I heard an unexpected roaring above my head and looked up and saw a huge balloon just clearing the treetops near where I was standing. The loud roar came from the gas burner attached to the base of the balloon, which, while operating, heated the air inside the balloon. This caused the air inside to become lighter than the colder air outside, thus lifting the balloon upwards and over the treetops. 

Seeing this large hot air balloon carrying passengers so close by reminded