The pressure difference also accelerates air upward around the leading edge. These accelerations occur in accordance with Bernoulli's law, but the greater upper surface velocity is more easily explained as resulting from pressure difference, rather than causing it as popular theories teach. As air is accelerated downward by wing passage, upward recirculation occurs all around the airplane, away from higher pressure below and toward lower pressure above. Thus recirculation occurs forward and upward around the wing, and laterally outward,upward and inward to produce twin trailing vortices which can be made visible in smoke behind the wings. For photographic evidence of this circulatory movement, check this picture and this one .
Forward recirculation carries upwash into which the wing flies.
Near stall condition, upwash velocity rounding a wing leading edge can be greater than the forward velocity of the airplane, pushing a stall warning switch tab, as shown here on a Cessna 182, forward and upward to operate a stall warning horn. Leading edge suction is sometimes used to operate a stall warning horn, as on this Cessna 175. Leading edge pressure reduction produces forward thrust on the wing, but curvature of circulation around the rear produces opposing rearward thrust. If these were equal they would cancel, but energy lost into lateral recirculation around the wing ends causes forward thrust to be less than rearward thrust. There is much more to the story of downwash and recirculation, and how they relate to design and performance of wings and airplane configurations.