Contrails are generally classified into two types. Exhaust contrails and aerodynamic contrails.
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.
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 trails that aircraft leave in the sky are called “contrails”, which is short for “condensation trails”. They are formed by the condensation of the water vapor in the aircraft exhaust.
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 desublimationor 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.
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.
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
This link here will take you to the interactive map, but I suggest you first watch the video below which explains how it works (but watch it full screen, in HD), or look at some rather nice visualizations here. And to the point of the XKCD article, here’s the worldwide distribution of air traffic. It’s mostly over Europe and the US, but is still worldwide, and a goodly proportion of the oceans, especially in the north.
I’ve programmed a visualization of air traffic that you can interact with in a web browser
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”).
Contrails are the white lines that sometimes form behind high flying aircraft. They are actually a type of cloud. The cloud forms because jet exhaust contains quite a bit of water. If the humidity is high, then the contrails can persist for a long time, like clouds do.
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.
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:
The speed of the plane
The speed of the exhaust
The temperature of the surrounding air
The temperature of the exhaust
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)
For threshold conditions, contrails become visible about one wing span behind the engines. For lower temperature, contrails can be seen forming already a few meters behind the engine.
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.
Contrails are the white trails that planes leave in the sky. They are a type of cloud, and are comprised of very fine ice crystals, like a cirrus cloud. If the air at high altitude is humid, then the contrail will continue to accrete ice, and can spread out into a layer of cirrus cloud.
There’s two main reasons why we might want to avoid contrails. Firstly for military planes, contrails act like a giant arrow in the sky, pointing directly to the plane. This is particularly a problem for stealth and spy planes, which might otherwise go undetected. From a military perspective, even a short non-persistent contrail is something that should be avoided. Consider this image, if it were not for the contrails, then the fighter escorts (“top cover”) would be basically invisible
The second reason is that contrails are having an effect on the weather and the climate. It’s thought that the warming effect of contrails (they trap in heat at night) is greater than warming caused by the CO2 the flights emit. While this is a relatively small fraction of global warming, it’s still enough to be a concern. From this climate perspective, we are only concerned with the persistent contrails. Short contrails have no real effect on the climate or the weather.
The simplest method of contrail avoidance, and one that has been used since WWII, is to not fly in regions of air that support contrail formation. You can do this by flying low enough that it’s too warm, or by making smaller modifications in altitude to avoid flight levels with high humidity.
Unfortunately high altitude humidity is hard to measure. So the U2 spy plane designers resorted to a rather low tech solution, a small rear-view mirror that allowed the pilot to see if he was making a contrail. Later planes used automated contrail detection techniques such as LIDAR. (See patents US5546183 in 1992, and US5285256 also in 1992)
For commercial flights changes to altitude are an expense they do not want. Jets tend to fly at the altitude which is most economical for them. That’s a combination of the ideal air density for the engines, and how much power it takes to get up there. Flying lower or higher will reduce engine efficiency, and will create more pollution and CO2, as well as burning more expensive fuel. This paper by Sridar et al, calculates an optimal reconfiguring of air traffic could result in a six fold decrease in contrails, for a 2% increase in fuel. That’s quite good, but it’s a best case, and still quite costly.
It is possible though that a fully computerized air traffic control system might be able to improve on the contrail situation in a more cost effective way by calculating the most optimal routes for everyone accounting for both engine efficiency and contrail formation. But such a system is still years in the future. See Patent US20090319164, filed in 2008 – this relies on automated spotting of contrails from the ground, and then telling other planes to avoid that region.
Contrails form due to moisture in the aircraft exhaust. So an obvious way of preventing contrails would be to remove the moisture. As you can imagine, this is not a trivial thing, jet exhaust is not some static thing – it’s what comes out of the back of the jet engine. Extracting the moisture is inevitably going to severely degrade the performance of the engine.
