The origin of counter-rotating wing tip vortices is a direct and automatic consequence of the generation of lift by a wing. Lift is generated by the creation of a pressure differential over the wing surface. The lowest pressure occurs over the upper wing surface and the highest pressure under the wing. This pressure differential triggers the roll up of the airflow aft of the wing resulting in swirling air masses trailing downstream of the wing tips. After the roll up is completed, the wake consists of two counter-rotating cylindrical vortices. For further detail, see the separate article: Induced Drag.
Wake Vortex Turbulence
The strength of the vortex is governed by the weight, speed, and shape of the wing of the generating aircraft. The vortex characteristics of any given aircraft can also be changed by extension of flaps or other wing configuring devices as well as by change in speed. However, as the basic factor is weight, the vortex strength increases proportionately.
Vortices usually persist for between one and three minutes, with survival often greatest at low level in calm or very light wind conditions and at higher altitudes in thinner air. Once formed, vortices descend until they decay (or reach the ground). Decay is usually rapid and occurs more quickly in windy conditions over land because of the greater variation in both components of wind velocity which are induced by frictional effects of terrain. Cross-winds can carry a vortex away from the flight path which the aircraft generating them has followed.
Aircraft wing configuration (Flap setting etc.)
Closeness to the ground - vortex ceases to be hazardous when ground contact occurs
Wind velocity - light winds delay decay
Turbulence, from sources other than wake vortex, accelerates vortex decay