Summer has had a hard time letting go this September. In the D.C. region and many parts of the country, relentless humidity has kept us sweating for weeks, and signs of fall have been few.

But if you’re ready to don your favorite sweater with a mug of hot apple cider in hand, time is on your side. The autumnal equinox arrives Saturday at 9:54 p.m. Eastern, and that means cooler weather is all but inevitable.

The fall equinox, traditionally considered the first day of fall, happens each year around Sept. 22 or 23. It’s the exact moment when the sun appears directly above Earth’s equator, bathing our entire planet in roughly equal amounts of daylight and darkness.

After the equinox, the sun’s direct rays shift into the Southern Hemisphere, where winter is ending and spring is now getting started. Here in the Northern Hemisphere, we’ll see daylight continue to dwindle for another three months until we reach the winter solstice Dec. 21.

Every year around the fall equinox, we lose daylight at our fastest clip of the year. The District is now losing 2 minutes and 30 seconds of daylight per day. In the northern United States, such cities as Minneapolis and Seattle see more than three minutes of the sun’s light vanish with each passing day.

The change in sunlight is the main reason trees burst into brilliant shades of red, orange and yellow before dropping their leaves for the winter. (To find out when fall foliage will peak in your area, check out this great reference.)

We still have a few more days before we enter the dark half of the year — when trees start responding to the waning daylight. Even though equinox literally means “equal night” in Latin, both of Earth’s hemispheres still get more than 12 hours of daylight Saturday. One reason for this is because Earth’s atmosphere refracts, or bends, light, which causes the sun to appear higher in the sky than it is.

In the District, sunrise and sunset on the equinox are at 6:56 a.m. and 7:05 p.m., respectively, giving us 12 hours and 9 minutes of daylight. Not until Sept. 26 will sunrise and sunset fall exactly 12 hours apart, at 6:59 a.m. and 6:59 p.m.

As the nights grow longer and the sun drops lower in the sky, temperatures rapidly begin their seasonal descent. The District’s average high temperature, now 77 degrees, dips to 64 degrees by the end of October. The average low temperature drops below 60 degrees Sept. 24 and won’t get there again until late May.

As if on cue, Mother Nature may deliver our first taste of crisp, autumnal weather late next week. The latest forecast from the National Oceanic and Atmospheric Administration’s Climate Prediction Center shows below-normal temperatures in the central and eastern United States as October arrives on our doorstep.

If you’re waiting for a true taste of autumn after our long swampy summer, this forecast will surely bring a smile to your face.

You can finally indulge in that pumpkin spice latte, and it will actually feel right.

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