Mexico City (CDMX) weather in September

Regulars call this a dry, pleasant month, squarely inside the November-to-May dry season, but are specific that the city's altitude produces a bigger day-to-night temperature range than a similar spot at sea level would see - a pleasant 77F afternoon can drop fast into the 50s by early evening. Our row backs that swing, 27.5C reading as 26C over a real 12C night, with rain minimal at 25.2mm across 5.4 days and humidity down to a dry 48 percent. Regulars call March and November the two best overall months, with lip balm named as a small but genuinely useful packing item given how dry the air stays. The record: 10.9 hours of sun, a 12.8 kph wind and AQI 87.

Regulars give December here as highs in the low seventies and lows in the low forties, which our 23.5C over 9.9C matches almost exactly - and then they explain why the number understates it. The altitude makes the cold penetrating, and almost nothing here has heating: apartments and small hotels included, with indoor temperatures reported at 16 to 18C overnight. Their consolation is that the building warms through the day, so the room does not start cold. A sweater in the evening and an extra blanket is the standing advice. Air is very dry, a dew point of 3.4C at 64 percent humidity, and rain has stopped at 16.7mm. The record: 9.6 hours of sun, a 9.9 kph wind and AQI 76.

Mexico City-forum threads on February weather all land on the same shape: warm, sunny days in the mid-70s Fahrenheit against genuinely cold mornings in the 40s, cool enough that people in the city bundle up before the sun climbs. The altitude gets its own warning thread every few months - at 7,350 feet the sun is more intense than the cool air suggests, and more than one poster describes feeling the elevation for a day or two after arrival, with the standing advice to keep the first day light rather than pushing straight into sightseeing. The record: 25.8C days feeling like 24C, 11.7C nights, just 17mm of rain across 4.2 days and 10.3 hours of sun - one of the driest, sunniest months on the calendar, with 52 percent humidity that keeps the heat from building through the day.

Mexico City forum threads draw the seasons plainly: the rainy season runs from about late May until late October, the dry season from November to late May, and the months locals rate best overall are March and November. That places October at the very end of the wet stretch - expect the tail of the season rather than its middle. The record for October: 24.3C days and 13.3C nights, humidity around 78 percent, 14 precipitation days and roughly 89mm across the month with 9.3 hours of sun. Layers matter in the evening, since the city cools quickly once the sun is down at this altitude.