Wake Turbulence

Light crosswinds represent the greatest risk (with respect to a wake turbulence encounter) to departing aircraft, as explained in this extract from the Pilot's Workshop Series:

Vortices tend to move outward from the aircraft. So if you are behind a departing aircraft, the vortex from the right wing will tend to move to the right. The vortex from the left wing will tend to move to the left in no wind conditions.

If we have a crosswind, the wind will tend to influence the movement of the vortices. A crosswind of about 3 knots will hold the upwind vortex pretty much in place at the runway where it was created, while the downwind vortex will rapidly move away from the runway.

Crosswinds greater than approximately 5 knots will tend to break up the vortices. So stronger crosswinds are good things, as far as vortices are concerned. At least the way we look at it, from our perspective; we want the vortices to begin to break up and decay. So light crosswinds require maximum caution, and I'm talking about a light crosswind of maybe 3 knots.

References:

Pilot's Workshop http://www.pilotworkshop.com/tips/wake_turbulence_takeoff.htm

See also Safety Sense Leaflet 15: http://publicapps.caa.co.uk/docs/33/20130121SSL15.pdf

Atmospheric Picture

FL45, English Channel.