Explanation: Honestly, I have no idea and I've never learned this, but I think it's because of air resistance and currents? Going downwind would mean the plane's rate would be the speed of the wind + the speed of the plane itself. Going upwind, the plane's rate would be the speed of the plane - the speed of the wind. Thus, going one way (e. g. California to New York) could take less time than going the other way.
