"Wake turbulence is turbulence that forms behind an aircraft as
it passes through the air. This turbulence includes various
components, the most important of which are wingtip vortices and
jetwash. Jetwash refers simply to the rapidly moving gasses
expelled from a jet engine; it is extremely turbulent, but of short
duration. Wingtip vortices, on the other hand, are much more stable
and can remain in the air for up to three minutes after the passage
of an aircraft. Wingtip vortices make up the primary and most
dangerous component of wake turbulence.
Wake turbulence is especially hazardous during the landing and
take off phases of flight, for three reasons. The first is that
during take-off and landing, aircraft operate at low speeds and
high angle of attack. This flight attitude maximizes the formation
of dangerous wingtip vortices. Secondly, takeoff and landing are
the times when a plane is operating closest to its stall speed and
to the ground - meaning there is little margin for recovery in the
event of encountering another aircraft's wake turbulence. Thirdly,
these phases of flight put aircraft closest together and along the
same flightpath, maximizing the chance of encountering the
phenomenon."
-http://en.wikipedia.org/wiki/Wake_turbulence
