The United States is the world's third or fourth largest nation by total area,before or after the People's Republic of China,depending on how two territories disputed by China and India are counted.Including only land area,the United States is third in size behind Russia and China,just ahead of Canada.The continental United States stretches from the Atlantic Ocean to the Pacific Ocean and from Canada to Mexico and the Gulf of Mexico.Alaska is the largest state in area.Separated by Canada,it touches the Pacific and Arctic Oceans.Hawaii occupies an archipelago in the Pacific,southwest of North America.The commonwealth of Puerto Rico,the largest and most populous U.S.territory,is in the northeastern Caribbean.With a few exceptions,such as the territory of Guam and the westernmost portions of Alaska,nearly all of the country lies in the western hemisphere.The coastal plain of the Atlantic seaboard gives way further inland to deciduous forests and the rolling hills of the Piedmont.The Appalachian 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 land of the Great Plains stretches to the west.The Rocky Mountains,at the western edge of the Great Plains,extend north to south across the continental United States,reaching altitudes higher than 14,000 feet (4,300 m) in Colorado.The area to the west of the Rockies is dominated by deserts such as the Mojave and the rocky Great Basin.The Sierra Nevada range runs parallel to the Rockies,relatively close to the Pacific coast.At 20,320 ft (6,194 m),Alaska's Mount McKinley is the country's tallest peak.Active volcanoes are common throughout the Alexander and Aleutian Islands and the entire state of Hawaii is built upon tropical volcanic islands.The supervolcano underlying Yellowstone National Park in the Rockies is the continent's largest volcanic feature.Because of the United States' large size and wide range of geographic features,nearly every type of climate is represented.The climate is temperate in most areas,tropical in Hawaii and southern Florida,polar in Alaska,semiarid in the Great Plains west of the 100th meridian,desert in the Southwest,Mediterranean in coastal California,and arid in the Great Basin.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 continental United States.参考资料：http://en.wikipedia.org/wiki/United_States#Geography