On a recent trip to Asia, our longest flight was one from Los Angeles to Shanghai which took nearly 14 hours to complete. Although my children were excited to fly for the first time, they both thought that particular leg of our trip was far too long.
On our way back home my eleven year old son was not looking forward to another 14 hour flight. I informed him that the flight back to the United States was going to be 2 hours shorter because in the mid-latitudes the upper-level wind generally blows from the west so we would have a tail-wind on the way home. Airlines of course take this all into account when scheduling flights.
The problem is, the wind is never the same from day to day, so often times a flight will either arrive earlier or late do to the wind speeds aloft which can bring much angst to both airlines and passengers.