Hot air balloons are using the basic scientific principles that warmer air rises in cooler air because hot air is lighter than cool air as it has less mass per unit of volume. Raising the air temperature inside the envelope makes it lighter than the surrounding air. The balloon floats because of the buoyant force exerted on it. This force is the same force that acts on objects when they are in water and is described by Archimedes' principle. The amount of lift provided by a hot air balloon depends primarily on the difference between the temperature of the air inside the envelope and the outside the envelope. The envelope has to be very large as it takes a large amount of heated air to lift it off the ground, for example, to lift up 723.5 kg you need nearly 100,000 ft³ feet air heated to 99 °C (210 °F).
Standard hot air balloons are called Montgolfier balloons and rely only on the lift of hot air from the burner and contained by the envelope. This kind of balloon was developed by the Montgolfier brothers, and had its first public demonstration on 4 June 1783 with an unmanned flight that last 10 minutes in the air.
The 1785 Rozière balloon, a type of hybrid balloon created by Jean-François Pilâtre de Rozier, has a separate cell for a lighter than air gas ( helium,) as well as a cone below for hot air (as is used in a hot air balloon) to heat the helium at night. All modern Roziere balloons now use helium as a lifting gas.
A solar balloon or solar airship, is a hot air balloon which gains buoyancy by using only the haet from the sun, the balloons are made from black material, which helps them to heat up in the sunshine. This causes the air present inside the solar balloon to expand and reduce in density compared with the surrounding air. As such, the balloon functions like a hot air balloon. Some solar balloons are large enough for human flight. A vent at the top can be opened to release hot air for descent and deflation.