Every day several hundred subsonic aircraft travel between the European and North American continents. Because of noise limitations at major airports, aircraft turn-around times and the time difference between the continents, the direction of the main traffic flow changes through the day. In the afternoon (GMT) the flow is predominantly westbound, while in the early hours of the morning the flow is predominantly eastbound. Oceanic air traffic control for the North Atlantic region has evolved into what is now known as the Organized Track System; traffic is divided into streams, each stream flying at a specified set of flight levels on one of several adjacent tracks. Figure 1 shows an example of the subsonic track system during the westbound peak. To secure operating economy the layout of the track system on any day is matched to the pattern of the upper winds.