Why does fall start September 1 for meteorologists?

Hello, fall!

Now wipe the sweat off your forehead and try saying that again with a straight face.

Meteorological fall begins September 1, even though much of the United States may still feel very much like summer. There could be swimming pools open, air conditioners humming and temperatures that have absolutely no interest in cooperating with the calendar.

So why are meteorologists apparently trying to rush pumpkin-spice season?

There’s actually some pretty good science behind it.

Why does fall start September 1?

So why does fall start September 1 for meteorologists? It comes down to the annual temperature cycle and a much simpler way of organizing weather data.

Meteorological fall is September, October and November. Winter is December through February, spring is March through May, and summer is June through August.

And those groupings aren’t arbitrary.

Meteorological seasons tend to line up pretty well with the temperatures we actually experience. December, January and February generally capture the heart of winter in the Northern Hemisphere, while June, July and August capture the heart of summer. September through November captures the transition from summer heat toward winter cold.

There’s also a practical advantage.

Weather scientists spend a lot of time comparing one season with another. Using three complete calendar months makes it much easier to calculate seasonal averages and compare this fall’s temperature or rainfall with falls from decades past.

September 1 arrives, and boom.

Fall.

Your swimming pool is allowed to file an objection.

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But isn’t the first day of fall later in September?

That’s astronomical fall, and your calendar isn’t lying either.

Astronomical seasons are based on Earth’s journey around the Sun. In 2026, the autumnal equinox occurs September 22 at 8:05 p.m. EDT, marking the astronomical beginning of fall in the Northern Hemisphere.

That’s when the Sun crosses the celestial equator heading south. Around the equinox, day and night are roughly equal in length, although they’re not exactly equal because Earth’s atmosphere bends sunlight and sunrise and sunset aren’t instantaneous events.

So there aren’t two groups arguing about when fall starts.

There are simply two different clocks.

Astronomers use Earth’s relationship with the Sun.

Meteorologists use the calendar and the annual temperature cycle.

But here’s the interesting part.

Is anything actually changing by September 1?

Absolutely.

The atmosphere doesn’t flip a switch Tuesday morning, but the ingredients that eventually produce fall have already been changing for weeks.

Since the June solstice, the Northern Hemisphere has been gradually losing daylight. The Sun is taking a lower path across the sky, nights are getting longer, and there’s less time each day for sunlight to heat the ground.

In many parts of the country, those changes are becoming increasingly noticeable. Cooler air masses can occasionally push south and deliver one of those first mornings that makes you walk outside and think:

Oh. There you are.

Then summer may come roaring back three days later because September enjoys messing with us.

Why can September still feel so much like summer?

Because seasons don’t arrive like somebody opening a door.

The land, oceans and atmosphere have stored enormous amounts of summer heat, and it takes time to lose it. That’s why some places can experience temperatures in the 80s, 90s or even hotter well into meteorological fall.

In Florida and other subtropical areas, the change can be especially subtle. Your maple tree may not be putting on an October fashion show, but the earlier sunset and gradually shortening days are still telling the same seasonal story.

Fall isn’t necessarily something you suddenly feel on September 1.

It’s something you can start to notice.

So when does fall really begin?

Take your pick.

If you’re a meteorologist, September 1.

If you’re following Earth’s position around the Sun, September 22 this year.

And if your personal definition requires crisp mornings, orange leaves and a sweatshirt?

Mother Nature will get back to you.

But tomorrow evening, notice when the Sun disappears. Pay attention the next time an unusually cool morning sneaks into the forecast. Watch how much earlier darkness arrives over the next few weeks.

Summer may still be sitting on the couch with its feet up.

But fall has already started walking toward the front door.

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