power, with infrequent severe drought in the major agricultural regions, a general lack of widespread flooding, and a mainly temperate climate that receives adequate precipitation.

The main influence on U.S. weather is the polar jet stream, which brings in large low pressure systems from the northern Pacific Ocean. The Cascade Range, Sierra Nevada, and Rocky Mountains pick up most of the moisture from these systems as they move eastward. Greatly diminished by the time they reach the High Plains, much of the moisture has been sapped by the orographic effect as it is forced over several mountain ranges.

Once it moves over the Great Plains, uninterrupted flat land allows it to reorganize and can lead to major clashes of air masses. In addition, moisture from the Gulf of Mexico is often drawn northward. When combined with a powerful jet stream, this can lead to violent thunderstorms, especially during spring and summer. Sometimes during late winter and spring these storms can combine with another low pressure system as they move up the East Coast and into the Atlantic Ocean, where they intensify rapidly.

These storms are known as Nor'easters and often bring widespread, heavy snowfall to the Mid-Atlantic and New England. The uninterrupted flat grasslands of the Great Plains also leads to some of the most extreme climate swings in the world. Temperatures can rise or drop rapidly and winds can be extreme, and the flow of heat waves or Arctic air masses often advance uninterrupted through the plains.

The Great Basin and Columbia Plateau (the Intermontane Plateaus) are arid or semiarid regions that lie in the rain shadow of the Cascades and Sierra Nevada. Precipitation averages less than 15 inches (38 cm). The Southwest is a hot desert, with temperatures exceeding 100 °F (37.8 °C) for several weeks at a time in summer. The Southwest and the Great Basin are also affected by the monsoon from the Gulf of California from July–September, which brings localized but often severe thunderstorms to the region.

Much of California consists of a Mediterranean climate, with sometimes excessive rainfall from October–April and nearly no rain the rest of the year. In the Pacific Northwest rain falls year-round, but is much heavier during winter and spring. The mountains of the west receive abundant precipitation and very heavy snowfall. The Cascades are one of the snowiest places in the world, with some places averaging over 600 inches (1,524 cm) of snow annually, but the lower elevations closer to the coast receive very little snow.

Another significant (but localized) weather effect is lake-effect snow that falls south and east of the Great Lakes, especially in the hilly portions of the Upper Peninsula of Michigan and on the Tug Hill Plateau in New York. The lake effect dumped well over 5 feet (1.52 m) of snow in the area of Buffalo, New York throughout the 2006-2007 winter. The Wasatch Front and Wasatch Range in Utah can also receive significant lake effect accumulations from the Great Salt Lake.

Natural Disasters

The United States is affected by a variety of natural disasters yearly. Although drought is rare, it has occasionally caused major disruption, such as during the Dust Bowl (1931–1942). Farmland failed throughout the Plains, entire regions were virtually depopulated, and dust storms ravaged the land.

Tornadoes and Hurricanes

The Great Plains and Midwest, due to the contrasting air masses, sees frequent severe thunderstorms and tornado outbreaks during spring and summer with around 1,000 tornadoes occurring each year. The strip of land from north Texas north to Kansas and Nebraska and east into Tennessee is known as Tornado Alley, where many houses have tornado shelters and many towns have tornado sirens.

Hurricane are another natural disaster found in the US, which can hit anywhere along the Gulf Coast or the Atlantic Coast as well as Hawaii in the Pacific Ocean. Particularly at risk are the central and southern Texas coasts, the area from southeastern Louisiana east to the Florida Panhandle, peninsular Florida, and the Outer Banks of North Carolina, although any portion of the coast could be struck.

Hurricane season runs from June 1 to November 30, with a peak from mid-August through early October. Some of the more devastating hurricanes have included the Galveston Hurricane of 1900, Hurricane Andrew in 1992, and Hurricane Katrina in 2005. The remnants of tropical cyclones from the Eastern Pacific also occasionally impact the western United States, bringing moderate to heavy rainfall.

Flooding

Occasional severe flooding is experienced. There was the Great Mississippi Flood of 1927, the Great Flood of 1993, and widespread flooding and mudslides caused by the 1982-1983 El Nino event in the western United States. Localized flooding can, however, occur anywhere, and mudslides from heavy rain can cause problems in any mountainous area, particularly the Southwest. Large stretches of desert shrub in the west can fuel the spread of wildfires. The narrow canyons of many mountain areas in the west and severe thunderstorm activity during the summer leads to sometimes devastating flash floods as well, while Nor'Easter snowstorms can bring activity to a halt throughout the Northeast (although heavy snowstorms can occur almost anywhere).

Hurricane Irene, the seventh costliest hurricane in U.S. history, in late August 2011.

The West Coast of the continental United States and areas of Alaska (including the Aleutian Islands, the Alaskan Peninsula and southern Alaskan coast) make up part of the Pacific Ring of Fire, an area of heavy tectonic and volcanic activity that is the source of 90% of the world's earthquakes. The American Northwest sees the highest concentration of active volcanoes in the United States, in Washington, Oregon and northern California along the Cascade Mountains. There are several active volcanoes located in the islands of Hawaii, including Kilauea in ongoing eruption since 1983, but they do not typically adversely affect the inhabitants of the islands. There has not been a major life-threatening eruption on the Hawaiian islands since the 17th century. Volcanic eruptions can occasionally be devastating, such as in the 1980 eruption of Mount St. Helens in Washington.

The Ring of Fire makes California and southern Alaska particularly vulnerable to earthquakes. Earthquakes can cause extensive damage, such as the 1906 San Francisco Earthquake or the 1964 Good Friday Earthquake near Anchorage, Alaska. California is well known for seismic activity, and requires large structures to be earthquake resistant to minimize loss of life and property. Outside of devastating earthquakes, California experiences minor earthquakes on a regular basis.

There have been about 100 significant earthquakes annually from 2010 to 2012. Past averages were 21 a year. This is believed to be due to the deep disposal of waste water from fracking. None has exceeded a magnitude of 5.6, and no one has been killed.

Other Natural Disasters

Other natural disasters include: tsunamis around Pacific Basin, mud slides in California, and forest fires in the western half of the contiguous U.S. Although drought is relatively rare, it has occasionally caused major economic and social disruption, such as during the Dust Bowl (1931–1942), which resulted in widespread crop failures and duststorms, beginning in the southern Great Plains and reaching to the Atlantic Ocean.

Extremes

Rose Atoll, American Samoa, the southernmost point belonging to the United States.

West Quoddy Head, Maine, easternmost point in the contiguous United States.

Point Udall, Guam, westernmost point belonging to the United States.

• https://en.wikipedia.org/wiki/Extreme_points_of_the_United_States
• https://en.wikipedia.org/wiki/Geography_of_the_United_States