Aerodynamics is the science of air in motion and explains how and why an aircraft flies. It can be compared to, and contrasted with, aerostatics.
Contrasted with aerodynamics, aerostatics is the science which explains how and why a hot air balloon, or similar craft, attains lift. These aerial vehicles do so by means of the buoyancy principle.
Air is compressible-that is, its own weight compresses it. The lower its location in the atmosphere, the more air-and therefore weight-is above it, rendering it densest at or near the ground. Conversely, as it rises, it becomes thinner.
Hot air balloons utilize these varying conditions to attain lift. Heated air, or lighter-than-air gas, within a balloon’s envelope causes the balloon itself to rise, because its internal air is less dense than the surrounding air. When it reaches the altitude where the density of its internal air equals that of the surrounding air, it ceases to rise and attains a state of internal and external equilibrium-that is