Contrail Avoidance and Mitigation Techniques

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.

A detailed overview of contrail avoidance technologies is given by Frank Noppel: in his PHd thesis, and in a paper with Riti Singh and Mark Taylor.

Altitude and Mirrors

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.

Engine Modifications

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)

Anti-Contrail Rays

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.

Things That Are NOT Contrails (or Chemtrails)

Contrails are long thin clouds of ice crystals that form behind planes that fly through freezing cold air. Usually you see them behind jets at around 30,000 feet.  If the air they fly though has enough moisture in it already, then these contrail clouds can last for a long time before they evaporate.  Sometimes you get a lot of them at once in the sky, if the weather is right. They look like this:

Note: the trails in the above photo ARE CONTRAILS (and some natural clouds).  That’s a photo taken by NASA scientist Louis Nguyen from I-95 in northern Virginia, January 26, 2001.    This is the only photo of contrails in this article.  The remaining photos are NOT contrails.

There are several things that a superficially somewhat similar, in that they involve stuff coming out of the back of an airplane and/or lines in the sky.  But these things are not contrails.


Skywriting is making patterns in the sky using smoke trails.  Done at a low altitude using small planes, it can look very like a contrail, but it’s very different as skywriting is made from smoke (made from injecting oil into the hot exhaust), and contrails are made from ice crystals.


Read More

How Long do Contrails Last?

Q) How long do contrails last?
A) According to all the books on clouds in the last 70 years, contrails last anywhere from less than a second, up to several hours. If depends on the atmospheric conditions at the altitude the plane is flying. It’s unrelated to the weather on the ground.

This is an oft-asked question. The answer is reasonably straightforward, but misunderstanding is common. To understand why a contrail can last as little as a fraction of a second, or as long as several hours, you need to understand what a contrail is, and how it forms.

Here’s another question, which has the same answer:

snowflakecvr2.jpgQ) How long does a snowflake last?

Why is this basically the same question? Because contrails are generally made of ice crystals. Jet exhaust contains a lot of water vapor (the chemical reaction actually produces more water than there was originally jet fuel), and when this gets shot out of the back of the engine at 2000MPH, it hits the frigid air (typically colder than -40 degrees), and the water vapor condenses and freezes, very quickly, into tiny ice crystals, just like snowflakes.

So why do these ice crystals sometimes stay around for a long time, and sometimes vanish in seconds? The temperature is well below freezing, so they can’t melt, can they? This is puzzling, because it involves something that most people know nothing about: “sublimation“.

Sublimation is when a substance (in this case, water), goes directly from being solid to being vapor (a gas), without actually melting into a liquid. It’s like evaporation, except instead of a liquid evaporating, it’s a solid (ice). If the air is dry (i.e. there is little water vapor in it), then the ice crystals will quickly sublimate into vapor, and the trail will vanish quickly.

However, if the air already has a lot of water vapor in it, then the ice will sublimate slower, and the trail will last longer.

If the air has so much water vapor in it already that it can’t hold any more (i.e. it’s “supersaturated”), then the ice crystals can’t sublimate, and so the contrail will stay around for a long time. The ice crystals might even attract water from the air, if there is enough, and the contrail will get thicker. Winds might make the contrail spread out to even cover the whole sky.

The above is a simplification, as other factors like temperature, pressure, and sunlight have an effect. But it explains the basic reasons why some trails last only a few seconds, and some can last for hours, and spread out to cover the sky.

Finally, there is one more way of asking the question:

Q) How long do clouds last?

This is the same question because contrails are clouds. Contrails are physically very similar to cirrus clouds (except they are long and thin), and so they act almost exactly the same. You see cirrus clouds that last for hours, so why not contrails?

References – note the dates

“A Color Guide to clouds” from Richard Scorer & Harry Wexler, 1963

1972: Richard Scorer – “Cloud of the worlds”



Pre WWII Contrails

The Wright Brother’s first achieved powered flight in 1903. They only got a few feet off the ground, so of course did not leave a contrail. However it was barely more than a decade before planes inched into colder air, and the first reports began.

The earliest known account of a contrail is a passing mention attributed to Ettenreich, who in 1919 reported of an incident from 1915 where he saw “the condensation of a cumulus stripe from the exhaust gases of an aircraft”

In a fascinating article: Wakes of war: contrails and the rise of air power, 1918-1945 Part I–early sightings and preliminary explanations, 1918-1938, (Air Power History. 54.2 (Summer 2007): 16(16).), Donald R. Baucom give an account of the rise of high altitude flight, and the inevitable accounts of contrails, both persistent and otherwise, that occurred during that time. In the autumn of 1918, in an account of Captain Ward S. Wells, Army Medical Corps, who was serving with the 60th Infantry, 5th Division, American Expeditionary Force, during the Meuse-Argonne campaign.

Our attention was first drawn to the sky by the sudden appearance of several strange and startling clouds–long, graceful, looping ribbons of white. These were tapering to a point at one end and at the other where they dissolved into nothingness 60 degrees across the sky, were about as broad as the width of a finger held arm’s distance from the eye. On close observation we noticed some distance ahead of each cloud point the tiny speck of a chasse [sic] plane…. [N]ever before had I seen a plane writing in white upon the blue slate of sky.

This is very interesting, as it’s the earliest known account of contrails (note that these are non-persistent). It was also very novel and interesting at the time, and was reported in a letter to Scientific American, (“Clouds formed by Airplanes“, Jun 7, 1919, p 60) by Wells’ brother, Everett Wells, and provoked some discussion of what might be forming these clouds.

