Exhaust contrails are formed by the mixing of the hot humid exhaust of the engines with cold humid surrounding air, creating long streamers of clouds. If the conditions are right then these can persist and spread. These are the most common type of contrail observed.

A typical exhaust contrail. There are initially four, one for each engine, then they mix together.

Aerodynamic contrails are  formed by the temporary reduction in pressure of the air moving over the surface of the plane, or in the center of a wake vortex. Reducing the pressure of the air means it can hold less water, so condensation occurs.

An aerodynamic contrail on a landing jet – condensation is visible above the wing surfaces, and in the center of the vortices coming from the outside ends of the deployed flaps, but nothing from the engines. This type of contrail is seen in high local humidity, as indicated here by the misty conditions.

I propose a useful new classification for a type of contrail, the Hybrid Contrail, defined as two distinct thin cylindrical portions of an exhaust contrail that have larger ice crystals due to wake vortices. A hybrid contrail is formed in a narrow range of atmospheric conditions, specifically with temperature below -40F, and a relative humidity with respect to ice slightly below 100%. When RHI is below 100% then a contrail that forms will not be persistent, and will eventually sublime away. The low pressure in the wake vortex core allows for a longer period of time in which the mixing air is above 100%, and hence the ice crystals in that portion of the contrail will grow larger and/or more numerous.

The entire evolution of a hybrid contrail can be see in this video. Notice the trail starts out as a large dense regular exhaust contrail, then this fades away leaving the hybrid contrail which separates away from dissipating exhaust contrail, breaking up into loops and segments.

Hybrid contrails will not form when RHI > 100%, as the entire contrail, including the vortex cores, is above the threshold for ice accretion, and so will accrete (gain ice) at the same rate. Hybrid contrails will  not form at values significantly below RHI of 100%, as the relative increase from the vortex core is small, and cannot push the ambient RHI over 100% after initial mixing. Hence hybrid contrails will only form in marginal conditions with RHI only slightly below 100%. A similar narrow range may also apply to temperature.

The resultant region of greater contrail densities will initially be indistinguishable from the exhaust contrail. However as the exhaust contrail sublimates (turns from ice back to water vapor) then the hybrid contrail will be revealed as two thin rope-like regions running along the contrail. The hybrid contrail will sometimes sink away from the exhaust contrail, due to the large size of the ice crystals. Usually the hybrid contrail will persist for a few minutes longer than the exhaust contrail. Since the hybrid contrail is much smaller in cross-section than the exhaust contrail, then the effects of turbulence and crow instability cause the hybrid contrail to twist into loops and curls that often resemble chromosomes.

A hybrid contrail below the parent exhaust contrail. The larger ice crystals in the hybrid contrail have caused it to fall quicker than the Exhaust Contrail, leading to considerable separation, even though they were originally part of the same trail.

The reason this new classification is needed is that people frequently mistake these hybrid contrails as being regular exhaust contrail, and they cannot understand why these particular contrails loop and twist in such a dramatic and asymmetric manner. In addition hybrid contrails are often spotted within regular exhaust contrails, and this is presented as evidence of something being sprayed within the cover of the contrail. Hybrid contrails also often look very unusual, and this is taken as evidence of some novel propulsion mechanism.

Hybrid contrails often end up looking like a string of loops or chromosome pairs. This looping and twists would be less apparent with the much larger exhaust contrail, as it would simply happen within it. The loops are actually the wake vortices themselves twisting, and as the hybrid contrail exists in the center of the vortex, the effect is much more pronounced.

While I’m suggesting a new classification, this is not in any way a new type of contrail. In fact it has been observed for many decades, such as in the 1972 book: Clouds of the World:

This 1972 book, Clouds of the World, discusses the formation of Hybrid contrails. But does not give them a particular name.

