The science that enables hot-air balloons to soar through the skies
comes down to one fundamental principle: heat rises. To turn this
fact of physics into flight, hot-air balloons are equipped with powerful burners. When fired up, these heat the air molecules inside the huge canopy. The energy and distance between each of these molecules increases with the heat, causing the air within the balloon to expand. The result is a pocket of hot air that’s less dense than the colder air surrounding the balloon.
The hot air within the balloon is then displaced by colder, denser air, causing the balloon to move upwards. This upward force is known as buoyancy. Much like how a submerged rubber duck in a bathtub will rise to the surface of the water, buoyancy lifts hot-air balloons through the air.