A physics principle that states the pressure exerted by air perpendicular to its direction of travel decreases with an increase in velocity. This is the theoretical principle upon which airplane wings work. An airplane wing's airfoil is unsymmetrical, its upper surface having a greater curve than its lower surface. This causes air flowing across the upper portion of the wing to speed up, since it must cross a greater surface area in the same length of time. The increase in air velocity on the upper surface causes the air pressure in the region just above the wing to be reduced. The higher pressure below the wing presses upward on it, creating lift.