Why Do Planes Get There Faster Flying East?
The First Person to Find the Sky's "River of Wind" Was a Weather Station Chief in Tsukuba
The distance is the same, yet the trip out and the trip back can differ by one to two hours. The cause is a very fast wind that blows from west to east about 10 kilometres up. And the first person to record this wind was a Japanese scientist who, about 100 years ago, released small balloons into the sky day after day.
You are looking up plane tickets for a trip to Hawaii, and something catches your eye. The flight out takes about 7 hours. The flight home takes about 8 and a half.
The two airports are the same, so the distance must be the same. Is the plane taking a detour on the way back?
In fact, the plane is not taking a detour. High in the sky, an invisible "river" is flowing.
Only two things explain the difference
About 10 kilometres up, where planes fly, a strong westerly wind called the jet stream winds its way around the globe. Over Japan in winter, it can be faster than a bullet train.
A plane moves at a set speed relative to the air around it. If that air is itself flowing east, the wind's speed is added for eastbound flights and subtracted for westbound ones.
It is just like an ordinary river, where a boat going downstream is fast and one going upstream is slow. The difference is that, until 100 years ago, nobody knew this river in the sky existed.
The Tateno weather station that chased balloons every day
In 1920, an upper-air observatory was built in what is now Tsukuba, Ibaraki Prefecture, to study the air high above. Its first director was a meteorologist named Wasaburo Oishi.
At that time, planes could not yet fly high, and there was almost no way to measure the upper air directly. So Oishi and his team released small hydrogen-filled balloons and followed them with a surveying instrument fitted with a telescope. A balloon rises at a steady rate, so the time tells you its height. From the viewing angle and the height, you can work out how far the balloon has been blown sideways.
Oishi and his team are said to have repeated these observations more than a thousand times over several years. They showed, in numbers, that a strong westerly wind reliably blows high up every winter. In 1926, they put the results together in a report.
Yet the discovery hardly became known to the world. Oishi wrote the report in Esperanto, which was then hoped to become an international language, but few meteorologists could read it.
Figure 1 shows the method. All they needed was a balloon, a clock and an instrument for measuring angles.
Wind speed is added to or subtracted from the flight time
A plane moves at about 900 km/h relative to the air around it. Its speed over the ground is this figure with the wind speed added or subtracted. With a 100 km/h tailwind, it is 1000 km/h over the ground. With a 100 km/h headwind, it is 800 km/h.
Use the slider in Figure 2 to change the strength of the upper westerly wind. The stronger the wind, the shorter the eastbound trip and the longer the westbound trip. You will also notice that the westbound trip stretches by more. That is because the slower you go, the faster the time for the same distance grows.
On real flights, airlines check the daily wind forecast. They pick routes that ride the jet stream going east and avoid it as much as possible going west. Even so, timetables often show eastbound flights taking one to two hours less.
The air near the equator is warm and the air near the North Pole is cold, so air high up tries to flow from the warm side to the cold side. But because the Earth is spinning, the northward flow gets bent toward the east. This is why the upper westerly wind is stronger in winter, when the north–south temperature difference is larger.
Around 1944, American heavy bombers flying over Japan are said to have been shocked by unexpectedly fierce headwinds. At about the same time, Japan is said to have used Oishi's observations in a plan to send balloons across the Pacific on this westerly wind. The wind high above became known to the world in a sad way.
Summary
Eastbound planes arrive sooner because they ride the jet stream, which flows from west to east about 10 kilometres up. A plane is carried along with the air, so the wind speed is simply added or subtracted. The first person to show this river of wind in numbers was Wasaburo Oishi, who kept releasing balloons into the sky over Tsukuba.
Written into every flight timetable
is the speed of a river in the sky, measured with balloons 100 years ago.
How the Earth's spin bends the wind is covered in the article on bathtub vortices and the Southern Hemisphere. Why weather changes arrive from the west is covered in the article on fine weather after a red sunset, and how upper air leaves behind condensation trails is covered in the article on contrails.
- On an airline timetable or booking site, look up the travel time for flights between Japan and the US (or Hawaii), both out and back. To avoid the effect of time zones, look at the listed "flight time", not the departure and arrival clock times.
- Compare the out-and-back difference for winter flights and summer flights. If the difference is larger in winter, that is evidence the westerly wind strengthens in winter.
- Work backward from the difference to the wind speed. Put the distance and 900 km/h into the formula in the fold-out section below, and you get the average wind speed.
Real flights change routes to avoid wind, and listed times can include waiting at the airport, so the calculation is only a rough guide. Even so, the seasonal difference can often be read clearly from timetables.
For those who want to know more: terms, formulas and links to textbooksWe mark clearly which level each part belongs to, from middle-school science to university courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics" and "Earth Science"
- High school+Advanced high-school material, or textbook sidebars
- UniversityUniversity-level specialist material (atmospheric dynamics) not taught in high school
- ResearchTopics researchers are still investigating, not taught as settled fact even at university
Middle schoolTerms: this phenomenon has names
- Westerlies: a band of wind that keeps blowing from west to east high above the mid-latitudes.
- Jet stream: a particularly fast, narrow ribbon within the westerlies. It lies around 10 km up, and near Japan in winter it is said to sometimes exceed 100 metres per second.
