Boeing is testing crosswind takeoff and landing performance of the Boeing 787. A crosswind take off/landing is one where the prevailing wind is blowing at an angle to the runway (as opposed to along it) – which can create dangerous conditions. Crosswind landings are especially tricky because pilots must fly the aircraft at an angle to the runway instead of directly towards it, with the crosswind carrying it like a boat on a current. Here is a great video of the tests at Keflavik (chosen for the high number of crosswind days in an average year):
(Video via Flightblogger)
Note landings especially – the aircraft is pointed at a massive angle relative to the runway, and at the last moment the pilot drags the nose to point down the runway for the rollout.