Air at a temperature of around 90°C is filled into the hot-air balloon. The hot air keeps the balloon in the air because its density is lower than that of the surrounding cold air. Before starting the balloon, a gas burner is ignited below the opening of the huge balloon cover and the flame heats up the air in the balloon. When the balloon rises to the desired height, the burner is switched off; if one wants to rise further, it is started again. The balloon must have a minimum size so that it can overcome the force of gravitational attraction of the Earth. Thus, hot-air balloons are able to float in air and look so beautiful in the sky.
