Air-seemingly weightless and all around us, really has mass and weight. Air can be compressed due to it's small molecular structure-air particles are less than a millionth of a millimeter in diameter! These small particles move very fast due to inernal kinetic energy. In a hot-air balloon, air is the primary focus. The air particles bounce around at high speeds inside the balloon. Hot air has a low density at atmospheric pressure, hot air weighs less and contains fewer particles than cold air, this makes hot air perfect for a balloon. The air pressure inside the balloon is the same on the outside. The hole in the bottom of the balloon lets colder air out and hot air in. The propane burner below the hole is what controls height. The hotter the flames from the burner, the higher you go. Letting the air cool makes the balloon go down. However if you go too high, the air becomes thinner and pressure decreases, leading to a decrease in height. Hot air balloons are neat, but history has shown some mistakes in engineering. The most famous balloon disaster was the Hindenburg-the hydrogen gas used to lift the largest aircraft in history is extremely flammable-and the balloon burst into flames while attempting to land in New Jersey on May 6, 1937.
The Hindenburg
