The equinox arrives Tuesday in Philly, and it finally feels like fall

Nature already is splashing its flecks of yellow on the trees, and soon the Technicolor waves of foliage will ripple toward Philadelphia in what the experts predict will be quite a spectacular season.

Right on schedule, at 8:04 p.m. Philadelphia time Tuesday, the astronomical fall begins with the instant of the autumnal equinox, when the sun beams its direct light on the equator and nature continues to turn down the dimmer switch in the Northern Hemisphere. It is as though the trees brighten to compensate for our daily loss of 2½ minutes of daylight.

It’s the season when the night skies are stunningly crowded with winged traffic, when a remarkable 2-inch-long, Halloween-hued creature has its days in the sun.

And on the nation’s 251st autumnal equinox, we take a look at what nature might offer in the only season with two widely accepted names, “autumn” and “fall.” And we peek at the coming winter — for which a lot of people around here have other names.

For the third time in the last four years, an equinoctial cooldown is due this week, with daytime highs in the 60s, as much as 10 degrees below normal.

But the federal Climate Prediction Center on Thursday had the odds tilted toward above-normal temperatures in October in Philly.

Monthly readings have been above normal in 15 of the last 16 Octobers. In 2024, it hit 84 degrees on Halloween. In that period, annual average temperatures at Philadelphia International Airport have been about 1.3 degrees above normal, but the Octobers have been a full 2 degrees above.

The intensifying El Niño in the equatorial Pacific is a major player in the center’s winter outlook, posted Thursday. It favors above-normal temperatures here and in most of the nation. The center eschews snow forecasts, but it does foresee a potentially active storm track along the East Coast.

A cautionary note: This whole seasonal-forecast exercise remains quite the work in progress, and last winter was yet another victory for atmospheric chaos over science. No one foresaw the 30 inches of snow for Philly or a historic snow-and-ice cover.

The so-called woolly bear caterpillar had long held a legendary place nationally in weather folklore, but in 1948 the forecasting reputation of the incipient Isabella tiger moths took wings.

That year a respected entomologist with the Natural History Museum in New York City ventured to a state park and announced that, based on his analysis of the caterpillars’ black-and-orange-ish rings, the subsequent winter would be mild in the New York region. His forecast was dutifully reported in a New York newspaper, and, indeed, the winter was quite mild.

He continued reporting his annual analyses (however waggishly) that correctly foretold of several mild winters.

As it happened, some of the winters in that period were so snow-deprived in the Northeast they became the impetus for the snow-making industry that has forever changed skiing.

Don’t credit or blame the woolly bears, said Douglas S. Richmond, entomology professor at Purdue University. Their ubiquitous presence this time of year all over North America indicates little more than a 100% probability that it is autumn.

They are quite fascinating creatures, he notes, that survive winters by producing “body fluids that function as cryoprotectants — essentially biological antifreeze.”

Their bodies actually may contain clues about the behavior of a previous season, according to Richmond.

“It’s probably better to think about woolly bears as being much better equipped with a rearview mirror than a crystal ball,” he said.

Humans have had a long history of forgoing global forecast models and oscillation indexes in favor of consulting nature for winter clues.

One popular myth cited by The Old Farmer’s Almanac holds that if the acorns keep raining down from the trees like hailstones, get ready for a big chill.

Leaves would be another indicator: If they drop early, winter will be a breeze. If they linger late, those breezes might be cold.

Among Native Americans, beavers were bellwethers. The thicker their autumnal coats and stronger their lodges, the harsher the winter.

The natural world is where to look for clues about the season to come. The waning sun may dim human energy, but it is an ultra-busy time for a lot of other species. It is prime mating season for animals such as birds, bears, deer, moose — and porcupines. We won’t ask how they hug.

And birds — billions of them — are on the move.

The bird feeders and baths are getting ever emptier these days, but something quite amazing is happening after nightfall about 3,000 feet over our heads.

An estimated 1.3 million birds flew across Philadelphia on Friday night and early Saturday. And Chester County was the regional hot spot, with 3.8 million heading south-southwest, according to the Cornell University Ornithology Lab’s radar-powered BirdCast dashboard, With a tail wind from the north, they were zipping along at 25 to 30 mph about 1,600 to 1,800 feet above our heads. On the site, you can track bird traffic live, county by county.

This is the peak migration season, continuing into the first week of October, says Keith Russell, urban-conservation program manager for Audubon Mid-Atlantic, with about 300 species racing through the region, including ruby-crowned kinglets and black-throated green warblers.

To travel thousands of miles, the birds evidently use a remarkable assortment of clues from the sun, the stars, the Earth’s magnetic field, and even landmarks, according to the Cornell lab.

If anyone calls you a “bird brain,” be flattered.

Our planet is arguably in the solar system’s sweet spot for equinoxes, which occur twice a year.

By contrast, Saturn gets one only every 15 Earth years, and Neptune every 82 years, says Harry J. Augensen, director of the Widener University Observatory and a physics and astronomy professor emeritus.

Mercury has four a year, but temperatures range from 800 degrees Fahrenheit to 290 degrees below zero.

Earth never looked so good.