Why are subway/metro stations sometimes really windy?

37 views
0

Especially on the escalators that go down? My guess is that it’s something about different air temperatures but really don’t know

In: 68

Trains push the air ahead of them through the tunnels. When the conditions are right (like when two trains are approaching the station from both sides at the same time) that air gets out through the escalator shafts to the outside because it doesn’t have anywhere else to go.

Trains pushing/pulling air into/out of the tunnels is the main cause, creating pressure differences.

As the train goes down the tunnel, it just keeps pushing the air forward. There are ventilation shafts, but most of that air will escape through the station.

While trains pushing wind ahead of them is one reason there are several others.

1. The main reason why the escalators going down/up are windy is because even small changes in air pressures in a space as large as the waiting platforms&tunnels downstairs and the hall upstairs can cause very high changes in airpressure in small spaces, like the comparitively narrow tunnels that connect the two. So if for example it’s getting colder upstairs (because it’s getting later in the evening, and cold air shrinks slightly) then the airpressure difference means that wind is going to stream from below to try to even out the pressure.
2. Lots of people means that ventilation needs to keep the air flowing. That contributes to windy spots in buildings as complicated as a subway station.
3. There is generally a large difference in temperature below and the temperature above. That’s going to make the air change as cold air shrinks and wants to get as low as possible while hot air expands and wants to go up.
4. The subway station connects to long tunnels. That is going to increase the felt winds since when air pressure in the tunnels change the air can’t expand in every direction (or pull in air from every direction), just along the path of the tunnel. For example, if you were just gently blowing into the air you’re not going to feel that 2 meters away, but if you did that through a hose then you would.

All of these also interact. For example if ventilation is pumping hot (but fresh) air from the surface it’s going to shrink as it’s cooled by the subway and you’re going to have a constant change in the air dynamics. Which causes wind.

The trains are acting like the piston in a syringe. They push the air in front of them and draw a partial vacuum behind them. Air rushes in/out the escalator shafts to try and equalize pressures.