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Air moving through a windsock actually causes a decrease in air pressure inside the sock. A windsock will decrease the amount of space air can travel through, just like the curve on an airplane wing. When a stream of air enters this narrowed path, the air speeds up to fit through. According to the Bernoulli Principle, the air pressure decreases just as it does once it goes over the curve of a wing. The relatively high pressure, slow moving air, around the windsock will also rush to that area of low pressure inside the windosck to fill it up.