Contrails also form because the exhaust cools quickly enough so that the water vapor condenses before it’s been too diluted by the surrounding air to do so. So you could prevent contrails either by making the exhaust hotter (so it takes longer to cool, and hence mixes more, avoiding the critical balance of temperature and humidity), or by mixing it with the ambient air quicker. Unfortunately here there’s no easy way of doing this without degrading the performance of the engine. Either way require more energy to be wasted in the exhaust and not providing thrust. So again you’d be making your engine less efficient, and more polluting.
Contrails also generally require condensation nuclei. Often this comes from soot and sulphur and other byproducts in the engine exhaust. If we could make less soot that would be great, as it’s less carbon particulates. However it would not prevent contrail formation, as it’s very unlikely we could get it down to zero, and there’s other condensation nuclei in the air anyway. Patent US20100122519 from 2010 claims to achieve this by using ultra-low sulphur fuel. But it’s only likely to provide a reduction in the initial density.
Another technique that has been suggested and tried over the year is adding some kind of chemical to the exhaust to prevent contrail formation. The most common approach is to add many times MORE condensation nuclei to the exhaust. The large number of condensation nuclei create lots more ice crystals, but they are very small, and if the size can be kept under half a micron then they will be sub-visible. It’s not a guaranteed fix though, as the contrail can still continue to grow visible if condition are humid enough. Presumably though by the time the trail become visible, the plane will be long gone. So that’s great for stealth, but might not be that useful for climate concerns. Here’s an early test, although it’s not clear what technology they are using:
Several Patents have been filed for such technology, and as patents reference each other we can see somewhat how the technology evolved.
One of the earliest was filed fifty years ago in 1962 by the US Air Force, US Patent 3517505, Method and Apparatus for Suppressing Contrails. Which consist of a chlorosulphuric acid spray, powered by compressed nitrogen, that is mounted near the engines. This is sprayed into the exhaust, the heat breaks it down into hydrogen chloride and sulphur trioxide, which acts as the nuclei.
The problems with this approach were that the acid was very corrosive, and you needed a lot of it – as much as 3% of the weight of the fuel. It’s also not really environmentally friendly, spraying tons of chemicals into the air, just to prevent the formation of some clouds. All of these chemical approaches have the downside of adding additional complicated machinery to the aircraft. So this approach was only investigated by the military, and on some planes it was dropped in favor of more low-tech approaches, like the rear-view mirror.
US3289409 in 1964 has a different approach, using the less chemically reactive “carbon black” (hydrocarbon soot). The patent describes a secondary combustion chamber that burns a small amount of normal fuel very inefficiently to create large amounts of soot, the theory being that the soot will absorb light, and re-radiate it as heat, essentially masking the white contrail with a black cloud. The downsides here are pretty obvious – the large amount of soot is itself a pollutant (soot blackens snow, contributing to global warming), and the amount of extra fuel needed is given the rather wide range of 0.01% to 5%. In addition the technique seems highly unlikely to work.
Then we have US4766725, in 1985, which used an exotic blend of surfactants (wetting agents, soap-like things). US5005355 in 1988 uses alcohols. Both use the approach of reducing the surface tension of water, preventing it from forming ice (effectively it reduces the freezing point)
Some of the more interesting and recent contrail suppression technologies comes from Noppel, Singh, and Taylor, who suggest that the ice crystals in the contrail can be broken down by zapping them with either electromagnetic radiation like microwaves (US20100132330) or ultrasonic sound waves (US20100043443).
The idea is the the radiation or sound energy will melt or break up the ice crystals into sub-visible sizes, or delay cooling past the crucial mixing point. This has a great advantage over the chemical methods in that it’s very clean. No additional pollution is added beyond the extra energy required from the engines. The method is combined with automated contrail detectors, so it’s only activated when contrails would form. This greatly reduces the energy needed.
The primary reason why anti-contrail technology is not being implemented is one of cost. Airlines have zero incentive to do this. Legislation would be required, and there is very little political will for such a thing – there is not even any really compelling evidence yet that it would be, on balance, a good idea, since all the solutions result in an increase in CO2 emissions, and/or additional chemical pollution. Still, it’s seems likely that such a determination may be made in the future, and we might, in a few decades, see far fewer contrails than we do now.
