Why does it take longer to fly East to West than West to East?
Which factors affect this?
Does Earth's spinning affect the time difference? Maybe the air circulation?
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9$\begingroup$ prevailing non-tropical winds are westerlies. Tailwinds flying east, headwinds flying west. $\endgroup$– caseyMar 21 '14 at 17:11
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3$\begingroup$ It has nothing to do with the rotation of the earth, since the atmosphere rotates with it. $\endgroup$– David RicherbyMar 21 '14 at 22:20
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12$\begingroup$ @DavidRicherby Actually, the jet streams are caused (indirectly) by the rotation of the earth. $\endgroup$– Paŭlo EbermannMar 22 '14 at 8:47
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7$\begingroup$ 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 $\endgroup$– SimonMar 23 '14 at 11:42
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$\begingroup$ @DavidRicherby Here is a seven year late comment: The prevailing winds are affected by the rotation of the Earth via the Coriolis effect. See my answer below. $\endgroup$– PilotheadSep 4 at 18:41
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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:
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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2$\begingroup$ "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. $\endgroup$ Mar 22 '14 at 8:09
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3$\begingroup$ @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 :-) $\endgroup$– voretaq7Mar 22 '14 at 18:51
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2$\begingroup$ @user2357112 you can also find those "handy flight planning programs" online for free! Here is one example but there are many more: fltplan.com/AwMPToQuickInfoEntry.exe?a=1 This one will give you the differences in travel time based on different headwinds or tailwinds. :) $\endgroup$– CanukOct 8 '14 at 23:59
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$\begingroup$ Hi @voretaq7: Its a good answer. But I have a small question. Lets consider the following question. When u r travelling against the air by some 500knts, ur speed is eventually reduced because of the barrier. But this will not be the case when, the air is tailgating u. This tailgating air will add some extra cruising speed even when pilot is not increasing the speed right ? But travelling in the opposite side of the air can be satisfied by increasing the cruise speed when he want to reach in the same 4 hrs. Right.. ? $\endgroup$ Jun 23 '16 at 4:15
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$\begingroup$ @NANDAKUMAR Airlines plan flights to cruise at roughly the same fuel-efficient airspeed regardless of winds. The difference in flight times is therefore due to the movement of the air over the ground, i.e. wind. Even if planes had sufficient reserve thrust to negate a headwind (and they don't), the increase in fuel cost would be prohibitive. $\endgroup$– StephenSMar 30 '19 at 19:50
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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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6$\begingroup$ Also en.wikipedia.org/wiki/Jetstream $\endgroup$– Michael BorgwardtMar 21 '14 at 12:01
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$\begingroup$ Wind speed is not directly affecting true airspeed but if an airline knows the next leg has a lot of tail wind and they are not in a rush they might as well fly a slower TAS and save fuel and still arrive on time. And for headwinds they will probably fly faster to make it to the destination on time. $\endgroup$– JanMar 17 '19 at 20:33
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$\begingroup$ @Jan Trade winds are predictable by season, so that’s baked into the schedule. Crews would only need to burn more fuel to mitigate unpredictable factors like a late departure or a reroute due to storms. $\endgroup$– StephenSJul 31 at 22:00
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$\begingroup$ The Trade Winds act in opposition to the OP's premise, that east to west travel is slower and which is only true between 30-60deg latitude. The Westerlies are what make east to west air travel slower. $\endgroup$ Sep 4 at 18:33
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Seven years late!) but none of these answers gives a comprehensive explanation, particularly in that the OP's premise "westbound travel is slower" is only correct at certain latitudes. Both the prevailing winds and the jet stream affect duration of air travel, and both depend on differential heating and the rotation of the Earth.
Prevailing Winds
This diagram shows the macro air circulation patterns around the globe. Generally, air is heated at the equator and rises, then travels at high altitude towards the poles. At about 30deg latitude the air mass has cooled sufficiently that it wants to descend, then heads at low altitudes back towards the equator. This is the Hadley cell. From space, these airflows appear largely Northerly and Southerly. On Earth, the Coriolis effect makes the airflow also appear Easterly and the result is called the Trade Winds. Here, westbound air travel is faster than eastbound.
Similarly, air is cooled at the poles and descends, then travels at low altitude toward the equator to approximately 60deg latitude where warming makes the air rise. This is the Polar cell. The same low altitude direction toward the equator and the same Coriolis effect makes these wind directions match the Trades, and they are called the Easterlies.
