Why it’s faster flying east, than it is west — the incredible aviation science of jetstreams
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We’re now mid-winter in the northern hemisphere and its effects can be seen right around the globe. Frosty mornings in the UK, extensive snow cover across the northern United States and near coast-to-coast snow in northern Japan.
However, there’s one element at its peak during this time of the year — though you’d do well to see it. We’re talking, of course, about jetstreams. High above the earth’s surface, these fast-moving bands of airflow from west to east have a significant effect on your flight time.
By knowing where the jetstreams are, airline operations departments can plan flight routes to take advantage of them when flying east and to avoid them when flying west.
However, any time there are strong winds, it also increases the likelihood of turbulence. As a result, pilots must balance the benefits of using the strong winds to reduce the flight time and as a result the fuel burn against the downsides of turbulence and passenger comfort.
What is a Jetstream?
Jetstreams were first discovered by Japanese meteorologist Wasaburo Ooishi in the 1920s. He recorded some 1300 observations of these high altitude winds between 1923 and 1925. However, due to his eccentricity and publication of his study in Esperanto (a constructed international auxiliary language), his findings were not taken seriously.
It wasn’t until WW2 that American bomber crews flying missions over the Pacific found that they were encountering headwinds of up to 230mph. Not only did this massively reduce the accuracy of their bombing, but it also meant that they were using far more fuel to reach their targets. So much so that some aircraft ran out of fuel on the return leg.
In simple terms, a jetstream is a narrow band of fast-moving air situated several miles above the earth’s surface. The position of a jetstream depends on the location of warm and cold air masses on the surface, so it varies from day-to-day. Some days they will be quite far north, other days they will be quite far south.
So what actually creates a jetstream? To understand this, we need to break things down and look at how weather systems operate.
The movement of air over the earth’s surface can be explained by Global Circulations. If the earth was stationary, had no water and wasn’t titled, when the sun heated the earth the area where the sun is directly overhead, the equator, would be heated the most.
As the hot air rises, it would reach a point in the atmosphere where it could rise no more and would have to spread out towards the poles. As this air reached the ends of the earth, it would cool and then sink back down to the surface. It would then flow back towards the equator where the whole process would start again. If this was the case, the weather on earth would be pretty simple and benign.
However, the earth isn’t as simple as this and the rotation, combination of land and sea masses and tilt all play a part in making things a lot more exciting.
Instead of there being a single airmass between the pole and the equator, there are actually three.
Just above the equator is the Hadley Cell where air rises, moves towards the pole and then sinks down to the surface where it moves back towards the equator. It is this movement of air that causes the giant thunderstorms seen over the tropics. At the poles, there is, unsurprisingly, the Polar Cell. Here, air rises and moves towards the poles where it then sinks back down to earth and completes the cycle by moving back towards the equator.
In between these two cells is the Ferrel Cell where air moves towards the poles at the surface and towards the equator aloft.
The rotation of the earth
With the three air masses defined, the rotation of the earth now plays a part and our first piece of physics — the speed/distance/time equation.
The speed at which the earth spins depends on where you are standing because the circumference of the earth is bigger at the equator than at the poles. As it all has to make one whole rotation at the same time – to cover the greater distance at the equator – it has to be moving quicker (speed = distance/time). If you are standing at the equator, the earth is spinning at around 1040mph whereas if you were standing on a pole, the rotation speed is virtually zero.
The second piece of physics to consider here is the conservation of momentum. Due to Newton’s laws of motion, the energy of the moving air masses, or their momentum, can not be created or destroyed but only changed by the action of forces.
As the air at the equator starts moving towards the poles, its eastward motion due to the spinning of the earth remains constant due to the conservation of momentum. However, as that air moves towards the poles, the surface of the earth below it is spinning at a slower speed, in effect getting ‘left behind’.
The result is that the further you go from the equator, the more the earth’s surface gets ‘left behind. Or, as seen from the surface, the faster the winds above become.
The Air Masses
The final part of creating a jetstream comes from the three air masses described above.
‘Wind’ is how we experience the movement of air from an area of high pressure to an area of low pressure. Hot air is less dense so has lower pressure. Cold air is denser so has a higher pressure. As a result, air will move from areas of cold air to areas of warm air.
The areas between the air masses are where the temperature differences are the greatest. As the difference in temperature increases, the difference in pressure increases and so the wind speed increases.
Because of this the areas where the difference in temperature and pressure is the greatest is where you’ll find the greatest wind speeds. Looking at the diagram above, the area at 60° North is one of these and where we find one of the most prominent jetstreams in aviation.
Significant Weather Charts
As part of our preflight briefing pack, we receive what is known as a Significant Weather chart for the route. This depicts any jetstreams, areas of associated turbulence and also thunderstorms that we may encounter.
