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........

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