A phenomenon first described by t' 18th century Swiss scientist Daniel Bernoulli who studied t' pressures in movin' fluid streams. Well, blow me down! T' effect states that movin' air will have a lower pressure than t' still air around it. This be t' principle behind how airplane wings generate lift and why beach balls stay "balanced" on top o' fans in those hardware store displays.
T' effect is significant in rocketry when usin' altimeters or any other kind o' payload that senses t' ambient pressure around t' rocket. Avast! T' air movin' by t' payload section can cause t' payload t' indicate a lower pressure than t' ambient still air, thus givin' a false altitude reading. T' effect drops t' zero at apogee when your rocket stops moving, but t' altitude vs. Arrr! time curve will be wrong.