- Pilot balloon (wind-measuring balloon): a small balloon released to measure the wind high above. Oishi's team used this method, and it is still used today as a supplement.
Middle schoolHigh schoolCheck it with a formula: how many hours' difference between out and back?
Consider a plane flying between Tokyo and Los Angeles at a constant speed relative to the air. We assume an average westerly wind of 100 km/h along the way, and calculate the flight times out and back. The meaning and units of each symbol are in the table just below.
| In symbols | t = L ÷ ( V ± U ) (+ for eastbound, − for westbound) |
| In words | Time taken = distance ÷ (plane's speed relative to the air ± wind speed) |
| Where the formula comes from | It is the addition of velocities (relative velocity). The plane moves at speed V relative to the air, and that air flows at speed U relative to the ground, so the speed over the ground is the sum or difference of the two. After that, it is just "time = distance ÷ speed". |
| t | Flight time (in hours) |
| L | Distance flown (in kilometres) |
| V | Plane's speed relative to the air (in kilometres per hour) |
| U | Average westerly wind speed along the route (in kilometres per hour) |
| Distance between Tokyo and Los Angeles, L | About 8800 kilometres |
| Airliner's speed relative to the air, V | About 900 kilometres per hour |
| Average westerly wind on the route, U (assumed) | 100 kilometres per hour |
| Core of the winter jet stream (near Japan) | Said to sometimes exceed 100 metres per second |
| Eastbound speed over the ground | 900 ＋ 100 ＝ 1000 |
| Eastbound flight time (hours) | 8800 ÷ 1000 ＝ 8.8 |
| Westbound speed over the ground | 900 − 100 ＝ 800 |
| Westbound flight time (hours) | 8800 ÷ 800 ＝ 11 |
| Difference between out and back (hours) | 11 − 8.8 ＝ 2.2 |
| Difference converted to minutes | 2.2 × 60 ＝ 132 |
| Speed of the jet stream core, in km/h | 100 × 3.6 ＝ 360 |
Even with an average westerly wind of just 100 km/h, the out and back trips differ by a little over 2 hours. The core of the jet stream reaches 360 km/h, so a plane that rides it well gets an even bigger tailwind boost. This is also why westbound flights take detours north or south to avoid this flow.
High schoolHigh school+Why is the westerly wind stronger higher up?
High schoolIn Earth science, you learn that the temperature difference between low and high latitudes drives the general circulation of the atmosphere, and that westerlies blow high above the mid-latitudes. A column of warm air is tall and a column of cold air is short, so the higher you go, the larger the north–south pressure difference becomes.
High school+Air set in motion by a pressure difference is turned by the Coriolis force from the Earth's rotation. When the two balance, the wind blows at right angles to the pressure difference, that is, from west to east. This is called the geostrophic wind. Over Japan in winter, the subtropical jet stream on the low-latitude side and the polar-front jet stream on the high-latitude side tend to overlap, making it one of the places with the strongest westerlies in the world.
UniversityThe thermal wind relation
In atmospheric dynamics, how the geostrophic wind changes with height is expressed by a formula called the thermal wind relation. It says that the larger the north–south temperature gradient, the stronger the westerly wind becomes as you go up. This westerly wind is strongest a little below the tropopause (around 10 km up). The large north–south meanders of the westerlies are treated as Rossby waves.
📖 Derivations of the formula and further reading: Thermal wind (Wikipedia, Japanese) ／ Jet stream (Wikipedia, Japanese)
ResearchWhat is still not clear
- How will the jet stream change with global warming? One idea is that as the Arctic warms faster, the north–south temperature difference shrinks and the westerlies meander more widely. Other analyses do not support this, and the debate continues.
- Will turbulence high up increase? Near the jet stream, "clear-air turbulence" shakes planes even when there are no clouds. Some studies say it will increase with warming, but estimates of how much vary widely.
- Why was Oishi's discovery buried? Researchers in the history of science disagree, not only about the language barrier but also about how much interest Western researchers of the time had in upper-air winds.
In other words, what this article says is also "an explanation within what is known today". Our picture of the winds high above is still being redrawn bit by bit through observation and calculation.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science, Grade 8: "Weather and its changes" | The westerlies, and weather changing from the west |
| High school | Basic Physics: "Composition of velocities, relative velocity"; Earth Science: "General circulation of the atmosphere" | The flight-time formula, and stronger westerlies higher up |
| High school+ | Earth Science (advanced) | Geostrophic wind, the two jet streams |
| University | Atmospheric dynamics | Thermal wind relation, Rossby waves |
| Research | Climate science, history of science | Warming and the jet stream, why Oishi's discovery was buried |
| ― | Links to daily life | Flight times on tickets, winter vs. summer differences |
- Lewis, J. M. (2003) Ooishi's Observation: Viewed in the Context of Jet Stream Discovery. Bulletin of the American Meteorological Society, 84(3)
- Wasaburo Oishi (大石和三郎), Japanese Wikipedia
- Jet stream (ジェット気流), Japanese Wikipedia
- Westerlies (偏西風), Japanese Wikipedia
- Yoshimitsu Ogura, General Meteorology (『一般気象学』), University of Tokyo Press (東京大学出版会)
*This article is a popular-science explanation for general readers. The figures given are rough estimates to help you understand the mechanism. Actual flight times vary with the route, aircraft, the day's winds and airport congestion.