The astronomical start of fall arrives Tuesday evening in the Northern Hemisphere, but forecasters say anyone hoping the change of season will bring a quick break from summer heat may be disappointed, with a combination of El Niño and climate change expected to keep U.S. temperatures running warmer than normal through much of the fall.

The autumnal equinox occurs precisely at 8:05 p.m. Eastern time Tuesday, the moment worldwide at which the sun sits directly above the equator. The word “equinox” derives from the Latin words “aequalis” and “nox,” meaning equal night, reflecting the fact that day and night run roughly 12 hours each across most of the world on the date of the equinox. That near-equal split will hold true Tuesday from as far north as Utqiagvik, Alaska, formerly known as Barrow and located north of the Arctic Circle, all the way south to Wellington, New Zealand, the world’s southernmost capital city. Day and night are not precisely equal on the equinox itself because Earth’s atmosphere refracts, or bends, sunlight in a way that creates the appearance of slightly more daylight than technically exists at that moment, meaning the date when day and night are truly equal in length typically falls a few days after the equinox.

South of the equator, Tuesday’s equinox marks the opposite seasonal transition, signaling the start of spring rather than fall for the Southern Hemisphere.

Despite the equinox’s popular association with the “official” start of fall, no government or scientific administrative body actually designates it as such in any formal capacity. Meteorologists and climatologists instead define the seasons differently, using a system based on the calendar month rather than the sun’s position relative to Earth. Under that meteorological definition, summer, defined as the three hottest months of the year in the Northern Hemisphere, June, July and August, officially ended on August 31, more than three weeks before Tuesday’s astronomical equinox.

Whichever definition is used to mark summer’s end, forecasters say the transition into fall is unlikely to bring the kind of widespread cooldown many might expect. Zach Labe, a climate scientist at Climate Central, said the combination of a developing El Niño pattern and the broader effects of human-caused climate change points toward a high likelihood of warmer-than-normal temperatures continuing through the fall season. “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.”

According to Labe, global temperature records set this summer were substantial in scope. Worldwide, August ranked as the warmest such month on record when accounting for combined land and ocean temperatures, and the U.S. national average for August set records across all three standard measurements: maximum, average and minimum temperatures. Labe explained that El Niño’s influence on those record-setting figures stems from the pattern’s underlying mechanics. “El Niño is all about redistributing heat,” he said, describing how the phenomenon transfers stored ocean heat into the atmosphere, a process that tends to provide an additional boost to global temperatures during El Niño years.

The scale of this year’s warmth has extended well beyond isolated regional records. Through August, the average temperature across 2026 has ranked as the warmest on record for the contiguous United States as a whole, according to Labe. At the state level, Arizona, California, Colorado, Montana, Nevada, New Mexico, Oklahoma, Texas, Utah and Wyoming have each recorded their warmest start to a calendar year on record through the same period.

Even as El Niño draws attention for its role in this year’s heat, Labe was careful to emphasize that the underlying driver of the broader warming trend lies elsewhere. “It’s not really El Niño that is breaking those records,” he said. “It’s human-caused climate change. That’s what’s driving the long-term trend.” Federal forecasters expect the current El Niño pattern to peak later this fall or in early winter and to rank among the strongest El Niño events on record, though Labe cautioned against assuming that intensity alone will translate directly into more severe weather impacts. “That doesn’t always necessarily mean stronger impacts,” he said. “Every El Niño has its own flavor. Each one is different. And so that’s really important to keep in mind.”

For much of the country, the practical upshot of Tuesday’s equinox is likely to be more symbolic than immediately noticeable in day-to-day weather. While the sun’s angle and daylight hours will continue shifting toward shorter days and longer nights as the Northern Hemisphere moves deeper into fall, the broader warmth that has defined this record summer appears poised to linger well past the calendar’s official seasonal marker, according to the forecast outlined by Labe and other climate researchers tracking the current pattern.

With El Niño’s peak still weeks or months away and the long-term warming trend tied to climate change continuing to shape baseline temperature expectations nationwide, forecasters say residents across much of the United States should prepare for a fall season that, despite its astronomical start Tuesday evening, may look and feel considerably warmer than the season traditionally has in years past.

Copyright ©2026 Dow Jones & Company, Inc. All Rights Reserved. 87990cbe856818d5eddac44c7b1cdeb8