This year the autumnal equinox – which marks the point when the sun crosses the celestial equator – falls on September 22 in the United States (10:29 PM Eastern time, and earlier for the other time zones.) People seem to refer to this as the “first day of fall”.
But this is the astronomical definition. Meterologists define summer as June through August, and fall as September through November (so it’s been fall for a while now). I noticed this this morning when on my local news they were talking about today as the “last full day of summer” in reference to the weather. But if summer is to be defined meteorlogically, we can take a look at the climate normals for Georgia (thanks to Golden Gate Weather). (I’m using Atlanta Hartsfield Airport here.) The hottest days of the year are July 16 through 19, with an average high of 89.3 degrees, in agreement with NOAA’s warmest day of the year map. If we want a period of three months that’s the hottest possible, we should find two days three months apart which have the same normal high temperature; these are June 7 and September 7. This is the period of the year where the average high temperature is above 85 degrees.
We could find “winter” the same way; a three-month winter in Atlanta would be November 27 through February 27, when average highs are below 59.5 degrees. Fall and spring are intermediate between those. However, in snowy places I’d be a bit more inclined to define winter as “the time when it snows a lot”. You may recall that Atlanta is not a snowy place.
The benefit of thinking of things this way is that it captures the insight that seasons come “late” or “early” in certain places. Take for example a San Francisco summer; late September is actually the hottest time of year in San Francisco. Indeed the warmest three-month period in San Francisco is August 2 through November 1 (when highs are 66.7 or higher), which corresponds with my intuition that July just isn’t summer there. The coldest three-month period is November 23 through February 23 (when highs are 61.1 or lower). In other words, fall in San Francisco is a few weeks in November, and spring lasts nearly half the year. San Francisco, by the way, is less snowy than Atlanta.
For climate charts that are prettier than anything I could make on the fly, see WeatherSpark for Atlanta and San Francisco. These dates that I’ve given roughly correspond with the “cold season” and “warm season” they report, although not exactly because they don’t appear to have constrained the lengths of those periods.