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Why does it take longer to fly West to East than East to West?

Which factors affect this?

Does Earth's spinning affect the time difference? Maybe the air circulation?

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migrated from astronomy.stackexchange.com Mar 21 at 15:49

This question came from our site for astronomers and astrophysicists.

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prevailing non-tropical winds are westerlies. Tailwinds flying east, headwinds flying west. –  casey Mar 21 at 17:11
    
It has nothing to do with the rotation of the earth, since the atmosphere rotates with it. –  David Richerby Mar 21 at 22:20
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@DavidRicherby Actually, the jet streams are caused (indirectly) by the rotation of the earth. –  Paŭlo Ebermann Mar 22 at 8:47
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To add a little more "concreteness" to the question, many ocean crossings use tracks (routes) defined on a daily basis. The forecast wind is a major factor in defining these routes so that east bound traffic can take advantage of the winds aloft westerlies and west bounds can minimise the impact of head winds. See here for examples - pilotweb.nas.faa.gov/common/nat.html and here en.wikipedia.org/wiki/North_Atlantic_Tracks –  Simon Mar 23 at 11:42

4 Answers 4

up vote 17 down vote accepted

As Robbie's answer implies, the answer is wind - Trade Winds govern what happens on or near the surface and were historically important for sailing ships, and today in aviation we deal with the the Winds Aloft at whatever altitude your plane is flying.

As an example, consider two hypothetical flights at 30,000 feet with the winds shown below:
Winds Aloft at 30,000 ft

With these winds a flight from A to B (East-to-West) will be fighting headwinds the whole way. My handy flight planning program tells me that at 500 knots (airspeed) you'd be in the air for about 5 hours 30 minutes.

Traveling in the opposite direction from B to A you'd have a tailwind, and with the same 500 knot airspeed you would make the trip in roughly 4 hours.


The winds aloft vary seasonally, which can affect flight times for summer versus winter trips. According to one of our folks who is regularly up at those altitudes even the daily variations can be noticeable, and may make the difference between being able to make a nonstop trip or having to stop for fuel on the way.

Pilots and flight dispatchers will often review the wind data prior to flight and try to select an altitude that affords a "good ride" (free of turbulence) and favorable winds (either a tailwind or the lowest headwind they can find).

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"My handy flight planning program" - right, because you're a pilot, so you just have one of those lying around. I wonder if laymen browsing the other StackExchange sites feel like this when someone busts out a boolean satisfiability solver or a 270-dollar official copy of the C11 standard. –  user2357112 Mar 22 at 8:09
    
@user2357112 "handy flight planning program" as opposed to "Taking out a pen and paper and figuring out where the wind speed/direction changes, how long I'll be flying with each wind component, and what effect that will have on my ground speed" - Anyone who has taken High School physics can do the math (it's vector addition), but nobody wants to :-) –  voretaq7 Mar 22 at 18:51

The main reason for the difference in time are trade winds.

Trade winds generally travel East to West, and so aircraft travelling in this direction have a faster ground speed, that is the speed relative to the ground. The true air speed of any aircraft is not affected by the wind.

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Also en.wikipedia.org/wiki/Jetstream –  Michael Borgwardt Mar 21 at 12:01

The jet stream moves from West to East. At the altitude an airliner flies the speed of the tail wind or head wind will have a significant impact on actual ground speed. This was first observed when B-29 bombing raids on Japan during WW2. Before that there was no notion of a 'jet stream'.

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Actually, the Japanese were aware of the jet stream already in the 1920s. They used it during WWII to send balloon bombs across the Pacific to the US: some of these made it as far as the Mid-West. –  David Richerby Mar 21 at 22:16
    
They never understood it the way it is understood today, although, yes, they did take advantage of it. They never once knew that it was a global phenomena. –  user1698 Mar 22 at 3:31
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Well, obviously, knowledge has improved in the 90 years since then... But the Japanese understood in the 1920s that there are consistent, strong winds from west to east across the Pacific at the latitude of Japan so it's not true to say that the first observation of the jet stream was by Americans in the 1940s. –  David Richerby Mar 22 at 3:41

Without even looking at the winds I'd just add something like 20 kts ground speed if going east and subtract 20 kts ground speed if going west. Amazing how often that came close enough to actual flight plan.

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