Here's a quiz!
Flying west to east is:
A) Slower than flying east to west
B) Faster than flying east to west
C) The same as flying east to west
If you answered B, congratulations! Flying west to east generally takes less time than flying East to West. Why? Because of the jet stream.
Actually, there are several jet streams that encircle the earth: two near the equator and one each near the earth's polar regions. Jet streams occur where different air temperatures meet. Cutting across the northern half of the United States, for example, cold air from the North Pole meets warmer air from more southern reaches of the planet. The difference in air pressure creates a contained body of fast-flowing air that we call a jet stream. Due to the direction of the earth's spin and the fact the colder air lies to the north and warmer air to the south, the stream flows from west to east.
Wind speeds are typically 100-150 mph during summer and can increase to 300 mph during winter. That's why it's faster flying west to east. A plane flying from California to New York, say, goes with the jet stream, which cuts flight time by about 30 minutes. Flying against the jet stream adds about a half hour to the trip. For overseas flights the time difference is even greater.
The next time you fly coast to coast, remember the jet stream.