what is the steam that comes out of manhole covers in New York City?

Answer 1

I had to look this up. Apparently the steam is caused by external water being boiled by contact from the steam pipes, not the steam pipes themselves leaking. But im not sure i just read it, maybe the other guy is right.

Answer 2

There are pipes of steam running underground throughout the city along with the electric, water, sewr, phones, and subways. It's really fascinating to think of what all is underground in Manhattan. It's like it's a huge ship or something. Anyway the steam travels to buildings that are heated by it or heat their water with it (not all buildings are customers), and at times these pipes need to be vented to let some of the pressure out.
It would be cool if the steamers were somebody's idea to heat the city outdoors in the winter, and of course they do a little. .
Look at the book, "The Building of Manhattan" for a fascinating story of its building. It has great sketches, maps and diagrams.

Answer 3

ConEd provides steam to some buildings in Manhattan. The "smoke" from the manholes are relief valves for the steam distribution system.