Read: D.C. Metro: From construction to operation

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 airflow 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.