Originally Posted by
CargoDane
Relative windspeed.
And you're wrong when you say "Required power only increases as the cube of speed in still air."
Actually, he's correct about this. When riding against a headwind, the power required to overcome aerodynamic drag scales like the square of (ground speed + wind speed) multiplied by ground speed. In other words, it's proportional to the square of the air speed multiplied by ground speed. In the absence of wind, the equations reduce to a simpler form, and the power scales with the cube of ground speed.
With airplanes, the power required to overcome aerodynamic drag simply scales with the cube of air speed.