Rain douses another Philly rush hour, but it might just rescue the foliage show

Yet another round of rain arrived just in time for Tuesday’s peak commuting period, as the region’s reversal of drought fortunes continues.

In the last week, the 2-plus inches of rain measured officially in Philadelphia exceeded the total that had fallen in the previous two months. As rush hour was getting underway, rain crept into the region and spread eastward, with 0.27 inches measured between 6 and 7 p.m. at Philadelphia International Airport.

While the recent dreariness hasn’t been exactly mood-enhancing, and braking on wet leaves can be a hazardous exercise, the rains evidently are having a benign effect on the region’s annual foliage show, which appears to be rallying as it approaches its annual peak.

“I notice the colors coming on strong now,” said William Cullina, executive director of the University of Pennsylvania’s Morris Arboretum in Chestnut Hill.

The recent rains have been a shot in the autumn palette, said Marc Abrams, a Penn State forestry professor and foliage connoisseur who has been observing the color change for four decades.

Earlier this month he had warned that the flash drought across Pennsylvania that began in the summer would have a dampening on the colors, which he described as “quite muted” in the early-going.

However, the foliage season in the Philadelphia region usually doesn’t peak until the end of October, so the atmospheric faucet might have been turned on just in time.

Continued extreme dryness would have compromised the coloration by accelerating leaf-fall, said Cullina.

Autumn hues are governed by three pigments: carotenoids (think pumpkins), anthocyanins (red apples), and green chlorophyll.

As sunlight weakens during the rapidly shortening days of fall, chlorophyll recedes and yields to yellows and oranges. Eventually, the nourishing veins at the leaf base close off, and the sugars remaining in the leaves make those showy anthocyanins.

The basic processes occur every year. However, the behavior of trees can be as elusive as that of the atmosphere, and it is impossible to forecast how brilliant any given season would be in any given region.

It is known that in addition to the absence of drought, Abrams said that ideal conditions include warm, sunny days in late September and October, and cool-ish nights without killer frosts.

Those criteria have been met around here, and the rest of the week should be dry with high temperatures in the 60s and lows in the 40s. Some parts of the region were covered with frost during the weekend, but Abrams noted that recent frosts in central Pennsylvania, where he works, have not affected color.

One fortuitous development in recent years is that the peak-foliage seasons have been lasting longer. In part that is due to generally increasing fall temperatures coinciding with worldwide warming, but another big factor has been a foreign invasion.

Foreign trees, particularly Asian species, have become more prevalent in urbanized areas, said Cullina. Those species tend to shed their leaves later.

That has been evident in New England, according to one non-scientific source. The people who run the immensely popular Polly’s Pancake Parlor, in Sugar Hill, N.H., have been tracking foliage onsets, peaks, and leaf-falls since 1975. The last two years, leaf-fall has been later than ever — Oct. 27 and Oct. 22, respectively.

Cullina said he recently returned from a trip to New England, where it had not been as dry as it had been around here, and the colors were “spectacular.”

Right now, all signs indicate that the Philadelphia region is about to experience what naturalist Edwin Way Teale called “the glorious, flaming sunset of the year.”