Fall starts Thursday. Here's your forecast

If you're tired of the swamp heat of summer, the calendar says relief is coming soon: The autumnal equinox, which marks the astronomical start of fall, is Thursday at 10:21 a.m. ET (that's 9:21 CT, 8:21 MT and 7:21 PT).

But don't look for a dramatic cool-down anywhere that day. Warm, summer-like temperatures are expected nearly everywhere except the northwestern and north-central U.S.

Each September, on either the 21st, 22nd or 23rd, the sun appears directly above the equator, marking the exact moment of the autumnal equinox.

Earth roasts through hottest summer ever recorded

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 over most of the world.

Though astronomers say summer ends Thursday, meteorologists and climatologists said summer ended Aug. 31, the final day of the three months of June, July and August.

And what a brutal summer it was, weatherwise: Worldwide, the Earth endured its warmest summer on record, the National Oceanic and Atmospheric Administration said Tuesday. Here in the U.S., we sweltered through the nation's 5th-warmest summer on record.

Folks in the Midwest, Northeast and Mid-Atlantic will have to wait until the end of the week and into the weekend for some truly autumnal weather.

Warm fall predicted, but 'La Nada' is a challenge for the forecast

The transition to the more typical late-September conditions will begin Friday night in the eastern Great Lakes and northern New England and will progress to the Mid-Atlantic by Sunday, AccuWeather said.

But for long-suffering residents of the Deep South and southern Plains, no such relief is in the forecast. Muggy and warm conditions are expected at least through the weekend and likely beyond.

Overall, the Climate Prediction Center said that the three months of October, November and December should see warmer-than-average temperatures across much of the country.

For the 12% of the world's pop

ulation that lives south of the equator in the Southern Hemisphere, Thursday morning marks the vernal or spring equinox, which is the beginning of spring.

So if you're in need of guaranteed warmth and sunshine in the months ahead, head way down south to countries such as Argentina, South Africa or Australia.