Air density and temperature. Helium balloon.

The buoyancy of an helium balloon depends on the temperature of the air where it is floating or sinking. In this experiment, the balloon is ballasted to  desdend slowly. If we put the balloon at a colder place, the surrounding air is less dense, so the balloon goes up. It happens the same way with hot air. Hot air ascends, well not that hot air ascends but really colder air (denser) goes down attracted by gravity force and then the hot air (or helium, hydrogen, etc.) is displaced upwards.