The yellow lines are jetstreams and can all be seen to be moving in an easterly direction. Along with these lines, there are two more pieces of information. Firstly, is the speed of the jetstream.
Along the yellow line is a white arrow with a number of markings on it. The large solid triangle indicates 50kts of wind and the smaller lines 10kts of wind. As a result, we can see that the small jetstream in the middle of the Atlantic is 100kts fast.
Underneath the same line is the altitude where the core of the jetstream is situated. The jetstream off the coast of Novia Scotia, Canada is showing a core at FL310, or 31,000ft. This is important when it comes to planning the altitude we fly at and also for predicting turbulence.
One of the downsides to jetstreams is the turbulence that often comes with them. The red dotted lines around them indicate where turbulence might be experienced and the adjoining black box indicates the altitudes and severity of the turbulence.
However, these forecasts are only released every 6 hours and the actual weather can be quite different.
Apps are now available that, when connected to inflight wifi, allow you to download the very latest actual weather reports and forecasts. It then puts this information in a dynamic graphical format, which enables us to see what weather we can actually expect at any given time of the flight. This is particularly useful when it comes to thunderstorms and turbulence as they are often reported by other pilots just minutes ahead.
How do pilots use jetstreams?
Jetstreams can be of huge benefit but also a considerable disadvantage to airlines and pilots, depending on where they are located and where the flight is headed. As a result, accurate forecasting is key.
Before each flight, the flight planning department asses the weather forecast carefully so that they can file a flight plan that takes the aircraft on the most economical route. For the most part, this means using the jetstreams as much as possible on eastbound flights and avoiding them as much as possible on westbound flights. It’s for this reason why we can sometimes end up with some quite odd routes.
The Best Route
The shortest route between two points is what’s known as a great circle. Put one end of a piece of string on San Francisco and the other on London and you’ll find that this takes you up over northern Canada, over Greenland, down through Scotland and onto London.
However, there are quite often strong jetstreams across the USA so sometimes we will take a longer route by distance but will actually be a lot quicker timewise as we take advantage of the strong tailwinds.
In still air, an airliner cruises at around 550mph. Add a 150mph jetstream, a speed fairly common throughout the winter and all of a sudden we are travelling over the ground at 700mph. Not only does this get us to our destination quicker but it can massively reduce the fuel used, even if flying a longer route by distance.
The Best Altitude
Not only do the jetstreams affect the route that we take, but they also play a part in how high we decide to fly. Normally, we prefer to fly as high as the weight of the aircraft will allow us. The higher we are the thinner the air is meaning that there is less drag to slow us down. The engines are also more efficient the higher we fly. This means that as a long haul flight progresses and the aircraft gets lighter due to burning off fuel, we will normally climb to a new, more efficient, altitude every few hours.
However, when there is a good jetstream available to help us along our way, we may decide to fly lower than we would do normally to utilise this extra boost. Like with a longer routing, the benefits of flying at a lower altitude to make the most of the strong tailwind may outweigh the negatives of flying at a lower altitude. The core of a jetstream may only be a few thousand feet thick. Outside this, the wind strength can start to drop off pretty quickly. In this instance, we will endeavour to fly right in the core of the jetstream.
When crossing the Atlantic, if there is a strong jetstream at a lower level, we may spend the first portion of the flight over mainland USA at a higher altitude and then descend for the crossing to take advantage of the winds.
The Downside to Jetstreams
So far it may seem that once you’ve found a nice strong jetstream, everything is perfect until it runs out of steam. However, the one negative part of jetstreams is the turbulence that often comes with them. This is why it is important to know where the core of the jetstream is located.
To use an analogy — the water flowing in the middle of a fast-moving river tends to be quite smooth. Here, unobstructed by rocks and trees, the water can move unimpeded. However, at the edge of the flow, where the water comes into contact with the riverbank, rocks and other items it becomes more turbulent. It’s the same in the air.
The best place to be when flying in a jetstream is right in the core. As we start to move towards the edge, the fast-moving air starts to come into contact with the slow-moving air and it is this interaction that causes turbulence. As a result, we will always endeavour to either fly right in the core of the jetstream or away from it altogether.
However, on busy nights crossing the pond, there are often a number of aircraft all vying for the same optimum flight level and not everyone can be accommodated.
Jetstreams are an important part of aviation, particular for trans-Atlantic crossings. So much so that authorities are changing the way in which we cross the pond. As traffic levels pick up, instead of being restricted to the North Atlantic Track system, some flights are able to file ‘random’ routes which, for the most part, track exactly long the path of a jetstream.
This means that they can take advantage of the strong tailwinds for longer, reducing flight times, fuel usages and carbon emissions as a result.
Featured Image – Getty Images
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