Weather or Not | It's warm in cities, wet outside them

Posted: July 02, 2007

With the addition of the 57-story Comcast Center, the Philadelphia skyline is dramatic evidence that Center City has become a hot venue.

Less noticeable is what all that pavement does to your weather.

If you work in town, you don't need a meteorologist to tell you that temperatures in the urban canyons can be 10 degrees warmer than those of surrounding areas.

That's the most-obvious result of the so-called heat-island effect. Buildings and paved surfaces do a fabulous job of absorbing solar energy during the day and are reluctant to release it during the steamy nights. So they heat up in a hurry once the sun comes up again.

It is impossible to say how much one more building adds to the impact, says J. Marshall Shepherd, a scientist who is one of the nation's top experts on heat islands, but unmistakably it does something.

What has become increasingly clear in recent years is that the effects aren't confined to the "islands" or to temperature. Several studies have documented urbanization's wide-ranging effects on precipitation outside of cities all over the country, and perhaps the world.

This is most prominent downwind of core downtowns, but the phenomenon could be spreading with sprawl.

How do cities affect rainfall?

Philadelphia, New York and other big cities are generous sources of warm air. Rain forms when warm air rises over cooler air and condenses. If the updrafts are strong enough, they will shake out the thunder from the sky.

The contours of the city are also a complicating influence on storms. They can cause storms to split, and then come together again somewhere downwind. Shepherd, a former NASA researcher who is now a University of Georgia professor, calls this confluence of warmth and tall buildings "urban rain effect."

Winds can displace the city-related moisture. While meteorologists caution that it is impossible to prove that any one heavy rainfall is the result of this phenomenon, the intuitive and circumstantial cases are compelling.

"When you watch storms on radar," says James Eberwine, a veteran meteorologist at the National Weather Service in Mount Holly, "when they reach the hot pockets, Philly and New York, you'll see them intensify a little bit."

The data support the hypothesis that some of the juice that storms are getting from the cities gets wrung out over the suburbs.

Using satellite data, Shepherd found that summer rainfall in cities such as Atlanta and Memphis was as little as half what it was 20 to 40 miles away. He cited other research that showed more modest differences in a league with what I found around here.

My analysis of National Weather Service data showed summer rainfall in Burlington, Bucks, Chester and Montgomery Counties to be about 10 percent higher than the amounts measured at Philadelphia International Airport.

Interestingly, the airport and Delaware County evidently are in something akin to a rain shadow. Prevailing winds in summer are from the southwest, so they would protect those areas from Center City moisture. What about the areas to the west?

In summer, sea breezes off the ocean can jog a storm to the west, and meteorologists believe that is one reason that the Pennsylvania suburbs have been hit with flooding downpours in the past.

Recall that in late June last year, about 3½ inches of rain fell at the airport during a week-long wet spell, but 8 to 12 inches was measured in parts of Bucks, Chester and Montgomery Counties.

It is unclear whether the urban effects are causing a net increase in the world's precipitation, Shepherd says.

If you live in the Northeast Corridor, you may find it hard to believe that less than 2 percent of the nation's land is paved over. Compared with greenhouse gases, urbanization is not considered a major factor in warming the world.

But global warming aside, the urban heat effect rebuts the old adage that we can't do anything about the weather.

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Animation of an urban heat island: .

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