Is autumn foliage falling later?

If it seems as though the fall foliage seasons around here have lingered ever longer - all but bumping into Thanksgiving - they have.

For one thing, the experts say, the region has added more and more tree species that can hold their leaves into December.

For another, this has been a remarkable period in the region's climate.

While it is a 100 percent certainty that fall will arrive at 11:44 a.m. today and that the sun's angles will become progressively more oblique, it may be several weeks before the region sees its first hard freeze.

The official Philadelphia temperature hasn't dropped to 32 before Halloween in 15 years, the longest such streak in the 136-year period of record.

While climate specialists caution against drawing conclusions about global warming from fall temperatures, the local trend is impressive.

In that 15-year stretch, on average the first freeze date has occurred on Nov. 12. By contrast, in the 1960s the average was Oct. 28.

Since the measuring site is located at the airport next to the Delaware River and a swamp, temperatures are likely to be lower in most suburbs. However, a 32 reading at the airport would suggest a widespread "hard" freeze, and that's critical to when leaves fall, said Paul L. Meyer, executive director of Morris Arboretum in Chestnut Hill, one of Philadelphia's chillier locations.

"A hard frost will bring them down very quickly," he said.

WSI Corp., a private weather service in Massachusetts that supplies forecasts to energy interests, has called for a mild fall throughout the Northeast, and that may be welcome news for energy consumers.

But it is highly unlikely that it will match last fall. In Philadelphia, the temperature nudged past 80 as late as Oct. 23, and the leaf fall was as much as three weeks behind schedule.

David Robinson, the Rutgers University professor who has been the New Jersey state climatologist since 1991, said he had never seen anything like it. "Last year just blew me away," he said.

Meyer, too, said he couldn't recall a more unusual season. With the proliferation of late-dropping species such as Bradford Callery pears and Norway maples, the foliage season would tend to last longer these days than in earlier years. "The species mix in Philadelphia is quite different from what it was 75 years ago," he said.

But last year was something else. While color changes are driven primarily by changes in sunlight, October's unusual weather conditions were an important factor, he added.

The onset of color evidently was slowed by the lack of chilly nights, and that phenomenon was not confined to Pennsylvania and New Jersey, said Paul G. Schaberg, research plant physiologist for the U.S. Forest Service in Vermont.

Schaberg said that early in the season, some chilly weather is important to persuade the greens, which contain chlorophyll, to take a hike and make way for the brilliant reds, which come from the tree's anthocyanins. That hasn't been happening.

"In recent years, we have seen a tendency for later frosts, and thus a later fall color season," he said.

It is unclear, however, whether the tardier foliage seasons have anything to do with global warming.

Over the long term, fall temperatures haven't shown much change. "Globally, you see nothing," Robinson said.

Nor does fall show any extraordinary tendencies in temperature and precipitation in the index of climate extremes, maintained by the National Climate Data Center.

In Pennsylvania and New Jersey, however, "You can make a case that we've been on a run," Robinson said.

In Philadelphia, last autumn was the 14th-warmest on record, and seventh-warmest statewide in New Jersey.

Ten of the top 15 warmest summers have occurred since 1991 in Philadelphia. New Jersey has had five of its seven hottest summers since 1999.

Whether the run of warmth will continue is uncertain, Robinson said, even if the world keeps warming.

Philadelphia has had other clusters of warm fall seasons.

During a 15-year period at the turn of the century, for example, the average first freeze date was Nov. 17.

"There has always been natural variability," Robinson said, "and there always will be."