The full development can be seen here:

http://www.airliners.net/photo/Arik-Air-(Hi/Airbus-A340-542/1820435/L/&sid=ecfd72e0685325752848fb2b4cad1867

Development of a the hybrid portions of a contrail are shown from the initial four separate exhaust contrails, through to just the two hybrid contrails and the crow instability breakup. The hybrid contrail is probably sinking below the exhaust contrail, but since it’s viewed in line there’s no visible separation.

In a four engined jet the contribution to the hybrid contrails comes mostly from the outside engines. This is because they are much closer to the ends of the the wings, and so feed almost directly into the vortices. The inner engines contrails are pushed down by the vortex sheet, and are greatly spread out before they might contribute. The following animation shows this initial separation:

The contrails from the inner engines are greatly spread out before they become entrained with the wingtip vortices, the outer engine’s contrails flow into the vortices at a much earlier stage.

The post Hybrid Contrails – A New Classification appeared first on Contrail Science.

When you breath out on a cold day, you see a little cloud of condensation form from your breath. This is the same kind of thing, your damp warm lungs add moisture to the air, and when you breath out, you get condensation.

But the condensation from your breath quickly evaporates, usually in less than a second. Condensation trails from a jet can last for many minutes, even for hours sometimes. So why is there this difference? Why do jet contrails sometime persist, but your breath condensation quickly evaporates?

The difference is because a contrail freezes.

It’s really that simple. Contrails form at -40 degrees Fahrenheit (which is also -40 Celsius), or colder. At that temperature the tiny drops of condensed water will instantly freeze. Once frozen they can not evaporate. They also can’t melt, as it’s -40. They can however fade away through a process known as “sublimation” – where a solid turns into a gas.

You’ve seen sublimation before. Dry Ice is frozen carbon dioxide. It does not melt, it just sublimes directly into the gas. If you take a bit of dry ice, and just leave it in the sun, it will just kind of fade away. That’s what happens to the ice in a contrail.

Ice will only sublime if the humidity (at that altitude) is lower than around 60% to 70%. So if it’s a bit higher then the contrail can last for a long time, just like clouds do sometimes. If the humidity is low, then the sublimation happens very fast, and the contrail only lasts a minute or so. If the humidity is high (above 70%) then you get reverse sublimation (also called desublimation, accretion or deposition, where water vapor turns directly to ice, but only when in contact with ice), and even more ice will form on the frozen condensation, the ice crystals will get bigger, and sink faster, causing the trail to spread out as it sinks through altitudes with different wind speeds.

So, the difference can be summed up as:

Contrail = Condensation + Freezing + Sublimation
Breath = Condensation + Evaporation

This isn’t really a property of your breath though, it’s a property of temperature. It you breath out at -40 degrees or colder, then your breath will freeze, and it will not evaporate. Instead of a little cloud that quickly evaporates, your breath at -40 degrees will look like smoke. Like these guys in Siberia, at -52C, you can’t tell the difference between their breath, and cigarette smoke.

Note: when we say a contrail freezes we are generally talking about an exhaust contrail – from the engine. There’s another type of contrail that’s the aerodynamic contrail you sometimes see when a plane is landing, streaming from the wing tips or flaps. That’s actually liquid water condensation, like your breath, and that’s why it quickly vanishes.

The post Contrails Are Condensation, But Not Like Your Breath appeared first on Contrail Science.

When you look up in the sky and you see a contrail, how far away is it? How far away are these contrails, a mile? two miles? Would you believe they are actually 20 to 100 miles away?

Contrails typically form above 30,000 feet, or around six to eight miles straight up. It’s quite hard to judge exactly how high it is, unless you know what type of plane it is. But assuming six miles is a pretty safe bet.

So if we assume it’s at least six miles above the ground, then all we need to do to find out how far away it is is to measure the angle of elevation. It’s quite a common child’s math problem, but usually applied the other way around, to measure the height of things if we know how far away they are.

To measure the height …

So it’s a real simple bit of maths, height = distance * tan(angle).

But of course if you know the height, then you can calculate the distance = height * (1 / tan(angle)).

