Here is an interesting article describing the effects of tailwinds:

The summary is:

While each one knot of headwind component improves landing/takeoff performance by about one percent, each knot

of tailwind component degrades landing/takeoff distance by about three to five percent. Tailwinds are roughly three to five times as effective at altering landing/takeoff performance than headwinds…and the alteration is not in your favor.

Tow pilots should think twice about taking off downwind even with just a slight breeze.  Likewise landing downwind to speed up launches needs to be done very carefully.  Glider pilots should be understanding.