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 Yucatán 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 California 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 Gilbert passed directly over Cancún in September 1988, with winds in excess of 200 kilometers per hour, producing major damage to hotels in the resort area. It then struck northeast Mexico, where flooding from the heavy rain killed dozens in the Monterrey area and caused extensive damage to livestock and vegetable crops.