Mexico City’s climate is influenced by its high elevation, its limited air circulation owing to the mountains surrounding it on three sides, and its exposure to both tropical air masses and cold northerly fronts. The latter make southward intrusions only during the Northern Hemisphere winter and spring. Like other high-elevation cities located in the tropics, Mexico City is relatively cool throughout the year. The mean annual temperature is 59 °F (14 °C), but temperatures vary seasonally and diurnally. The difference between summer and winter mean temperatures is approximately 11 to 14 °F (6 to 8 °C).
Winter is the driest time of year. Night frosts occur from December through January, primarily along the city’s elevated periphery. Snowfall is extremely rare at lower elevations, however, and winter temperatures can rise into the mid-70s °F (mid-20s °C) during the day. April and May are the warmest months because summer temperatures are ameliorated by a rainy season that begins in late May and lasts until early October. During that time the normally dry upland basin becomes verdant and its air cool and clean.
The city’s climate has changed since the surrounding lakes were drained and as the built-up area has increased in size. The lakes once had a temperature-moderating effect that prevented the basin from becoming either too cool or too warm, and they contributed moisture for a higher relative humidity than that which prevails today. Vast areas of paved surfaces now impede moisture from entering the soil and have a greater ability to retain heat than vegetated areas; furthermore, they reduce the cooling effects of evaporation. As a result, the city’s buildings, roadways, and machinery have created a thermal island—an urban heat island. Meanwhile, air circulation in the valley is stymied by temperature inversion, in which a blanket of hot polluted air blocks the normal vertical movement of air. "Mexico" © Emmanuel BUCHOT, Encarta, Wikipedia
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