Now most planes leaving contrails cruise at between 30,000 and 45,000. 6 miles is 31,680 feet, so it’s a pretty safe assumption that any contrail you see is at 6 miles or above. So for the purposes of calculating the distance, let’s just assume it’s six miles high. At worst we will underestimate.  Here’s what the figures work out as for various angles from the horizon:

The important colum there is in bold, the “Miles” column which tells you how far away horizontally the plane is. That is, it tells you how far away the point is that is directly above.

The column next to that, LOS (Line Of Sight) tells you the actual distance from you to the plane. So when it’s overhead it’s 6 miles. As it gets further away it gets closer to the horizontal distance.

Look at the angles below 45 degrees. In particular a plane that’s ten degrees above the horizon will be 34 miles (55 km) away, and one that is five degrees will be 68 miles (110 km) away.  Now ten degrees might not seem like a lot but it actually is surprisingly high if you go out and point your arm up at ten degrees.

Here I’m pointing up at ten degrees, I see plenty of planes “over there”, but it seems pretty unintuitive to think that they are 50 miles away. But they are.Notice, as you would expect that at 45 degrees, the plane is the same distance away as the height, six miles. And any angle above 45 degrees is within a six mile radius. But still, consider that many people will point to a plane at 45 degrees as being “overhead”, when really it’s six miles away.

Now let’s look at some actual contrails. These photos were taken on an iPhone using the “Theodolite” app, which tells you the angle of elevation.

Contrails at five degrees, so around 70 miles away

Contrails are generally at the same altitude. So if a contrail is “below” another, that means it’s further away. In this case 46 miles further away. Notice how much further away the contrails are than the mountains, but they seem visually closer.

Here you might describe this contrail as “flying right over the yellow house”, but at 20 degrees elevation it’s AT LEAST 16 miles away – way out away from the city.

You can also use this to get an estimate of the length of a contrail segment if it’s coming towards you. Here we can calculate the contrail goes from 12 to 27 degrees, so is about 16 miles long. It looks like it’s vertical, but it’s actually horizontal.

This math breaks down somewhat for below five degrees, as they the curvature of the earth is much more of a factor. But for most contrails you see you can get a reasonable estimate of the distance from the angle. And since most contrails do not fly directly overhead, the vast majority of visible contrails are going be at a low angle, and very far away. Much further than you might think.

Back to the photo we started with. It was taken from a bridge over the Thames, to the west of London. Using Google Earth, I’ve place at grid at six miles altitude, with five miles between each line.

Contrails, looking west from Kew Bridge over the Thames, to the West of London. It looks like the contrails are “over the city” but really they are much further away

Drawing a grid at six miles up shows just how far away. The closest would be ten miles away, the furthest over 100 miles. The number at the top of the buildings don’t show the distance to the building (actually less than a mile), but show the distance to something six miles up in the sky if it appeared directly behind the top of the building.

If we draw out the visible cone on a map, we see that the contrails are mostly far out over the countryside, nowhere near the city at all. Most likely they are intercontinental flights from Continental Europe to North America

The post How Far Away is That Contrail? appeared first on Contrail Science.

http://contrailscience.com/map

Here’s a quick video explaining how to use it:

You can use to to see how many potential contrails you will get over any particular area by filtering out all traffic below 30,000 feet.

Works best in the latest versions of Chrome or Firefox, or in Internet Explorer with Google’s Chrome Frame plugin. Will not work currently on Opera or iOS. If you have to use Safari, then you need to enable WebGL (Preferences/Advanced -> Show Develop menu, then from the Develop menu “Enable WebGL”).

Feedback, bugs etc appreciated!

The post Interactive Flight/Contrail Map Visualization appeared first on Contrail Science.

When jet exhaust comes out of the engine, it’s superheated. So the water is in the form of vapor, steam, and hence it’s invisible. As it mixes with the surrounding (freezing) air it very quickly cools down, and at a certain point it will condense out into water droplets, and then freeze into ice. Because it takes a fraction of a second to do this, then there’s a gap between the engine and the contrail.

