Why the flight back from New York is an hour shorter.
At around 10 km altitude, jet streams blow from west to east, often at 150 to over 300 km/h. Flying east, the wind pushes the aircraft along; flying west, it holds it back. New York–London therefore typically takes about 6.5 hours, London–New York around 7.5 to 8. Airlines plan routes daily around the jet stream's current position.
Useful: Expect westbound flights to take longer; timetables already price that in.
Source: en.wikipedia.org