Polar and Hadley are driven by polar cold and equatorial heat. If the Earth were smaller they would combine into one cell. At the existing scale an intermediate cell forms, the Ferrel cell, and its direction is driven by contact with Polar and Hadley at 30deg and 60deg.
Polar makes a "chimney" at 60deg which is shown on the chart as low pressure. Hadley makes a downdraft at 30deg, shown as high pressure. The Ferrel air mass flows from high pressure to low, or toward the poles at low altitude. With airflow reversed the Coriolis effect now makes the winds appear westerly, and are aptly called the Westerlies. This is where air travel in the US and Europe occurs, and this is where the OP's "westbound travel is slower" applies.
Jet Stream
This phenomena occurs in addition to the prevailing winds and is affected by the same factors. There are usually four jet streams, one located near each of the vertical air flows between the circulation cells. The 60deg latitude Polar jet streams are stronger and lower than the 30deg Subtropical streams, and this explanation focuses on the northern Polar jet.
From this diagram you can see circulation in the norther Polar Jet. Looking Eastward in the direction of the jet stream airflow there is a counter-clockwise rotation between the Ferrel and Polar cells. Though this entire column of air is ascending, the temperature differential between the colder air of the Polar cell and the warmer Ferrel air causes this weak rotation. Location and altitude fluctuate, but the lower reaches are around 30-40kft where transport aircraft fly.
You might think that the lower altitude flow toward the equator would combine with the Coriolis effect to create a mild easterly flow as happens in the Polar easterlies. It does attempt this but runs into the much stronger Westerlies and is deflected, accelerating as it moves in a westerly direction. Think wind blowing into a wall, deflected to move along its length. Depending on local temperatures, the deflected air can reach speeds as high as 200kt.
So both atmospheric heating/cooling and the Earth's rotation via the Coriolis effect determine the direction of the prevailing winds, and consequently the amount of time air travel takes in a particular direction.
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$\begingroup$ I'd upvote again for that addition if I could; really appreciate the detail & the clarity in this answer. $\endgroup$– Ralph JSep 4 at 23:41
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$\begingroup$ It'll be a day or two before the system lets me award it, but since there is no second up-vote available, it's a nice alternative. $\endgroup$– Ralph JSep 5 at 2:15
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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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2$\begingroup$ 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. $\endgroup$ Mar 21 '14 at 22:16
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$\begingroup$ 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. $\endgroup$– user1698Mar 22 '14 at 3:31
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5$\begingroup$ 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. $\endgroup$ Mar 22 '14 at 3:41
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Short answer: As you have indicated in the question description, the answer is winds. Better known as Jet Streams.
Long answer: https://alliknowaviation.home.blog/2019/03/29/why-do-planes-travel-faster-west-to-east/
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$\begingroup$ It's not just jet streams, it's the Earth's prevailing wind patterns. supposing a headwind component of 24kts on a 10 hour westbound flight $\endgroup$ Dec 24 '20 at 18:54
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$\begingroup$ Sorry, incomplete comment above: It's not just jet streams, it's the Earth's prevailing wind patterns. supposing a headwind component of a modest 24kts on a 10 hour westbound flight - that's 24nm more travelled per hour which is 240nm in 10 hours. Cruise TAS of 480 kts ~ 8nm per minute so 240nm means a difference of 240/8 = 30 minutes more than in 0 wind conditions (aka still air). If the return trip has a tailwind component of 24kts, anticipating a 9 hr flight works out to 24 x 9 = 216nm so 216/8 = 27 min less. These 2 approximations add up to 30 + 27 = 57 min between the Wbound and Ebound. $\endgroup$ Dec 24 '20 at 19:16
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Although the rotation of the earth and weather created by solar energy do affect the air circulation of the planet, and jet streams aloft, the main reason why west to east flights take "longer" is that flying east into new time zones adds an hour to your landing time for each time zone flown through. Opposite flying west.
Some of this is made up for when flying at the latitudes of west to east jet streams when travelling east, so the actual flight time going east under those conditions will be shorter, even though you arrive at a "later" time.
A closer look at your flight time based on your original time zone may give greater insight to your question, and maybe a little less "jet lag".
answered Mar 30 '19 at 18:18
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Both the winds and earth's rotation affect the difficulty of flight eastbound vs. westbound. If eastbound is slower and harder and winds carry you westbound, I take it you're within the tropics, where the tradewinds govern flight direction. Kentucky and Utah, and farther north latitude (and likewise south to Argentina) the winds work in the opposite direction.
