Originally Posted by
Wiirachay
The pilots wanted a change to scenery. They were tired of seeing polar bears.
The real answer is the jet stream, which goes in an eastward direction. It can increase the ground speed of the airplane by 100 mph. Going west, it's in the pilot's best interest to avoid it. Hence, a polar route is used going west.
Example, I flew IAD-NRT, and flight took the shortest path distance-wise. However, on the way back, the flight flew over SEA, which added around 300 miles to the trip. But the jet stream was very fast. Here's a visualization.
http://gc.kls2.com/cgi-bin/gc?PATH=i...OR=&MAP-STYLE=
And even though it added 300 miles to your trip, 12 hours with 100mph tailwind takes off 1200 "air miles" for an effective savings of 900 miles of air to travel through (math there isn't exact, but you get the idea). That's either an hour and a half shorter flight (and correspondingly less fuel burn) or a maybe 10%-15% reduction in speed (also saving fuel).
Airline dispatch offices have sophisticated computer programs which take into effect the winds aloft to plot out the best route.