Autumnal equinox marks the start of fall, but heat could stick around

Our long, hot summer is finally coming to an end this evening with the autumnal equinox, which marks the beginning of fall here in the Northern Hemisphere.

At a precise moment each September, usually on the 22nd or 23rd, the sun is directly above the equator, marking the autumnal equinox in the Northern Hemisphere.

(South of the equator in the Southern Hemisphere, it's known as the vernal or spring equinox and marks the beginning of spring.) 

But what weather will the U.S. see this fall, given the record hot August and a forecast that calls for El Niño to peak either in late fall or early winter?

Read on for the forecast and also a look back at some of the statistics that made the three-month summer so remarkable.

What time is the autumnal equinox?

The exact time of the equinox is 8:05 p.m. ET. The equinox occurs at the same moment worldwide.

The word equinox comes from the Latin words "aequalis" and "nox," meaning equal night. On the autumnal (and the spring) equinox, day and night are both roughly 12 hours long in most of the world.

This will be true Sept. 22 from as far north as Utqiagvik (formerly Barrow), Alaska – north of the Arctic Circle – to as far south as Wellington, New Zealand, the world's southernmost capital city.

Day and night aren't exactly 12 hours long on the day of the equinox because the Earth's atmosphere refracts, or bends, light in an optical illusion that brings more daylight than there really is. Because of this, the date when day and night are of equal length is usually a few days after the autumnal equinox.

Astronomical fall vs. meteorological fall

Although some people say the autumnal equinox is the "official" start of fall, no administrative or political organization actually designates that.

In fact, though astronomers say summer ends Sept. 22, meteorologists and climatologists say summer ended Aug. 31, the final day of the three hottest months of the year (June, July and August).

Cooler temperatures? Maybe, but warmer than normal

If you were looking forward to cooler temperatures this fall, you might be disappointed. Thanks to a combination of El Niño and climate change, there’s a high likelihood for warmer than normal temperatures to continue this fall, said Zach Labe, climate scientist at Climate Central.

"The summer that is just ending was one for the record books, with record ocean heat, the warmest month recorded globally," Labe said. "And extremes like these could be just the beginning."

Worldwide, August was the warmest on record for land and ocean temperatures combined and the August average in the U.S. set records in terms of maximum, average and minimum temperatures, Labe said.

"El Niño is all about redistributing heat," so the heat comes from the ocean and is transferred into the atmosphere, Labe said. That’s why El Niño years often add an additional boost to global temperatures.

Through August, the average temperature in 2026 has been the warmest on record in the contiguous United States, and Arizona, California, Colorado, Montana, Nevada, New Mexico, Oklahoma, Texas, Utah and Wyoming each had their warmest start to the calendar year on record, he said.

Even though we’re likely to hear more and more about warm temperature records being broken, "it’s not really El Niño that is breaking those records," Labe said. "It's human-caused climate change. That's what's driving the long-term trend."

Federal forecasters expect El Niño to peak later this fall and to be one of the strongest El Niño events on record, "but that doesn't always necessarily mean stronger impacts," Labe said. "Every El Niño has its own flavor. Each one is different. And so that's really important to keep in mind."

Doyle Rice and Dinah Voyles Pulver are national correspondents for USA TODAY focusing on weather and climate. Reach them at drice@usatoday.com and dpulver@usatoday.com.

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