The poles and equator differ in (eastward) rotation speed. The equator has farther to go and keeps up with the axis by going faster. So land nearer the axis (Canada, Argentina) takes it slow spinning east, slower than wind and weather. So that's where to take advantage of the winds eastbound. The tradewinds are at the equator, where the ground follows (east) a bigger circle, faster than wind and weather. There, the westbound is more efficient.
I'm not sure about Alabama/Arizona/etc, but I wager the eastbound and westbound are about the same, since they are in-between the north and tropics (and Bolivia the in-between of the southern hemisphere). The latitude makes quite a difference.
Unlike planes being pushed by wind, space shuttles leave the atmosphere. Being propelled only by earth's rotation, all rockets go east no matter the latitude.
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$\begingroup$ I'm finding this a bit confusing to follow... are you saying that rotation affects flight time apart from the wind? $\endgroup$– fooot ♦May 28 '15 at 21:56
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Unlike planes being pushed by windthat's valid only if it is a tailwind $\endgroup$– Federico ♦May 28 '15 at 22:06 -
1$\begingroup$ @Federico well a plane could be pushed in any direction. $\endgroup$– fooot ♦May 28 '15 at 22:25
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$\begingroup$ @fooot he's using it in the sense of "push forward", see the
propelleda few words later. $\endgroup$– Federico ♦May 28 '15 at 22:31 -
1$\begingroup$ While this answer is correct about the direction prevailing winds blow, it's completely off about the reason. The actual reasons are quite a bit more complicated than this and have to do primarily with global heating patterns. Long story short, there's a persistent cyclone at each pole (which is stronger in that pole's respective winter season.) The jet streams we usually refer to are the polar jet streams which occur between the mid-latitudes and the polar vortices. There's a weaker jet stream between the tropics and each mid-latitude zone, but they're too high to be useful for most flights. $\endgroup$– reirabMay 20 '16 at 3:06
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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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$\begingroup$ "winds ... add to ground speed going east" A quite practical answer. And it was a reliable heuristic when I did flight planning flying out of Texas. How does this not answer the question? More to the point, how is it wrong? Down voting is not about "liking" an answer. It's about correctness. If not the best answer then don't vote for it. But it's not wrong. And, if it is wrong then down voting w/out saying why is a dis-service to everyone who comes here for answers. $\endgroup$– radarbobJan 17 '16 at 3:25
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1$\begingroup$ I did not down vote your answer, however I can see why some people would. The question is why do flight times differ, and you did not answer that question. While your answer is interesting and potentially useful (depending on the type of aircraft being flown), it does not answer the question that was asked (see How to Answer for more details). This would be better suited as a comment than an answer. $\endgroup$ Jan 17 '16 at 11:57
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$\begingroup$ Well "the answer is wind." quoting above. But no one mentioned Matthew Fontaine Maury, what answer can be complete without starting with this guy? We can go on to how bombing Japan in the world war lead to the discovery of the jet stream. Your point about comment vs answer is well taken. $\endgroup$– radarbobJan 17 '16 at 21:21
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2$\begingroup$ I downvoted, because it includes no details that would make the answer useful. For example, it makes a huge difference whether you were an airline pilot doing cross-Atlantic routes or a GA pilot in that one valley where certain winds prevailed. Secondly, you don't address why you added those 20kts; was it because of wind, orbital effects, UFO interference, magnetic fields? Of course, you know it is the winds, but your answer doesn't actually say that. I'd happily upvote a more detailed version of this answer even though it doesn't answer the question exactly. $\endgroup$ Jan 19 '16 at 10:19
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2$\begingroup$ Note that downvoting is not reserved for when the post contains wrong information, but also when it's "poorly researched, or fails to communicate information." Also, please don't take my downvote as an attack on you, your willingness to help others or your capability as a pilot, but solely because I didn't think the answer was helpful at all without the details I mentioned in the last comment. I'm genuinely sorry if my negative feedback made you feel that way. $\endgroup$ Jan 19 '16 at 22:28
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It is simple. The Earth rotates from East to West. There is hence an eastward shear effect in the atmosphere. If you fly East, this shear adds to the cruising speed. Reverse case if you fly west. So more time if you fly westwards. But how much more depends on the prevailing wind-shear. This cannot be pre-determined except when you actually fly and meter the wind speed.
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2$\begingroup$ Also, the question was actually for the opposite effect (prevailing winds are not in the same direction at all latitudes, so the Earth-rotation cause is quite indirect). $\endgroup$– jcaronJan 16 '16 at 12:55