How is vertical control of a gas balloon primarily accomplished?

Vertical control of a gas balloon is primarily accomplished by valving gas or releasing ballast. This method directly impacts the balloon's overall buoyancy and allows for adjustments to ascend or descend. When gas is released from the balloon, it decreases buoyancy, causing the balloon to descend. Conversely, releasing ballast, such as sandbags or water, reduces the overall weight of the balloon, thereby increasing its buoyancy and allowing it to rise.

Variations in burner flame intensity can contribute to temperature changes within the envelope, affecting lift indirectly; however, this method is not the primary means of achieving vertical control. Adjusting the volume of the envelope by altering the envelope's shape or size is not typically how balloons are controlled, as they are generally designed to maintain a fixed envelope volume during flight. Finally, altering the weight of the passengers can affect weight distribution but is not a practical or primary method for achieving vertical control in operational terms. Thus, using gas valving or ballast release is the most effective and immediate technique for managing a gas balloon's altitude.