“It turns out that with carefully controlled conditions, you can create an actual indoor cloud. Artist Berndnaut Smilde creates the clouds with a fog machine and then carefully monitors the temperature and humidity of the room to get his artificial cloud to hang there long enough to snap a picture.”
Cracked.com http://www.cracked.com/article_19817_18-images-you-wont-believe-arent-photoshopped-part-9.html#ixzz1tfvN1BN4