Have you ever wondered why a westbound flight takes longer than the same route eastbound? It’s all to do with the rotation of the earth, but not as simple as it might sound. This article takes a quick look at why this happens.
Longer flight times
There are many factors influencing flight duration, but in general, you will always notice westbound flights scheduled to take longer. Compare most flight schedules and you will see the return leg of an eastbound flight is longer than the outbound.
For example, an eastbound flight from New York to London with British Airways takes approximately 6 hours 45 minutes, and the return from London to New York around 8 hours (according to flight schedules published by British Airways).
Likewise, a flight from London to Hong Kong is around 12 hours, while the return westbound is over an hour longer.
The effect of the rotation of the earth – but not that obvious!
The Earth rotates from west to east. And at quite some speed – at the Equator, rotational velocity is around 1,000 kilometers per hour. Thinking simply, that should make a westbound flight take less time, as the Earth is moving towards the aircraft.
This though does not happen. In reality, the aircraft is also moving away from the destination as it continues to spin with the earth (it’s not just the surface that rotates, but the atmosphere too). What matters then is the velocity of the aircraft in relation to the Earth.
If you need proof of this, just remember that if you jump up, the Earth does not rotate under you. If it did, you would land hundreds of meters away!
Adding in the effects of wind – the jet streams
It is the rotation of the Earth that is causing the longer flight times, but not because it’s moving towards or away from the flying aircraft. It is rather due to its influence on wind patterns – the so-called high-altitude jet streams.
A rotating object has a force perpendicular to the axis of rotation – this is known as the Coriolis force. This force pushes winds to the east in the northern hemisphere and to the west in the southern hemisphere. The strength of this is related to the speed of rotation. Points closer to the equator rotate faster than points close to the poles – as they have further to travel in each rotation in the same time.
In addition to this, winds are affected by solar heating and will flow from areas of high pressure to low pressure. Combined, these effects produce jet streams that move from west to east, but with an undulating pattern. The jet stream can vary in strength, altitude and routing over time and will usually be strongest closer to the poles.
For a more complete discussion about jet streams, and how they form, see this excellent Wikipedia article.
These jet streams have a significant effect on aircraft. An aircraft traveling east can effectively pick up a tailwind speeding its journey. Whereas a westbound flight may end up flying against the wind.
More than just affecting average flight times, these jet streams have a major impact on flight routings and scheduling. Airlines will look at jet stream patterns each day and alter routes for their flights accordingly. Taking a longer physical distance route might make sense if you can piggyback on a strong tailwind. And likewise, you can re-route westbound flights to avoid some of the effects of the jet streams.
So next time you are told your flight will be longer or shorter due to wind or jet stream effects, you will know why.