Do cities deflect rain storms?

Motorists normally pay scant attention to the mulch-colored dirt on the humble hillsides of the Schuylkill Expressway near Conshohocken. But not last Sunday: That's because the dirt was on the expressway.

"We could see very deep, muddy water just pouring on the road," said Robert Vonderheide, who had the misfortune of being eastbound near the Conshohocken curve when all mud broke loose.

The rains hammered areas to the west and east of Philadelphia, with up to 3.5 inches falling. The airport measured less than half that amount.

Nature's caprice? Very possibly.

But in the 21st century, summer thunderstorms often whack suburbs harder than cities, here and around the country.

And although last Sunday's storm might not have been a case in point, scientists say one reason for that phenomenon could be the cities themselves.

Recent research suggests that dense, urban building might enhance rain just outside cities. A NASA study based on satellite data showed that summer rainfall in Memphis and Atlanta was half what it was 20 to 40 miles away.

Other studies have found that urban heat might alter lightning-flash patterns and the paths of thunderstorms and perhaps even afford downtowns a small measure of protection against severe weather.

All this remains controversial. "You can get people almost at each other's throats," said Tim Oke, an urban-climate specialist at the University of British Columbia.

But climate change aside, it has long been established that as sure as asphalt is warmer than grass on a blistering afternoon, cities are warmer than their surroundings. That's the "urban heat island" effect. Buildings and paved surfaces soak up sun and are stingy about giving up heat at night.

"The urban heat island is known to occur on all cities, regardless of size," said Robert Bornstein, a meteorology professor and urban-climate expert at San Jose State University.

Precisely how urban heating affects rainfall and thunderstorms is uncertain, and people who predict weather say it might be a long time before it can be factored into forecasts. But it's there.

"It's reasonable to think that there would be some enhancement of activity over and/or downwind of the area," said Al Cope, senior scientist at the National Weather Service office in Mount Holly.

He added that no one yet had taken a hard look at Philadelphia, which has undergone a significant skyscraper boom since the building-height limit was lifted in the mid-1980s and, presumably, an urban-heating boom.

Thunderstorms are set off when warm, moist air rises rapidly through cooler air and water vapor comes out of hiding and condenses into raindrops. Urban heating is a likely source of booster fuel in launching the plumes skyward and thus producing heavier rain. Along with the heat, Oke said, the buildings create turbulence that "packs" the air and ejects it upward.

Though cities might add a kick to the rain, they appear to be exporting some of that mayhem and moisture.

"The urban environment itself may help to shield itself," said J. Marshall Shepherd, a professor of geography at the University of Georgia who is a heat-island expert. Tornadoes in city centers, he noted, are rare.

Shepherd first looked at precipitation patterns several years ago when he worked at NASA. His interest was piqued by what he saw in summer, when strong and geographically extensive fronts and other big players in the atmosphere often go on vacation. He noticed that smaller features - a sea breeze here, an air-lifting hill there - perturbed the atmosphere and set off showers in the absence of obvious storm systems.

When he detected a similar phenomenon around cities, he surmised that the perturbing influence might be urban heating and that the resulting rain and lightning were being displaced. Shepherd did the Memphis-Atlanta study by analyzing satellite data to compensate for the sparse measuring network at the surface. In Philadelphia, the effect isn't necessarily all downwind; sea breezes off the Atlantic can jog blobs of rain westward.

He called it the "urban rain effect."

Other research around Atlanta and Houston and in Brazil suggested that cities also were affecting lightning patterns. Scientists found that areas recording the most lightning flashes coincided neatly with places that were getting heavier rain.

City buildings might be deflecting or altering the course of storms, according to Bornstein. He found cases in New York in which the buildings induced storms to split as they approached the Big Apple, adding that he had seen a similar phenomenon in Paris. Such a splitting would spare the city proper, but areas downwind would get socked after bisected storms reunited.

Shepherd believes Bornstein is on to something. Think of the storm, he said, as a moving stream forced to flow around a massive stone - in this case, the skyline.

Could that have happened last Sunday? Meteorologists doubt it. The rains were wrung out of a decent-size storm system that affected a large area. It is more likely that the system encountered a particularly juicy environment out where Vonderheide was driving, said Ken Reeves, director of forecast operations at AccuWeather Inc.

Cope said he was skeptical of the split theory in general. After all, he said, the tallest Philadelphia buildings are about 1,000 feet high, and thunderstorms can reach several thousand feet into the atmosphere. Oke agreed.

"I doubt it has much effect, as far as being an obstacle in the flow," Cope said.

"I can't say every storm splits," Bornstein said. But he added, "whenever I present this at conferences, the forecasters come up and say they see it in their cities."

As for quantifying the effects of city heat on rain, another complicating factor is suburban sprawl, Reeves said. Suburbs have more malls, parking lots, and streets than ever. So how could one isolate the urban effect?

"As a scientific conundrum," Oke said, "it's very difficult to untangle."