Hot air balloon physics operation

If the balloon operator wishes to lower the hot air balloon, he can either stop firing the burner, which causes the hot air in the envelope to cool (decreasing the buoyant force), or he opens a small vent at the top of the balloon envelope (via a control line). This releases some of the hot air, which decreases the buoyant force, which also causes the balloon to descend. To maintain a steady altitude, the balloon operator intermittently fires and turns off the burner once he reaches the approximate altitude he wants. This causes the balloon to ascend and descend (respectively). This is the only way he can maintain an approximately constant altitude, since maintaining a strictly constant altitude by way of maintaining a net zero buoyant force (on the balloon) is practically impossible.

If the balloon operator wishes to move the balloon sideways (in a horizontal direction) he must know, ahead of time, the wind direction, which varies with altitude. So he simply raises or lowers the hot air balloon to the altitude corresponding to the wind direction he wants, which is the direction he wants the balloon to go.

The balloon stays inflated because the heated air inside the envelope creates a pressure greater than the surrounding air. However, since the envelope has an opening at the bottom (above the location of the burner), the expanding hot air is allowed to escape, preventing a large pressure differential from developing. This means that the pressure of the heated air inside the balloon ends up being only slightly greater than the cooler surrounding air pressure.

An efficient hot air balloon is one that minimizes the weight of the balloon components, such as the envelope, and on board equipment (such as the burner and propane fuel tanks). This in turn minimizes the required temperature of the air inside the envelope needed to generate sufficient buoyant force to generate lift. Minimizing the required air temperature means that you minimize the burner energy needed, thereby reducing fuel use.