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, you can see what’s escaping into the air. When you rub your hands, you can see the heat surrounding them.
Source: Gizmodo – This Is What it Must Feel Like to See the Invisible