Why does steam rise from the subway grates in the streets during wintertime?

Question

Anyone know?

Answer 1

probably it is the venting of excess pressure of the steam heating central system which pumps steam to different buildings

Answer 2

Both the steam system and the subway system are

subterranean conduits, one of which transports steam, and the other

people.

The steam vents assist in regulating the pressure of Con Ed's

huge network of pipes and valves, the world's largest steam system.