In NYC, manholes allow excess steam and vapor to escape the pipes below. Steam, along with the yellow cab, have come to visually symbolize New York, as every low-budget filmmaker knows. If you happen to be one, I’d recommend the full atmospheric effect – a nighttime shot with the cab driving through the billowing steam.
As we know from recent experience, a really cold day = more steam coming from the manholes.
Pix and other images 