Wind does funny things in a city. On an unusually pleasant summer morning in Manhattan, Kirk Clawson and his team of scientists are mapping the chaotic airflows of midtown by tracking a harmless gas. It's preparation for something serious: a hazardous waste leak or a terrorist attack. But an unexpected wind from New England messes up their plans. They thought they'd see flows of 10émph, but they're logging speeds closer to 15émph. "And it's shifted direction by 35édegrees," says Clawson, working in the faculty lounge of a high school on 46th Street between 6th and 7th avenues.
The wind change makes it all the more difficult to trace the plume of colorless, odorless sulfur hexafluoride that a colleague is discharging from two big tanks on a handcart up at 48th Street. In this drill, the SF6 plays the part of a biological or chemical weapon. Clawson is a researcher for the Department of Homeland Security's $10 million Urban Dispersion Project, which did some preliminary tests early this year. Today he has deployed five researchers throughout the city to call in with updates.
Clawson and his colleagues are at the forefront of an emerging discipline: urban micrometeorology. Their aim is to provide NYC officials with a map of how winds act in this neighborhood to help with planning evacuations under a range of weather conditions. (During the work week, midtown is one of the most densely populated square miles in the world.) Their success depends on Clawson having placed his researchers in the path of the SF6 cloud.
He decides not to move them. "All the gas is released," he announces after a few minutes. Clawson pulls out a map and waits for his team to start calling in with field data. The release is concentrated around 49th and 50th streets, and the majority of the collection points are positioned in a 10-block swath to the southwest.
Clawson's PDA phone chimes softly; he checks the caller ID and then holds it to his ear. "Go ahead, Roger," he says, typing on his laptop. Seven minutes have elapsed since the gas was dispensed, and Roger, whose van has just driven past 47th Street and 8th Avenue, is reporting significant amounts of SF6. "Good," Clawson laughs. "We put you in the right spot."
Ten minutes later, a report comes in from Jason. Clawson takes the call, then turns to Jerry Allwine, the project leader. "It's very erratic. Jason says his machine has railed [registered higher amounts than it can measure]." As the easternmost sampler, he should have been out of reach of most of the plume, but the wind shifted. This turn of events is nothing like the test they ran in Oklahoma City, Clawson says during a lull, marveling at his chaotic data. That city has a small, contained downtown with a predictable summer wind. In short: It's not New York.
Half an hour after the initial release, Clawson gets the strangest report of all. It's Jason, calling from a new location. "What have you got?" He pauses to listen. "Oh, wow. Wow." Allwine looks at Clawson from across the table. "Jason just railed again, after 20 minutes of being at baseline." In other words, long after the SF6 was thought to have dispersed, a cell of concentrated gas meandered through the city until it reached 44th Street. It will be impossible to say how until the team analyzes data from 175 additional automated air samplers that Allwine has set up around town. "On previous days, we've had lighter winds. We'd see a steady peak, and then a drop-off," says Allwine. "But today the wind is shredding and pulling the plume into pieces." This is the kind of weirdness that can kill everyone on the sunny side of a street but leave those on the shady side unscathed.
According to team member Julie Pullen, Manhattan's "corrugated" skyline makes it difficult to predict how winds will act at street level. "It's not the height of the buildings," but their relative locations, she says. "It's that you'll have a three-story building next to a 60-story office tower." That's bad news for scientists trying to build a model, but worse news for terrorists trying to plan a precise attack.
- Jeff Howe
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Going With the Flow
