The lowest pressure occurs over the upper surface and the highest pressure under the wing. This pressure differential triggers the rollup of the airflow aft of the wing resulting in swirling air masses trailing downstream of the wingtips. Viewed from behind the generating aircraft, the left vortex rotates clockwise and the right vortex rotates counterclockwise. The strongest vortices are produced by heavy aircraft, flying slowly , in a clean configuration. For example, a large or heavy aircraft that must reduce its speed to 250 knots below 10,000 feet and is flying in a clean configuration while descending, produces very strong wake.
Extra caution is needed when flying below and behind such aircraft.
While there have been rare instances where wake turbulence caused structural damage, the greatest hazard is induced roll and yaw. This is especially dangerous during takeoff and landing when there is little altitude for recovery. Short wing span aircraft are most susceptible to wake turbulence. The wake turbulence-induced roll rates can be extreme. Countering roll rates may be difficult or impossible even in a high performance aircraft with excellent roll control authority.