The Valley of Mexico is a highlands plateau in central Mexico roughly coterminous with the present-day Distrito Federal and the eastern half of Estado de Mexico. Surrounded by mountains and volcanoes, the Valley of Mexico was a center for several pre-Columbian civilizations, including the Teotihuacanos, Toltecs, and Aztecs. The ancient Aztec term Anahuac and the phrase "Basin of Mexico" are both used at times to refer to the Valley. Monte Roraima In geology and earth science, a plateau, also called a high plateau or tableland, is an area of highland, usually consisting of relatively flat open country. ... The Mexican Federal District, known in Spanish as Distrito Federal (D.F.), is an area within Mexico that is not part of any of the Mexican states, but an independent self-governing city-state and the seat of the Federal Government. ... The United Mexican States, or Mexico, is a federal republic, comprising 31 states. ... Teotihuacan was the largest Pre-Columbian known city in the Americas, and the name Teotihuacan is used to refer to the civilization this city dominated, which at its greatest extent included most of Mesoamerica. ... The Toltecs (or Toltec or Tolteca) were a Pre-Columbian Native American people who dominated much of central Mexico between the 10th and 12th century AD. Their language, Nahuatl, was also spoken by the Aztecs. ... The Aztecs were a Pre-Columbian Mesoamerican people of central Mexico in the 14th, 15th and 16th centuries. ... Anahuac is a geographical district of Mexico, limited by the traditional and vaguely defined boundaries of an ancient Indian empire or confederation of that name previous to the Spanish conquest. ...

Geography

The Valley of Mexico is roughly 35 miles (60 km) east-to-west and 50 miles (80 km) north-to-south. The valley is an endorheic basin, with no natural outflows. As a result, before the 20th century, much or most of the valley was covered with a chain of inter-related lakes: Texcoco, Chalco, Xochimilco, Zumpango, and Xaltocan. These have since been drained and little of their original expanse remains — today the greater Mexico City urban area covers nearly the entire valley. The shores of Lake Hart, an endorheic desert lake in South Australia In geography, an endorheic basin—also called a terminal or closed basin—is a watershed from which there is no outflow of water, either on the surface as rivers, or underground by flow or diffusion through rock or... Lake Texcoco is a lake in Mexico. ... Lake Chalco is a lake that was located in the mexican basin. ... Lake Xochimilco is a lake located in the Mexican Basin, this water body was an important element for the human development in central Mexico. ... Mexico City (Spanish: Ciudad de México, México D.F. or simply México, pronounced IPA: ) is the capital city of the nation of Mexico. ...

Pollution woes

Air pollution associated with this urban environment, consisting primarily of ozone, sulfur dioxide, and suspended particles, is trapped by natural contours of the valley. In winter, thermal inversions add to the woes. This has led many organizations, from the World Resources Institute to the Guinness Book of World Records, to designate Mexico City as the most polluted major city in the world. The air reached unhealthy levels for 355 days during 1991, improving to 170 days in 2003, and 141 in 2004.^[1] A temperature inversion is a meteorological phenomenon where air temperature increases with height. ... World Resources Institute (WRI) is an environmental think tank based in Washington, D.C. It was established by James Gustave (Gus) Speth. ... The Guinness Book of Records (or in recent editions Guinness World Records, and in previous US editions Guinness Book of World Records) is a book published annually, containing an internationally recognized collection of superlatives: both in terms of human achievement and the extrema of the natural world. ...

Climate

The Valley of Mexico averages roughly 7350 feet (2240 meters) above sea level. Although the valley is located within the tropical zone, the high altitude keeps it relatively temperate. The annual median temperature is between 61° - 64° F (16° - 18° C) and seasonal changes are small. Night frosts can occur in the coldest months, but even in January the average high is 70° F. April and May are the warmest months. The rainy season, which usually lasts from late May until early October, accounts for 80% of the annual 33 inches (850 mm) of rainfall and moderates the usual dryness of the air.

Footnotes

1. ^ http://www.azcentral.com/specials/special03/articles/0118Air18.html Article on pollution