a photo of man in a D.C. Metro station.
Where's the wind coming from exactly? Mark E. Gibson/Getty

Unpredictable and sometimes alarming, the weirdly powerful winds that sometimes whip through subway stations might be explained by some basic rules of physics.

The intense and mysterious winds of the Washington, D.C., Metro system seem to come out of nowhere. They’re not particularly bad when standing on the train platform, as one might expect; in fact, it’s the level between the train platform and the street where the infuriatingly powerful gusts are the strongest. Every now and then, my fellow commuters and I will be caught holding down our clothes and gripping our reading material, while our hair slaps us in the face. Occasionally, a small child is knocked over. But it’s impossible to predict when the winds will strike. One morning I noticed I was tensing my entire body, bracing myself for a blast that—of course, this time—never came.

It’s maddening, and no one can seem to figure out where the wind is coming from, or why it’s so variable. Plenty of Washingtonians seem content to clutch their hats and ignore this puzzling phenomenon. When I first moved here, I heard the tracks would be “constantly on fire” (they aren’t), construction would make weekend travel impossible (it doesn’t), and all the Metro cars have weird, vintage rugs (which, OK, some do). But what I wasn’t warned about—and wish I had been—was the wind.

After a few months of being consistently caught off guard, I needed to understand what was happening. So I called Kenny Breuer, an engineering professor at Brown University, to see whether he could help me figure out the possible factors.

Several interconnected variables could be at play here, according to Breuer, and one could be an air-pressure difference. Any large, underground environment with a lot of people—such as a Metro station—needs “a powerful air-conditioning or air-handling system.” This system creates an exchange of air from the ground to the upstairs, or indoors to outdoors, where there is also likely a pressure difference. Air also naturally moves from areas of higher pressure to those of lower pressure, to equalize a pressure difference. “If you’re in the right place,” he told me, “these relatively small pressure differences can generate a substantial flow.”

Furthermore, Bernoulli’s principle states that as air pressure changes, air speed reacts in the inverse, basically trading off. So, as Breuer explained, “if you have a high air pressure, you have a low speed, and if you have a low air pressure, you have a high speed.” Anytime there is airflow, like “air moving through the station, from large spaces to small spaces,” the velocities and pressures can change substantially.

Yet another factor could be what’s commonly called the piston effect, which refers to the air flow that a vehicle creates when it moves through a tunnel. In this case, trains are running through the tunnels at high speeds, pushing air out in front of them onto the platform— which spreads outward wherever it can (such as up escalators and into other sections of the station)—and creating suction by pulling air into the tunnel behind them.

On top of all this, Breuer pointed out that “incredibly complicated geometry” is involved in Metro stations: “There’s trains going in two different directions, or if it’s a big station, an interchange with multiple rails and multiple trains going.” In large cities like New York, building shape has a direct influence on wind strength. (Chicago also comes to mind here, although the Windy City did not get its name from weather, but from “the hot air bellowing from politicians.”) And because of the aforementioned pressure difference, aboveground air also affects the air belowground.

Finally, thermodynamics must be accounted for. “If the air in the Metro is warmer than the air outside or vice versa,” Breuer said, “then the temperature difference could cause a flow of air.” Again, it’s simple physics: Cold air is denser than hot, which means cold air will fall and hot air will rise. In the case of so many urban, underground train stations, which are (in theory) heated in the winter and cooled in the summer, cold air could be flowing down into the station, or warm air flowing upward out of the station, “and that could be a significant factor.”

So air-handling systems, Bernoulli’s principle, the piston effect, complex building shapes, and temperature shifts all possibly have a role. I reached out to D.C.’s transit agency, the Washington Metropolitan Area Transit Authority, for further clarification and comment, but it never responded.

My bizarre fascination aside, is there anything, really, to worry about here? My research turned up one long-term concern: Underground transportation hubs are uniquely vulnerable to airborne biochemical attacks, similar to what happened in Tokyo in 1995. These stations have “high populations of people in a confined space,” with dozens of openings to the city, as David Brown, of Argonne National Laboratory, explained to me over the phone. That means “material can move throughout the system and can come up all over the place.”

He must have heard the concern in my voice, because he followed up with reassurances about “a computational model that we can use to predict these [air]flows and how materials are transported through the system.” He added, “The really key things are to know how material dilutes from station to station, and at what speed it moves through the system.” In the past decade, Brown and his team have run “extensive” simulations in Boston, New York City, and Washington, D.C., to gain information that could help inform responses to a biochemical-terrorist incident.

But the short answer to my question is no—commuters can face Metro winds without fear. Apart from one very unusual case in 2014, when a child in a stroller was blown onto the tracks in London, and three cases in which gusts caused by the movements of the trains themselves blew people right over, the strong winds aren’t dangerous. At most they will ruin our hair, ruffle our clothes, and discourage at least some of us from wearing skirts.

This post originally appeared on The Atlantic.

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