Mexico City has a subtropical highland climate , due to its tropical location but high elevation. The lower region of the valley receives less rainfall than the upper regions of the south; the lower boroughs of Iztapalapa, Iztacalco, Venustiano Carranza and the east portion of Gustavo A. Madero are usually drier and warmer than the upper southern boroughs of Tlalpan and Milpa Alta, a mountainous region of pine and oak trees known as the range of Ajusco.
The average annual temperature varies from 12 to 16 °C (54 to 61 °F), depending on the altitude of the borough. The temperature is rarely below 3 °C (37 °F) or above 30 °C (86 °F). At the Tacubaya observatory, the lowest temperature ever registered was −4.4 °C (24 °F) on 13 February 1960, and the highest temperature on record was 33.9 °C (93 °F) on 9 May 1998.
Overall precipitation is heavily concentrated in the summer months, and includes dense hail.
Snow falls in the city very rarely, although somewhat more often in nearby mountain tops. Throughout its history, the Central Valley of Mexico was accustomed to having several snowfalls per decade (including a period between 1878 and 1895 in which every single year—except 1880—recorded snowfalls) mostly lake-effect snow. The effects of the draining of Lake Texcoco and global warming have greatly reduced snowfalls after the snow flurries of 12 February 1907. Since 1908, snow has only fallen three times, snow on 14 February 1920; snow flurries on 14 March 1940; and on 12 January 1967, when 8 centimeters (3 in) of snow fell on the city, the most on record. The 1967 snowstorm coincided with the operation of Deep Drainage System that resulted in the total draining of what was left of Lake Texcoco. After the disappearance of Lake Texcoco, snow has never fallen again over Mexico City.
The region of the Valley of Mexico receives anti-cyclonic systems. The weak winds of these systems do not allow for the dispersion, outside the basin, of the air pollutants which are produced by the 50,000 industries and 4 million vehicles operating in and around the metropolitan area.
The area receives about 820 millimeters (32 in) of annual rainfall, which is concentrated from May through October with little or no precipitation the remainder of the year. The area has two main seasons. The wet humid summer runs from May to October when winds bring in tropical moisture from the sea, the wettest month being July. The cool sunny winter runs from November to April, when the air is relatively drier, the driest month being December. This season is subdivided into a cold winter period and a warm spring period. The cold period spans from November to February, when polar air masses push down from the north and keep the air fairly dry. The warm period extends from March to May when subtropical winds again dominate but do not yet carry enough moisture for rain to form.