There are some wind/atmospheric conditions in which a wake vortex does not dissipate as expected but remains in almost a stable state. The separations are based on the probability of dissipation and the following aircraft's susceptibility to the vortices from the previous aircraft. With a light cross wind a wake that would have dissipated safely to the side of the runway may instead be carried along the runway by the ambient wind. These conditions where vortices are more stable are when LIDAR based systems like WindTracer (
WindTracerŽ ˇ Lockheed Martin ) can be extremely useful.