Here's why:

* Rising: The hot air inside the balloon is less dense than the surrounding air, creating buoyancy that lifts the balloon upwards.

* Descending: To descend, the balloonist releases some of the hot air, making the balloon denser and allowing it to sink.

While a hot air balloon might experience some horizontal movement due to wind currents, it's the vertical motion that is controlled by the balloonist and allows for the ascent and descent.