Hot-air balloons of all sizes are lifted in to the air by a a simple physical phenomenon - air becomes less dense as its temperature rises. Consequently, the air enclosed in the envelope of a hot-air balloon weighs less when heated by the burners than the surrounding air that it displaces. The balloon therefore rises. The main factors that control a balloon's flight are the volume of the envelope, the difference in temperature between the air enclosed in the envelope and the surrounding air, and of course the overall weight of the balloon and its payload. As a balloon rises, the surrounding air becomes thinner and the balloon's buoyancy decreases. This limits the altitudes that can be achieved. Larger capacity balloons are potentially more buoyant so generally have a higher altitude limit.
Modern hot-air balloons heat the air by burning propane, the gas used in camping and other outdoor grills and heaters.. The propane is compressed in to liquid form and stored in lightweight cylinders each of which is equipped with a hose extending to its base so liquid can be extracted. Once released from the pressurized cylinders, the liquid passes through the steel coil of the burner where it is heated and returns to gaseous form. The gas then burns fiercely and efficiently, heating the enclosed air and lifting the balloon in to the air. Once the balloon is rising the burners are turned off, removing the source of heat and allowing the air in the balloon gradually to cool. The rate of ascent decreases and before too long the balloon begins to sink back to earth. The pilot must then reignite the burners to increase the temperature of the enclosed air.