Our meteorologist Veronica Johnson reached out to Metro to find out the answer.
According to WMATA spokesperson Dan Stessel, that rush of air you feel when you enter a Metro station is caused by trains entering and leaving the station. The speed of the trains creates a vacuum, pulling air all the way in from the ground level entrance. That is also where the suction is most noticeable.
As far as the interior climate of Metro stations, Stessel said that’s controlled by a "chiller system." The station's air blows through coils of water, which dissipates some of the heat in the summer.
According to Metro, the chiller system helps to maintain a ten degree difference between outdoor and indoor temperatures in the warmer months.