John Rowland wrote:

Drains in varous London roads were emitting clouds of steam tonight. Why
tonight, when they don't normally do this?

Presumably because it was cold, and the contents of the drains were
warmer. It was *condensing water vapour* rather than steam, and doesn't
need to be nearly so hot. Steam, FWIW, is as invisible as the rest of
the air.
--
http://gallery120232.fotopic.net/p11654414.html
(Over-powered? 143 566 on push-pull duty at Birkenwirder, Berlin, 1999)