One often gets satellite images of contrails and it’s hard to visualize what’s going on, especially with racetrack contrails drifting in the wind. So I wrote a little contrail simulator applet to demonstrate the formation of a contrail under various conditions. Continue reading »
It’s interesting because it shows that back in 1980 people were noticing contrails persisting and spreading to cover the whole sky. At that point people had even noticed it contributed to increased cloudiness.
This is a useful video to show to anyone who thinks that this is a recent phenomenon. For more details, the research paper also makes it very clear that contrails in 1980 were just the same as contrails in the present day. Continue reading »
It’s a lovely Friday morning here in Los Angeles. Clear blue skies, not a cloud to be seen, and no contrails either.
For that matter, there don’t appear to be any planes at all. I look up into the clear deep blue sky, and I can’t see a single plane. What’s going on? Where have all the planes gone?
Of course the planes are still there. Airlines have pretty regular schedules, and at any given time there’s going to be a few planes visible somewhere from my house. The reasons why I’m having a hard time spotting them (I did spot one eventually) are several, but it boils down to this:
High altitude planes are very hard to spot on a cloudless sky with no contrails.
I’ll briefly list the reasons, then go into each one in more detail
Empty Field Myopia – The eye, when looking at a featureless field of vision, will focus just a few feet in front of you, so the planes will be out of focus.
Saccadic Masking – When your eye moves from one point to another, you don’t see anything while the eye is in motion.
Small Planes, Big Sky – a plane is about 1/100th to 1/10,000 the size of a contrail, making it proportionally harder to spot.
Invisible planes – Atmospheric conditions and the color of the planes can make some planes blend in with the sky so well that they are essentially invisible, or very indistinct.
The chemtrail conspiracy theory seems to frequently misidentify ordinary contrails as “chemtrails” – some kind of secret spraying program. This theory comes in many flavors, and there’s a large number of things people bring up as “evidence” to support this theory. I’ve tried to gather all the debunks of this evidence in one place here, for easy reference.
While the title of this post is “How to Debunk Chemtrails”, the actual debunking depends on what version of the theory needs debunking.
The most common version is simply that “normal” contrails should not persist, so the persistent trails must be “chemtrails”. The simplest way to debunk this is to note that all known books on clouds and weather say that contrails sometimes persist. As seen in this video:
After that, there’s a variety of common claims, and variations on those themes. The best approach is to debunk the individual claim (such as: contrails only last a few seconds), rather than trying to debunk the entire theory.
I’ve tried to arrange each section in the order of most useful links first.
Contrails Through History
Theorists claim: Contrails used to dissipate in a few seconds, minutes at most.
But they didn’t. There’s a huge amount of evidence (in addition to lots of people remembering persistent contrails). It dates back to 1918, and continues up to the present day.
Sometimes planes leave contrails, and sometimes they don’t. It depends on the weather, and specifically it depends on the weather at altitude. It’s also very localized. A plane might leave a trail in one region, and another plane a mile away might not leave a trail.
NASA have put together a contrail forecast page that you can use to roughly predict when contrails are likely for a given region, and a given altitude. The following image should be the latest forecast
I suggest also looking at a water vapor satellite image.
The presence of a circle at a particular point simply tells you if the conditions are favorable for contrail formation at the altitude indicated by the color of the circle. The size of the circle only varies so different altitudes can be shown at the same point, but the smaller the circle, the higher the altitude.
The Mb scale on the left is the measure of atmospheric pressure in millibars. This can roughly be translated to altitude, as pressure decreases fairly uniformly with altitude. Planes actually use the air pressure to measure their altitude using an altimeter, but you have to set it to the local sea level pressure in order to get an accurate result for landing and take-off. To avoid confusion, planes flying above 18,000 feet all set their altimeters to the same reference, 29.92 inchs of mercury, or 1013.25mb (for sea level).
The scale starts at 400mb, which is around 23,500 feet, and goes to 125mb, or about 48,500 feet.
