There is a quite a lot of water vapour in the exhaust gases from a petrol vehicle, so it's certainly likely that some of this water vapour will become visible as steam once it hits the cold air outside the car, and colder it gets the more visible this will be.

As long as you aren't smelling (and loosing) coolant and don't have an excessive build up of moisture in the oil systems, I'd say it was normal.

__________________
Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)