It’s called the Schlieren effect and it means that you can see stuff that’s invisible to the human eye,like changes in air density. So when you turn on a hair dryer, you can see the blast of air it shoots out. When you open a can of Coke, and you can see what’s escaping into the air. When you rub your hands,you can see the heat surrounding them. Read more...
Source: gawker.com