Most of Mexico consists of rough plateaus and mountains; only about a third of the country is flat. There are five major physiographic regions: the Central Plateau, the Sierra Madres, the Chiapas Highlands, the Gulf Coastal Plain, and the Lower California–Sonoran Desert region.

Central Plateau. This vast tableland juts southward from the United States almost to the Isthmus of Tehuantepec. The plateau increases in height from north to south, from elevations of little more than 3,000 feet (910 m) near the United States border to more than 8,000 feet (2,440 m) above sea level at the southern end. Some parts are flat, but in many places there are hills and mountains. These sometimes occur as parallel ranges with basins between them; in other instances the highlands encircle basins.

The vast northern part of the plateau resembles the land of southern New Mexico and western Texas. It is quite dry and has few inhabitants. The southern part of the plateau has more rainfall and is more fertile. Nearly half of the people in Mexico live in this region. Mexico City is in the Valley of Mexico at the southern end of the plateau. Just south of the valley is a belt of active and dormant volcanoes that stretches almost from coast to coast. Orizaba, rising 18,855 feet (5,747 m), is Mexico's highest peak and the third highest in North America. Volcanoes in this belt that have been active in the past century include Popocatépetl and Parícutin.

Sierra Madres. This region consists of the three mountain ranges fringing the plateau's V-shaped edge. They are the Sierra Madre del Sur on the south, the Sierra Madre Occidental on the west, and the Sierra Madre Oriental on the east. The seaward slopes of the ranges are greatly eroded and often exceptionally steep. On the Pacific side the mountains often rise abruptly from sea level to heights of 10,000 to 12,000 feet (3,050 to 3,660 m). The ranges are marked by deep gorges, canyons, and knifelike ridges. In the south the Sierra Madre del Sur drops abruptly to the lowlands of the Isthmus of Tehuantepec.

Chiapas Highlands. These rise on the opposite side of the isthmus and run southeastward to Guatemala. The region centers on a volcano-studded range, also called Sierra Madre, which forms the backbone of much of Central America. The land is high and rough, especially along the Pacific Coast. To the northeast, the mountains decline and merge with the plateau of Yucatan.

Gulf Coastal Plain. This flat to rolling, occasionally swampy, lowland runs along the eastern coast. It narrows as it extends southward from the Texas border and almost disappears, because of mountains, between Tampico and the Isthmus of Tehuantepec. Beyond the isthmus, the coastal plain fans out over much of Yucatan.

Lower California-Sonoran Desert. This region is a semiarid to arid area in the northwest, consisting of the peninsula of Lower California and the Sonoran Desert. The peninsula is made up of plateaus, terraces, and mountains similar to those of neighboring southern California. Peaks along the peninsula's mountainous backbone rise up to about 10,000 feet (3,050 m) above sea level in the north and 7,000 feet (2,130 m) in the south. The extremely dry Sonoran Desert, which lies east of the Gulf of California, is basin-and-range land similar to that of southwestern Arizona.

Water

Most of Mexico's rivers are relatively short and carry little water. Virtually all are unsuitable for navigation, but some of them are important for irrigation and hydroelectric power. Two chief rivers, both in the north, are the Colorado and the Río Bravo del Norte, known as the Rio Grande in the United States. Nearly all of the rivers in the northern part of the plateau have no outlets to the sea and eventually dry up inland.

Chief rivers descending from the plateau include the Río Fuerte, Río Yaqui, and Río Grande de Santiago in the west; the Río Balsas in the south; and the Río Papaloapan and Río Pánuco in the east. Some reach the sea through deep canyons that rival the majestic Grand Canyon of the Colorado in the United States. The Río Usumacinta and Río Grijalva are the main rivers draining the Chiapas Highlands.

Numerous lagoons line the coast, the chief ones being Términos and Madre on the Gulf coast. Natural lakes are found mainly in the basins in the southern part of the plateau. The largest is Lake Chapala near Guadalajara.

Climate

Although Mexico straddles the Tropic of Cancer and lies mainly in the subtropics, it has great climatic variety, largely because of differences in altitude and latitude. Temperatures steadily decrease as altitude increases; rainfall, in general, decreases from southeast to northwest. Climates range from perpetually cold on the highest peaks to tropical rainy or dry in the lowlands.

Temperature. On the basis of temperature, Mexico is usually divided into three regions. From sea level up to about 3,000 feet (910 m) is a region known as the tierra caliente (hot country), which has tropical and subtropical climates. The tierra templada (temperate country), roughly between 3,000 and 6,000 feet (910 and 1,830 m) above sea level, is a more temperate zone, with springlike weather virtually all year. Most of Mexico's large cities are situated in or slightly above this region. Above 6,000 feet lies the cool zone, the tierra fría, which becomes progressively colder with increased elevation. On a number of volcanoes, there is a permanent cover of ice and snow at altitudes above 14,000 feet (4,270 m).

The hottest weather occurs in the north, particularly around the Gulf of California, where summer temperatures occasionally rise to more than 110° F. (43° C). The coldest winter weather, excluding high mountain areas, occurs on the plateau in the northern states, where below-freezing temperatures are frequent. Rarely does the cold weather extend as far south as Mexico City. One of the outstanding features of the climate throughout much of Mexico is the great daily range of temperatures. Days are warm to hot, but nights are chilly to cool. In Mexico City, for example, the difference between daytime and nighttime temperatures is often about 25 Fahrenheit degrees (14 Celsius degrees).

Precipitation. Most of Mexico receives scant rainfall. All of northern Mexico is either arid or semiarid; some parts of the Sonoran Desert are virtually rainless. Summer is the rainiest season throughout the country, except in the extreme northwest, where winter is the rainy season.

Areas that receive substantial amounts of precipitation—40 to 100 inches (1,020 to 2,540 mm) or more each year—are the southern Pacific coast, much of the Gulf coast, the state of Chiapas, and most of the Yucatán peninsula. Hurricanes sometimes occur along the Gulf coast, mainly in late summer and early autumn.