(Most of the images here come from the excellent contrail spotting forum at luchtzak.be  and ExtremeSpotting.com)

The causes of this gap are the same as the causes behind the gap you see when steam is coming out of a kettle under pressure:

The size of this gap varies quite a bit, based on various factors I’ll discuss below. Here’s some variations:

There are several variables that you need to account for in explaining these differences:

1. The speed of the plane
2. The speed of the exhaust
3. The temperature of the surrounding air
4. The temperature of the exhaust
5. The size of the plane.

Now the size of the plane does not actually affect the size of the gap unless you are measuring that gap in “plane lengths” – which you really should not. A large plane does not automatically produce a larger gap, so it’s not a good unit of measurement. The plane length of an A380 is 238 feet, the plane length of an A320 is only 123 feet. So all other things being equal, the gap on the the smaller plane will look like it’s twice as long as the gap on the bigger plane, if you measure it in plane lengths.

So, consider speed. If the plane were moving at 500 knots, and it were simply letting some water vapor steam out the back, then that steam would be blown away from the plane at 500 mph, so the length of the gap would be determined by how long it takes the vapor to condense.

If the plane was not moving at all, and just shooting out the jet exhaust, then the exhaust would be blown back at that initial speed, and then quickly slow down, but there would still be a gap.

Combine those two things, you’ve got #1 the speed at which the exhaust contrail eventually moves away from the plane, and #2, the initial speed at which it moves away.

Now consider temperature. The vapor has to cool below the temperature at which water will condense. This cooling happens by the exhaust gases (temp #4) mixing with the surrounding air (#3). The hot exhaust mixes with the cold air, just like if you pour a cup of  hot water into a cold bath. This mixing happens very rapidly due to the turbulence behind the plane.

So the length of the gap depends on how quickly this cooling happens. It will be quicker if the temperatures involved are low. Modern efficient engines have much cooler exhaust than older engines so will have shorter gaps. The higher you fly, the colder it gets and the shorter the gap gets

Causes of Short Contrail Gap

• Low speed plane
• Low power setting (low exhaust speed and cooler exhaust)
• High altitude (colder surrounding air)

Causes of Long Contrail Gap

• High speed plane
• High Power setting (high exhaust speed and hot exhaust)
• Low Altitude (warmer surrounding air)

Of these, probably the one that has the greatest effect is air temperature, which is generally determined by altitude. Prop planes like this C-130 don’t fly as high as jet planes, so when they do create contrails, it’s generally going to be near the warmest temperature possible, and hence the gap will be longer. Prop planes may also entrain the exhaust gasses in vortices behind each engine, resulting in slower mixing with the surrounding air than the more forceful turbulence of a jet engine.

Planes can also make aerodynamic contrails from the wing surfaces or propellors. These are very different to the normal exhaust contrails. Since these are caused by a lowering of pressure, the contrail formation is nearly instant, as the air immediately reaches the correct temperature, so there’s no cooling time required, so no gap. The following photo shows a C-130 like above, but with aerodynamic contrails coming from the tips of the propellors. It also illustrates the vortices that form behind the individual engines, which will slow down the mixing of engine exhaust with the surrounding air, lengthening the gap seen above.

One more thing that can affect the apparent gap is how it is illuminated by the sun. Contrails dont just spring into existence as solid white clouds, they start out quite faint and transparent. If this region is lit by direct sunlight, then it’s more visible, and the gap will seem shorter. If it’s not lit by the sun – like if the plane is in the shadow of a cloud, then the gap will seem longer.
The following photo illustrates this. The contrail on the left of the photo is being shaded by the body of the plane. Even though both contrails are the same behind the plane, meaning they have the same short gap, the gap on the left contrail looks much longer.

The post How Big is the Gap Between Contrails and Engines? appeared first on Contrail Science.