The Tropic of Cancer effectively divides the country into temperate and tropical zones. Land north of the twenty-fourth parallel experiences cooler temperatures during the winter months. South of the twenty-fourth parallel, temperatures are fairly constant year round and vary solely as a function of elevation.
Areas south of the twentieth-fourth parallel with elevations up to 1,000 meters (the southern parts of both coastal plains as well as the Yucatan Peninsula), have a yearly median temperature between 24°C and 28°C. Temperatures here remain high throughout the year, with only a 5°C difference between winter and summer median temperatures. Although low-lying areas north of the twentieth-fourth parallel are hot and humid during the summer, they generally have lower yearly temperature averages (from 20°C to 24°C) because of more moderate conditions during the winter.
Between 1,000 and 2,000 meters, one encounters yearly average temperatures between 16°C and 20°C. Towns and cities at this elevation south of the twenty-fourth parallel have relatively constant, pleasant temperatures throughout the year, whereas more northerly locations experience sizeable seasonal variations. Above 2,000 meters, temperatures drop as low as an average yearly range between 8°C and 12°C in the Cordillera Neovolcánica. At 2,300 meters, Mexico City has a yearly median temperature of 15°C with pleasant summers and mild winters. Average daily highs and lows for May, the warmest month, are 26°C and 12°C, and average daily highs and lows for January, the coldest month, are 19°C and 6°C.
Rainfall varies widely both by location and season. Arid or semiarid conditions are encountered in the Baja Peninsula, the northwestern state of Sonora, the northern altiplano, and significant portions of the southern altiplano. Rainfall in these regions averages between 300 and 600 millimeters per year. Average rainfall totals are between 600 and 1,000 millimeters in most of the year in most of the major populated areas of the southern altiplano, including Mexico City and Guadalajara. Low-lying areas along the Gulf of Mexico receive in excess of 1,000 millimeters of rainfall in an average year, with the wettest region being the southeastern state of Tabasco, which typically receives approximately 2,000 millimeters of rainfall on an annual basis. Parts of the northern altiplano and high peaks in the Sierra Madre Occidental and the Sierra Madre Oriental occasionally receive significant snowfalls.
Mexico has pronounced wet and dry seasons. Most of the country experiences a rainy season from June to mid-October and significantly less rain during the remainder of the year. February and July generally are the driest and wettest months, respectively. Mexico City, for example, receives an average of only 5 millimeters of rain during February but more than 160 millimeters in July. Coastal areas, especially those along the Gulf of Mexico, experience the largest amounts of rain in September. Tabasco typically records more than 300 millimeters of rain during that month. A small coastal area of northwestern coastal Mexico around Tijuana has a Mediterranean climate with considerable coastal fog and a rainy season that occurs in winter.
Mexico lies squarely within the hurricane belt, and all regions of both coasts are susceptible to these storms from June through November. Hurricanes on the Pacific coast are less frequent and often less violent than those affecting Mexico's eastern coastline. Several hurricanes per year strike the Caribbean and Gulf of Mexico coastline, however, and these storms bring high winds, heavy rain, extensive damage, and occasional loss of life. Hurricane Hugo passed directly over Cancún in September 1989, with winds in excess of 200 kilometers per hour producing major damage to hotels in the resort area. In September 1988, Hurricane Gilbert struck northeast Mexico. Flooding from the heavy rain in that storm killed dozens in the Monterrey area and caused extensive damage to livestock and vegetable crops.
Mexico faces significant environmental challenges affecting almost every section of the country. Vast expanses of southern and southeastern tropical forests have been denuded for cattle-raising and agriculture. For example, tropical forests covered almost half of the state of Tabasco in 1940 but less than 10 percent by the late 1980s. During the same period, pastureland increased from 20 to 60 percent of the state's total area. Analysts reported similar conditions in other tropical sections of Mexico. Deforestation has contributed to serious levels of soil erosion nationwide. In 1985 the government classified almost 17 percent of all land as totally eroded, 31 percent in an accelerated state of erosion, and 38 percent demonstrating signs of incipient erosion.
Soil destruction is particularly pronounced in the north and northwest, with more than 60 percent of land considered in a total or accelerated state of erosion. Fragile because of its semiarid and arid character, the soil of the region has become increasingly damaged through excessive cattle-raising and irrigation with waters containing high levels of salinity. The result is a mounting problem of desertification throughout the region.
Mexico's vast coastline faces a different, but no less difficult, series of environmental problems. For example, inadequately regulated petroleum exploitation in the Coatzacoalcos-Minatitlán zone in the Gulf of Mexico has caused serious damage to the waters and fisheries of Río Coatzacoalcos. The deadly explosion that racked a working-class neighborhood in Guadalajara in April 1992 serves as an appropriate symbol of environmental damage in Mexico. More than 1,000 barrels of gasoline seeped from a corroded Mexican Petroleum (Petróleos Mexicanos--Pemex) pipeline into the municipal sewer system, where it combined with gases and industrial residuals to produce a massive explosion that killed 190 persons and injured nearly 1,500 others.
Mexico City confronts authorities with perhaps their most daunting environmental challenge. Geography and extreme population levels have combined to produce one of the world's most polluted urban areas. Mexico City sits in a valley surrounded on three sides by mountains, which serve to trap contaminants produced by the metropolitan area's 15 million residents. One government study in the late 1980s determined that nearly 5 million tons of contaminants were emitted annually in the atmosphere, a tenfold increase over the previous decade. Carbons and hydrocarbons from the region's more than 3 million vehicles account for approximately 80 percent of these contaminants, with another 15 percent, primarily of sulfur and nitrogen, coming from industrial plants. During the dry winter months, untreated fecal matter also becomes airborne. The resulting dangerous mix is responsible for a wide range of respiratory illnesses. One study of twelve urban areas worldwide in the mid-1980s concluded that the residents of Mexico City had the highest levels of lead and cadmium in their blood. The volume of pollutants from Mexico City has damaged the surrounding ecosystem as well. For example, wastewater from Mexico City that flows north and is used for irrigation in the state of Hidalgo has been linked to congenital birth defects and high levels of gastrointestinal diseases in that state.
Beginning in the mid-1980s, the government enacted numerous antipollution policies in Mexico City with varied degrees of success. Measures such as vehicle emissions inspections, the introduction of unleaded gasoline, and the installation of catalytic converters on new vehicles helped reduce pollution generated by trucks and buses. In contrast, one of the government's most prominent actions, the No Driving Day program, may have inadvertently contributed to higher pollution levels. Under the program, metropolitan area residents were prohibited from driving their vehicles one day each work week based on the last number of their license plate. However, those with the resources to do so purchased additional automobiles to use on the day their principal vehicle was prohibited from driving, thus adding to the region's vehicle stock. Thermal inversions reached such dangerous levels at various times in the mid-1990s that the government declared pollution emergencies, necessitating sharp temporary cutbacks in vehicle use and industrial production.
Data as of June 1996