Originally Posted by
Lonely Flyer
My understanding a tail wind is just a headwind in the other direction so that is why they change the direction of takeoffs.
That is essentialy correct, but a tailwind's effect on takeoff distance is dramatic.
There are some runways, too, that are essentially one-way runways; most of these are, e.g., small airstrips in mountainous regions which, due to obstructions at one end, can only be used in one direction. If the wind is wrong, then no takeoff is possible (or prudent anyway!)