Fall might not officially begin until Sept. 22 with the autumnal equinox, but meteorologists and climatologists actually consider Sept. 1 the beginning of autumn.
This is because the astronomical seasons are based on the Earth’s position relative to the sun, while "climatological" or "meteorological" seasons are divided into three-month periods based on the temperatures that would be expected during each season.
For example, summer is the hottest time of the year and winter is the coldest time of the year, with fall and spring the transition seasons in between. So instead of focusing on the Earth’s orientation with respect to the sun, these so-called climatological seasons are solely classified by the annual temperature cycle across the world.
The hottest three months of the year in the Northern Hemisphere are June, July and August, so climatological summer runs from June 1 to Aug. 31. The tweet below from Alaska-based climatologist Dr. Brian Brettschneider shows this definition fits better than the astronomical definition for most areas in the U.S. and Canada.
Conversely, December, January and February are the Northern Hemisphere’s coldest three months, so climatological winter runs from Dec. 1 to Feb. 28 (or Feb. 29). Brettschneider’s tweet below once again shows this definition fits better than the astronomical definition for the majority of the U.S. and Canada.
The transition seasons are the three months in between summer and winter, so climatological fall is Sept. 1 to Nov. 30 and climatological spring is March 1 to May 31.
Splitting the seasons into these nearly equal three-month periods makes seasonal recordkeeping much easier since the start and end dates of the seasons are always the same. Additionally, each climatological season is always 90 to 92 days, depending whether it's a leap year or not.
The dates of the solstices and equinoxes can vary by a day or two each year, so an apples-to-apples comparison of seasonal temperatures or precipitation would become a nightmare for our friends at the National Weather Service if they didn't use climatological seasons.
Astronomical fall officially begins Sept. 22 with the autumnal equinox at 9:04 p.m. Eastern time. That's simply the precise moment in time when the sun will be in direct alignment with the equator. Therefore, everywhere on Earth will experience an equal 12 hours of day and night on that date.
Since the Earth is tilted approximately 23.5 degrees off a vertical axis, the most direct sunlight is aimed at the Northern Hemisphere during our astronomical summer. That means the days will continue to grow shorter through the fall until the winter solstice on Dec. 21 because the most direct sunlight shifts into the Southern Hemisphere during our astronomical winter. Thereafter, the days will turn longer until the summer solstice on June 21.