The primary lifting force in a hot air balloon is due to:

The primary lifting force in a hot air balloon is due to the interior air temperature being greater than the ambient temperature outside the balloon. When the air inside the envelope of the balloon is heated, it expands and becomes less dense than the cooler air surrounding it. This difference in density creates buoyancy, allowing the hot air balloon to rise.

The principle behind this is based on Archimedes' principle, which states that an object will float in a fluid if it is less dense than the fluid. In this case, the hot air acts as the fluid, and the cooler surrounding air provides the necessary lift. Therefore, the ability to manipulate the temperature of the air within the envelope is essential for controlling the ascent and descent of the balloon.