United States of America is a federal state of North America, whose territory is divided between 50 member states and the District of Colombia, in which the capital Washington is located. The territorial continuity of the S. was altered in January 1959, when the dignity of a member state was granted to Alaska, separated from the neighboring states (conterminous United States) by the interposition of a vast tract of Canadian territory; the entry, a few months later, of Hawaii (in the northern Pacific, about 4000 km from the Californian coast) as the fiftieth state added another relevant element of discontinuity. To the USA also belong: as non-incorporated territories, the US Virgin Islands (Caribbean Sea), the island of Guam and the American Samoa Islands (Pacific Ocean), the Associated Free State of Puerto Rico (Caribbean Sea); as the Commonwealth, the Northern Mariana Islands (Pacific Ocean); as a dependency, the Midway Islands and other small islands (Pacific Ocean); as a leased military base, Guantánamo Bay (Cuba).
● The conterminous United States border N with Canada along the parallel of 49 °, from the Pacific to the Laurentian Great Lakes region, then follow the central line of these and, almost to the Atlantic, the St. Lawrence River. The southern border with Mexico follows a SE-NW course, at a latitude of about 30 °; to the East it is marked for about 1200 km by the lower course of the Río Grande, in the western part it follows the straight lines agreed in the second half of the 19th century. The continental sector faces the Atlantic Ocean to the East, the Pacific to the West and the Gulf of Mexico to the S; with Alaska it reaches W on the Bering Sea and N on the Beaufort Sea, part of the Arctic Ocean.
Apart from Alaska, the US territory falls within the range of temperate latitudes and overlooks two oceans and a Mediterranean sea. hot; but it is characterized by more continental than oceanic climates. Mountain ranges on the Pacific coast limit the influence of the ocean and westerly winds to a narrow coastal strip. A greater supply of moisture comes from the Atlantic, as the obstacle of the Appalachian Mountains is overcome by the easterly winds. The coastal strip here is subject to precipitation of the order of 1000 mm per year, while along the Mississippi it drops to 600 mm. Proceeding westwards, beyond the Rocky Mountains, we pass to semi-arid values and there are even desert areas still a few tens of kilometers from the Pacific, while on the side exposed to the ocean the rainfall increases, from S (Los Angeles: 400 mm per year) a N (Seattle: 900 mm).
● The distribution of temperatures is influenced, more than by latitudes, by coastal sea currents, by the distribution of reliefs and by local atmospheric circulation. In Seattle it ranges from 5 ° C in January to 18 ° C in July, while in Los Angeles the corresponding values are 13 and 22 ° C. On the Atlantic coast, a sort of climatic limit is represented by Cape Hatteras: to the North flows the cold Labrador Current, to the South the warm Gulf Stream. The coast north of New York has January averages below 0 ° C, while in July it can exceed 25 ° C; in Savannah, it can stay at 10 ° C in January, but it is unlikely that it will exceed 28 ° C in July. Throughout the Mississippi Basin, in winter, high subpolar pressures push cold air masses up to tropical latitudes. coastal region of the Gulf, especially towards the end of the summer, such as the disastrous Katrina (August 2005). The supplies of hot-humid air also contribute to keeping the summer averages high: in July, in the S, they are recorded from 28 to 30 ° C in Baton Rouge or Dallas, while it remains around 25 ° C in Chicagoor Cincinnati or Indianapolis ; at the border with Canada, temperatures can still exceed 20 ° C. The winter situation is quite different, when in New Orleans and throughout the Gulf coast the January averages do not drop below 10 ° C, while the 0 ° C isotherm is around 30 ° N (the latitude of Cairo). In Chicago it drops to −4 ° C, at the Canadian border the January averages often reach −15 ° C, with an annual excursion always above 30 ° C. In the Rocky Mountains the climates obviously take on a mountainous character. In the great plateaus, the continental characteristics are enhanced: in Salt Lake City, at an altitude of 1300 m, it ranges from −3 ° C in winter to 25 ° C in summer; in Reno, at the same altitude, the January average remains below 0 ° C but in summer it generally does not reach 20 ° C. The large peninsula of Florida in the SE has very special conditions: in Miami, near the Tropic of Cancer, in January the average is close to 20 ° C, in July it approaches 30 ° C, and the rains normally exceed 1500 mm. 1.3 Flora and fauna. For the vegetation and fauna in the USA➔ America.
