Why is it so?

Zac showed me an experiment this morning. It works best on the morning after a wild party, when there are a few half-inflated helium balloons floating around. Hold your hand over a balloon, palm to one side, then rapidly fan your hand from side to side. The balloon rises - and as you fan faster, the balloon rises higher. Why is it so? Scientific explanation after the jump.

The principle at work here is Bernoulli’s Principle - it states that in fluid flow (which includes a moving current of air), an increase in velocity occurs simultaneously with a decrease in pressure. This is exactly the same principle that keeps airplanes in the air. While it’s probably not new to readers of this blog, it bears repeating: the faster-moving air above the wing (or the balloon) has a lower pressure than the air below, and this creates lift. Net result - the balloon rises, and as you wave your hand faster (creating more airflow) the balloon rises even higher.