Mexico City

Mexico City is the Federal District (Distrito Federal), capital of Mexico and seat of the federal powers of the Mexican Union. It is a federal entity within Mexico which is not part of any one of the 31 Mexican states but belongs to the federation as a whole. Mexico City is the country's largest city as well as its most important political, cultural, educational and financial center.

As an "alpha" global city Mexico City is one of the most important financial centers in North America. It is located in the Valley of Mexico (Valle de Mexico), a large valley in the high plateaus at the center of Mexico, at an altitude of 2,240 metres (7,350 ft). The city consists of sixteen boroughs.

The 2009 estimated population for the city proper was around 8.84 million people, and has a land area of 1,485 square kilometres (573 sq mi). According to the most recent definition agreed upon by the federal and state governments, the Mexico City metropolitan area population is 21.2 million people, making it the largest metropolitan area in the western hemisphere and the fifth largest agglomeration in the world.

Mexico City has a gross domestic product (GDP) of $390 billion US$ in 2008, making Mexico City the eighth richest city in the world. The city was responsible for generating 21% of Mexico's Gross Domestic Product and the metropolitan area accounted for 34% of total national GDP.

The city was originally built on an island of Lake Texcoco by the Aztecs in 1325 as Tenochtitlan, which was almost completely destroyed in the 1521 siege of Tenochtitlan, and subsequently redesigned and rebuilt in accordance with the Spanish urban standards. In 1524, the municipality of Mexico City was established, known as México Tenochtitlán, and as of 1585 it was officially known as La Ciudad de México (Mexico City). Mexico City served as the political, administrative and financial center of a major part of the Spanish colonial empire. After independence from Spain was achieved, the Federal District was created in 1824.

After years of demanding greater political autonomy, residents were given the right to directly elect the Head of Government and the representatives of the unicameral Legislative Assembly by popular vote in 1997. Ever since, the left-wing Party of the Democratic Revolution (PRD) has controlled both of them. In recent years, the local government has passed a wave of liberal policies, such as abortion on request to any woman up to 12 weeks into a pregnancy, a limited form of euthanasia, no-fault divorce and same-sex marriage.

Mexico City has a subtropical highland climate (Köppen climate classification Cwb), due to its tropical location and high elevation. The lower region of the valley receives less rainfall than the upper regions of the south; the lower boroughs of Iztapalapa, Iztacalco, Venustiano Carranza and the west portion of Gustavo A. Madero are usually drier and warmer than the upper southern boroughs of Tlalpan and Milpa Alta, a mountainous region of pine and oak trees known as the range of Ajusco.

The average annual temperature varies from 12 to 16 °C (54 to 61 °F), depending on the altitude of the borough. The lowest temperatures, usually registered during January and February, may reach -2 to -5 °C (28 to 23 °F), and are usually accompanied by snow showers on the southern regions of Ajusco. The maximum temperatures of late spring and summer may reach up to 32 °C (90 °F). Extremes range from -4.4 °C (24.1 °F) to 33.9 °C (93.0 °F).

Overall precipitation is heavily concentrated in the summer months, and includes dense hail. The central valley of Mexico rarely gets precipitation in the form of snow during winter; the two last recorded instances of such an event were on March 5, 1940 and January 12, 1967.

The region of the Valley of Mexico receives anti-cyclonic systems. The weak winds of these systems do not allow for the dispersion, outside the basin, of the air pollutants which are produced by the 50,000 industries and 4 million vehicles operating in and around the metropolitan area.

The area receives about 820 millimetres (32.3 in) of annual rainfall, which is concentrated from June through September/October with little or no precipitation the remainder of the year. The area has two main seasons. The rainy season runs from June to October when winds bring in tropical moisture from the sea. The dry season runs from November to May, when the air is relatively drier. This dry season subdivides into a cold period and a warm period. The cold period spans from November to February when polar air masses push down from the north and keep the air fairly dry. The warm period extends from March to May when tropical winds again dominate but do not yet carry enough moisture for rain.