Photojojo » Schlieren Photography: How to Photograph the Invisible
http://photojojo.com/content/guides/schlieren-photography-guide/

would LOVE to do this

Ever taken a picture of a cough? Not just somebody coughing. No, we mean the actual air currents as they’re being expelled. Well, they just did it at Penn State, thanks to the magic of schlieren photography.

Schlieren Photography: How to Photograph the Invisible