http://www.youtube.com/watch?v=ZiP-6YHIUnw
http://www.youtube.com/watch?v=-hsloC-ic1o
http://www.ust.hk/~webopa/news/2000_News/news060902.jpg
http://upload.wikimedia.org/wi ... _model-wingtip-vortex.jpg

It's not always white in some cases, too. http://www.youtube.com/watch?v ... vgqxU&feature=related I think I remember this from a book, but I think its either a special type of paint that doesn't effect the paint on the car, or a special type of fog. I will google it.

Funnily enough, I've been watching a documentary on the development of the Lotus Elise today. In the wind tunnel at MIRA they used vapourised paraffin oil. That was in 1994/1995 though, so I don't know if it's changed since then...and I don't know if all wind tunnels use that either.

how to make the fog?

Quote from scania :

how to make the fog?

I don't know exactly but they said the smoke comes out from the heated lance. Presumably there's a tube feeding the paraffin oil into the lance and there's a heating element that instantly vapourises the paraffin oil just before it exits the lance.

How about trying this method? http://www.youtube.com/watch?v ... Ba-LU&feature=related

buy a cheap smoke machine, pipe the compressor output onto a main tube, with little feed jets on it. Easiest way i can think of.