The page describes how this works, and I repeat it here in full:
The RUC model data are representations of the complete 3-dimensional structure of wind, temperature, and humidity over the USA at a resolution of 25 mb and 40 km. The horizontal resolution has been degraded to 1° latitude x 1° longitude to facilitate the computations. Because they are based on a sparse number of actual in situ (balloon sonde) data taken every 12 hours and satellite measurements, the RUC data are not a perfect representation of the various meteorological parameters, especially water vapor. The model humidity at upper levels of the atmosphere is often too low, reflecting the current biases known to exist in our measurement system. Persistent contrails require a relative humidity with respect to ice (RHI) that exceeds 100%. We know that contrails are sometimes observed in areas where estimates of the RHI are less than 100%. The existence of contrails in those locations highlights the “dry-bias” in the humidity fields.
Because the input data do not perfectly characterize the meteorological conditions, the diagnoses of persistent contrail conditions are only estimates and will not detect all of the areas where persistent contrails will form and may also add areas of formation that do not exist. All estimates of persistent contrail formation conditions are based on a modified Appleman curve using three different engine propulsion efficiencies. To give some idea of where contrails may form, but are not diagnosed, we have included information about RHI for values above 70% for single-level plots.
Two forms of results are presented.
Favorable contrail conditions, for a range of pressure levels between 125 and 400 mb, are represented as concentric circles – color coded with reducing diameter for each level. These results can be displayed for engine efficiencies of 0.2, 0.3, and 0.4.
Favorable contrail conditions at each level, represented by ‘X’, along with relative humidity w.r.t ice (RHI). These results are only available for engine efficiencies of 0.3.
There are more contrails now than there were 20 or 30 years ago, because there are a lot more flights now.
But it’s not entirely that simple. As well as more frequent flights, there are more routes. Areas that almost never saw a contrail would suddenly start seeing them on a regular basis when a new route opened up between two cities. Say Denver, CO and Medford, OR. In 1990 there were no direct flights between Denver and Medford, so you’d have to fly from Denver to San Francisco, and then SF to Medford. Then an airline opens up a new route, and suddenly some towns on the line between Denver and Medford start seeing contrails.
The map is from an unusual perspective, viewing the globe from overhead of Europe, so the US is above and to the left of the center. It does not show all the flights in the world, seeming concentrating on those that connect in some way to Paris, but it does illustrate the huge increase in air routes that is reflected across all airlines. See how sparse the US is in 1980, and how criss-crossed it is in 2008.Continue reading »
[Original Post Dec 7 2010, updated Feb 13th 2011 with German contrail]
This is a remarkably common news story: It’s just after sunset, someone looks towards the west and they see the short contrail of a jet plane illuminated by the sun. It looks red, like fire. They zoom in with their video camera. They don’t know what it is, thinking it’s a fireball, a meteor, or some kind of UFO, so they alert the local media. The local media published it, and occasionally the story grows.
One very popular photo amongst the “chemtrail” theorists is this one from NASA:
This enhanced infrared image from NASA’s Terra satellite shows a widespread outbreak of contrails over the southeastern United States during the morning of January 29, 2004. Satellite data are critical for studying the effects of contrails. The crisscrossing white lines are contrails that form from planes flying in different directions at different altitudes. Each contrail spreads and moves with the wind. Contrails often form over large areas during winter and spring. CREDIT: NASA
The image is quite striking, showing a very large number of contrails over an area five hundred miles wide. It’s an infrared image which has been enhanced specifically to bring out the contrails. This image has been used by NASA a few times:
This must be the least missile-like mystery-missile ever. Taken Sunday, Jan 2nd, 2011, at “about three in the afternoon. The video shows a jet contrail – barely moving relative to the clouds. It’s just a jet that happened to fly directly overhead, so the approaching contrail looks vertical.
But if you really want to go down the “chemtrail” route. Obviously if something were the result of spraying trails that are tens of miles long, a mile wide, and five miles up in the air, then the result would not be localized to one flock of bird in half a square mile.