A solar hot air balloon takes advantage of the fact that solar radiation provides about 1000 Watts per square meter. A balloon made of black plastic film and inflated with dry air absorbs enough solar energy to raise the temperature of the air on the inside ; this reduce the air density significantly.

This hot air bubble is lighter than the ambient air. The balloon rises.

Mass of 1 cubic meter of ambient air at 15°C : 1,225 kg.
Mass of 1 cubic meter of air at 35°C : 1,145 kg.
The climbing aerostatics force or lift is therefore 80 grams per m³.

A solar hot air balloon for human flight can be piloted vertically by using a valve at the top of the balloon : the solar balloon is oversized compared to it's load. To fly at a desired altitude (level flight), the valve is opened slightly. To ascend, the pilot closes the valve completely. To descend, he opens more the valve.
It is possible :

• to arrest a descent quickly (obstacles or strong descents) by closing the valve and dropping ballast : 20 kg of water or of sand),
• to descend quickly (turbulence, vertical movements) by opening a valve at the top and a panel at the equator,
• to use automatic altitude regulation for level flight.