The Jet Streams

The jet streams are currents of rapidly moving air that form where a cold air mass meets a warm air mass. In the Northern Hemisphere, they blow from west to east and are characterized by high winds. Jet streams are important to pilots because flying against one can significantly increase air time. However, flying in the same direction as the jet stream can reduce air time.
 

Which trip would be faster; a flight from Seattle to New York, or a flight from New York to Seattle?

The eastward flight, from Seattle to New York, is faster because you are working with the jet stream, not against it.

Jet streams typically take a meandering path and sometimes dip out of their normal zone of occurrence. In the diagram below, you can see how a meandering jet stream can bring unusually cold polar air into the U.S. Deviations of the jet stream from its normal journey can thus cause unexpected weather.