Mexico City - Climate

Fall (Autumn)
begins about October - weather-wise. The evenings draw in and darkness falls towards 7 p.m. The days can still be warm, so short sleeves may be worn during the middle of the day, but  with cooler evenings take another clothing layer. The rainy season that started in August will still continue to be felt.

Winter
sets in during mid November, early December. You can expect a few (only) frosts between now and the end of February. Some days you will need a jersey or warm jacket. Often by day, it is still warm enough for short sleeves, but by night, definitely a outer layer is required. When polar waves are announced, be sure to protect your most delicate plants and water the grass BEFORE dawn, to melt the frost that sets during the night. Otherwise, the mixture of frost and the morning sun may 'burn' the grass, leaving it dry and yellow.
It doesn't snow in Mexico City, but during a 'polar wave', temperatures may drop to -1 degrees centigrades for a few days at a time.
Personally, I find weather forecasts rather reliable, even though most Mexicans do not pay much attention to them and prefer to 'feel the weather' before going out (for years, weather forecasts weren't reliable, and Mexicans simply don't believe in them any more).
Air conditioners/heating are not generally in use.

Spring
begins late March, when days get warmer and evenings comfortably cooler. However, Mexicans have a saying: "Febrero loco, Marzo otro poco" ("February is crazy, and so is March"). This refers to the unpredictable and wide variety of temperatures and weather conditions that may be found in a given week and even in a day (It may be sunny in the morning, with ice-cold winds in the afternoon and even showers, then turning nice and warm by 6 pm, for example). People then tend to wear layers of clothing, to take them on/off as needed. By April, the weather stabilizes and Mexico City lives a glorious weather thru late May with isolated showers here and there.

Summer
is long from May through September, with longer evenings and the sun setting around 8 p.m. Some showers may be experienced, but usually the weather is warm until the rainy season starts, towards the end of August. With the rainy season, traffic worsens all throughout the city, and one can be stuck on the main highways for up to four hours due to floods.