Geography of the United States, Climate of the United States, and Environment of the United States

Geography, climate, and ecology

Main articles: Geography of the United States, Climate of the United States, and Environment of the United States

Composite satellite image of the contiguous United States

The land area of the contiguous United States is 2,959,064 square miles (7,663,941 km²). Alaska, separated from the contiguous United States by Canada, is the largest state at 663,268 square miles (1,717,856 km²). Hawaii, occupying an archipelago in the centralPacific, southwest of North America, is 10,931 square miles (28,311 km²) in area.^[73]

The United States is the world's third or fourth largest nation by total area (land and water), ranking behind Russia and Canada and just above or below China. The ranking varies depending on how two territories disputed by China and India are counted and how the total size of the United States is measured: calculations range from 3,676,486 square miles (9,522,055 km²)^[74] to 3,717,813 square miles (9,629,091 km²)^[75] to 3,794,101 square miles (9,826,676 km²).^[1] Measured by only land area, the United States is third in size behind Russia and China, just ahead of Canada.^[76]

The coastal plain of the Atlantic seaboard gives way further inland to deciduous forests and the rolling hills of the Piedmont. TheAppalachian Mountains divide the eastern seaboard from the Great Lakes and the grasslands of the Midwest. The Mississippi–Missouri River, the world's fourth longest river system, runs mainly north–south through the heart of the country. The flat, fertile prairie of theGreat Plains stretches to the west, interrupted by a highland region in the southeast.

The Bald Eagle, the national bird of the United States since 1782.

The Rocky Mountains, at the western edge of the Great Plains, extend north to south across the country, reaching altitudes higher than 14,000 feet (4,300 m) in Colorado. Farther west are the rockyGreat Basin and deserts such as the Chihuahua and Mojave. The Sierra Nevada and Cascademountain ranges run close to the Pacific coast, both ranges reaching altitudes higher than 14,000 feet.

The lowest and highest points in the continental United States are in the state of California, and only about 80 miles apart. At 20,320 feet (6,194 m), Alaska's Mount McKinley is the tallest peak in the country and in North America. Active volcanoes are common throughout Alaska's Alexander andAleutian Islands, and Hawaii consists of volcanic islands. The supervolcano underlying Yellowstone National Park in the Rockies is the continent's largest volcanic feature.^[77]

The United States, with its large size and geographic variety, includes most climate types. To the east of the 100th meridian, the climate ranges from humid continental in the north to humid subtropical in the south. The southern tip of Florida is tropical, as is Hawaii. The Great Plains west of the 100th meridian are semi-arid. Much of the Western mountains are alpine. The climate is arid in the Great Basin, desert in the Southwest, Mediterranean in coastal California, and oceanic in coastalOregon and Washington and southern Alaska. Most of Alaska is subarctic or polar. Extreme weather is not uncommon—the states bordering the Gulf of Mexico are prone to hurricanes, and most of the world's tornadoes occur within the country, mainly in the Midwest's Tornado Alley.^[78]

The U.S. ecology is considered "megadiverse": about 17,000 species of vascular plants occur in the contiguous United States and Alaska, and over 1,800 species of flowering plants are found in Hawaii, few of which occur on the mainland.^[79] The United States is home to more than 400 mammal, 750 bird, and 500 reptile and amphibian species.^[80] About 91,000 insect species have been described.^[81]

The Endangered Species Act of 1973 protects threatened and endangered species and their habitats, which are monitored by theUnited States Fish and Wildlife Service. There are fifty-eight national parks and hundreds of other federally managed parks, forests, andwilderness areas.^[82] Altogether, the government owns 28.8% of the country's land area.^[83] Most of this is protected, though some is leased for oil and gas drilling, mining, logging, or cattle ranching; 2.4% is used for military purposes.^[83]

Political divisions

Main articles: Political divisions of the United States, U.S. state, and Territories of the United States

Further information: Territorial evolution of the United States and United States territorial acquisitions

The United States is a federal union of fifty states. The original thirteen states were the successors of the thirteen colonies that rebelled against British rule. Early in the country's history, three new states were organized on territory separated from the claims of the existing states: Kentucky from Virginia; Tennessee from North Carolina; and Maine from Massachusetts. Most of the other states have been carved from territories obtained through war or purchase by the U.S. government. One set of exceptions includes Vermont, Texas, andHawaii: each was an independent republic before joining the union. During the American Civil War, West Virginia broke away from Virginia. The most recent state—Hawaii—achieved statehood on August 21, 1959.^[84] The states do not have the right to unilaterallysecede from the union.

The states compose the vast bulk of the U.S. land mass; the two other areas considered integral parts of the country are the District of Columbia, the federal district where the capital, Washington, is located; and Palmyra Atoll, an uninhabited but incorporated territory in the Pacific Ocean. The United States also possesses five major overseas territories: Puerto Rico and the United States Virgin Islands in the Caribbean; and American Samoa, Guam, and the Northern Mariana Islands in the Pacific.^[85] Those born in the major territories (except for American Samoa) possess U.S. citizenship.^[86] American citizens residing in the territories have many of the same rights and responsibilities as citizens residing in the states; however, they are generally exempt from federal income tax, may not vote for president, and have only nonvoting representation in the U.S. Congress.^[87]

The United States also observes tribal sovereignty of the Native Nations. Though reservations are within state borders, the reservation is a sovereign. While the United States recognizes this sovereignty, other countries may not.^[88]