Wake Turbulence is cause by wing tip vortices and is a by-product of lift.  The higher air pressure under the wings tries to move to the lower air pressure on top of the wings by flowing towards the wing tips where it rotates and flows into the lower pressure on top of the wings.  This results in a twisting rotary motion which is very pronounced the wing tips and continues to spill over the top in a downward spiral.   Therefore, the wake consists of two counter-rotating cylindrical vortices.  

Vortex Strength:  The strength of these vortices is governed by the shape of the wings, the weight and the speed of the aircraft; the most significant factor being weight.  The greatest vortex strength occurs under conditions of heavy weight, clean configuration, and slow speed.  Strength of the vortex shows little dissipation at altitude within 2 minutes of the time of initial formation.  Beyond 2 minutes, varying degrees of dissipation occur along the vortex path; first in one vortex and then in the other.  The break-up of vortices is affected by atmospheric turbulence, the greater the turbulence, the more rapid the dissipation of the vortices.  

Induced Roll: Aircraft flying directly into the core of a vortex will tend to roll with the vortex.  The capability of counteracting the roll depends on the wing span and control responsiveness of the aircraft.  When the wing span and ailerons of a larger aircraft extend beyond the vortex, counter-roll control is usually effective and the effect of the induced roll can be minimized.  Pilots of short wing span aircraft must be especially alert to vortex situations even though their aircraft are of the high performance type. 

Helicopter Vortices:  In the case of a helicopter, similar vortices are created by the rotor blades.  However, the problems created are potentially greater than those caused by a fixed wing aircraft because the helicopter's lower operating speeds produce more concentrated wakes than fixed wing aircraft and the size of the aircraft is not a factor on the intensity of the vortex.   Departing or landing helicopters produce a pair of high velocity trailing vortices similar to wing tip vortices of large fixed wing aircraft.  Pilots of small aircraft should use caution when operating or crossing behind landing or departing helicopters.   

Vortex Avoidance:  Avoid the area below and behind other aircraft, especially at low altitude where even a momentary wake turbulence encounter could be disastrous.