In aerodynamics, interference drag is a component of parasitic drag which is caused by vortices. Whenever two surfaces meet at a sharp angle on an airplane, the airflow has a tendancy to form a vortex. Accelerating the air into this vortex causes drag on the plane, and the resulting low pressure area behind the plane also contributes. Thus, the primary method of reducing interference drag is eliminating sharp angles by adding fairings which smooth out any sharp angles on the aircraft. As with other components of parasitic drag, interference drag follows the drag equation and rises with the square of the velocity.