What causes false lift during the launch of a hot air balloon?

The correct answer highlights the Venturi effect, which occurs when the wind flows around the envelope of a hot air balloon. This effect can create localized areas of lower pressure next to the envelope surface, leading to an increase in lift if the wind speed is high enough. As the balloon is launched, the interaction between the envelope and the moving air can inadvertently generate additional lift that is not solely due to the hot air inside the envelope.

Understanding this phenomenon is crucial for balloonists because it can lead to unexpected changes in altitude or control during the launch phase. The Venturi effect is a significant concept in fluid dynamics, illustrating how pressure varies with speed in a fluid, which applies directly to how air behaves around the balloon.

Other options, such as high humidity or improper ballast management, do not directly cause false lift in the same mechanical manner as the Venturi effect. The engine's lift thrust would not apply to hot air balloons, as they do not utilize engines for lift. Thus, the dynamics involving airflow and pressure changes around the balloon envelope are central to recognizing the causes of false lift during launch.