Burner heat raises the envelope's internal air temperature, lowering its density below the surrounding air — the resulting buoyant force is modeled as a fixed upward acceleration while the burner fires, scaled by the burner-power multiplier. With the burner off the envelope cools and the balloon sinks at a fixed rate; venting the crown valve releases hot air faster for a controlled quicker descent. Horizontal drift comes from wind that varies in direction and strength with altitude — pilots steer only by choosing which layer to fly in.