|Hundreds of hot air balloons - a lot of hot air!|
He snapped this spectacle at a ballooning event in Belgium in 2011.
Did you know that the air inside a typical hot air balloon weighs about 1 tonne? You might ask why it still flies.
The answer is that the cold air that it displaces is even heavier. Yes - air weighs more that you might expect. When the balloon is hovering at a steady height, the light, hot air inside plus the weight of the balloon and passengers is exactly the same as the weight of the cold air that it displaces.
To go up, the pilot makes the air in the balloon a little hotter (and therefore lighter), and to go down she allows it to cool a little. The trick with going down is to avoid doing it too fast, and being able to get back almost to the balance point just before hitting the ground.