On Fri, 18 Nov 2005 03:04:19 -0000,
John Rowland
wrote in :

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

The weather. I imagine the air would have been close to 100%
relative humidity -- the cooler the air the less water vapour it can
carry, so cooled air tends towards its saturation point. However, the air
immediately above a quantity of water is essentially saturated (100% r.h.)
at the temperature of the water. Say the water in the drains was still at
15 C while the air was 0 C[1]; the partial pressure of water vapour in the
outside air would be 4.6 mmHg, that in the drain 12.8 mmHg. Any movement
of the air from the drain to the outside would cool the air to super-
saturation, and the excess water would form droplets, visible as steam.

http://hyperphysics.phy-astr.gsu.edu...watvap.html#c1

[1] It's -3 C here at the moment; got to 0 C around 10 or 11 last night.

--
Ivan Reid, Electronic & Computer Engineering, ___ CMS Collaboration,
Brunel University. ] Room 40-1-B12, CERN
KotPT -- "for stupidity above and beyond the call of duty".