Read More

Barium Chemtrails on KSLA

Brief Summary:
  • Samples of water were collected in August 2007, in Stamps Arkansas, by leaving some bowls outside for a month
  • The resultant dirty water was tested by KSLA and was found to have the same amount of barium in it as most municipal tap water.
  • The reporter misunderstood the results, and said there was a lot of Barium
  • The reporter now admits he was mistaken, and that he found no evidence for chemtrails



Some conspiracy theorists think that persistent spreading contrails indicate some kind of deliberate aerial spraying, probably by the government. They speculate as to what could be in these trails, and one of the most common things they claim is barium.

Some people are so obsessed by this idea that they have rainwater tested to see if it has barium in it. They usually find some, and then trumpet this as evidence that their theory is correct.

Unfortunately they are wrong. I’ll explain why, but first, some basic science. Read More

Where are all the Chemtrail Photos?

Some people think that persistent spreading contrails are somehow unusual, and are actually something dangerous being deliberately sprayed on the US people by the government, or perhaps for weather control purposes. They call these persistent contrails “chemtrails”.

Some of them are very insistent that this is a practically constant assault, saying the sky is never blue any more, and there are “chemtrails” constantly criss-crossing the sky.

Read More

Broken Contrails and Contrails with Gaps

Contrails are clouds made from water vapor that condenses then freezes behind a plane engine. Since the engines are on constantly, it seems a bit odd when you see contrails with gaps in them, or even contrails that stop and start. If the engine is pumping out a constant amount of water, then why is the trail not constant? This puzzle is sometimes seized on by people who think that persistent contrails are actually “chemtrails”, or some kind of deliberate spraying operation. Read More

Contrails, Dark Lines, Black Beams, & “Chemtrails”

People who think persistent contrails indicate some kind of conspiracy (which they call “chemtrails”), sometime point to the “dark lines” that sometimes accompany contrails. Since they can’t immediately think of why these dark lines should be there, they assume it’s part of the conspiracy. Either there is some kind of “dark chemtrail”, or the plane is projecting a dark beam of some kind of negative energy, or it is following a dark beam.

In reality these “dark lines” or “black beams” are simply various kinds of shadows. There are actually three main kinds of dark line related to contrails.

1. Self Shadow

Where the contrail itself is in shadow and appears dark. This can happen a number of ways. The sun can be low on the horizon and the contrail can be shadowed by a mountain or a thick cloud bank (such as in this video). In rare cases the plane can be flying directly towards the sun, and the contrail will shadow itself. When self-shadowing, the leading edge of the contrail will be brightly lit, with a dark area behind it, such as parts of the contrail above.

2. Parallel Shadow

The contrail is simply casting a shadow on a layer of cloud beneath it. The cloud layer is thin enough so you can see through it, but it’s visible, so you can see the shadow on it. Theoretically you could cast a shadow on a cloud layer above or behind a contrail, if the sun were low enough, but this would be rather difficult to observe. Most “dark lines” are of this type. There’s an excellent explanation of these shadows over at Atmospheric Optics.

There’s nothing new about such shadows. Here’s some from 1955:

One interesting thing about these shadows is that it frequently looks like the contrail is below the clouds, when it’s actually above them.  You can see this illusion in the photo above. The reason this happens is that the white of the thin layer of clouds and the contrail are additively transparent, so they look identical, regardless of which one is in front of the other. The illusion happens because the brain interprets the bluer regions of the cloud as being darker regions in the same plane as the cloud, when they are actually holes in the cloud.The contrail will seem solid white when viewed through the holes, or when it is in front of the holes.

3. Edge Shadow (Volumetric Shadow)

The most interesting type of contrail dark line is when the contrail is lined up with the sun. This produces a slice of shadow through the atmosphere that looks like a dark line when viewed edge-on. It is quite difficult to visualize what is going on since you have to think in three dimensions, and we are accustomed to thinking of shadows as being flat, since they are usually cast on surfaces. You are not seeing a thin dark shadow here, you are actually seeing a huge slab of very faint shadow, but it’s viewed from looking along the edge. Imagine you have a thick sheet of glass. Viewed head on, it’s transparent, but if you look at it from the edge, it seems a lot darker.

The above image is an excellent demonstrating of the fleeting nature of these edge shadows.  Two photos taken just a few minutes apart, int the first the camera is more fully in line with the plane of the contrail.  In the second the contrail (or the photographer) has moved, and we are viewing the edge shadows from a slight angle which reveals they are made up of two or three segments, caused either by the plane turning slightly, or variations in upper level winds bending the contrail.

Most photos of these odd shadows don’t include the sun.  But when they do, you’ll see that the sun is lined up with the contrail (or part of it)

The photo on the right is a rather dramatic illustration of this effect. The “dark line” here is caused by the slab of shadow cast by the portion of the upper part of the exhaust trail of the space shuttle Atlantis that is lined up roughly in a flat plane with the camera and the sun. The sun has just set, so the rays of the sun are almost parallel to the ground, so the upper portion of the plume is casting a long tall shadow through the air towards the horizon. This is viewed edge on from below, and so looks like a dark line. Since it’s a full moon, the sun is directly opposite the moon, so the “shadow” looks like it’s pointing at the moon (if you look closely, you’ll see it continues past the moon). This is particularly dramatic because of the combination of the setting sun and a vertical exhaust trail. With normal contrails, the sun has to be higher in the sky to cast the slab of shadow downwards.

The image above is another great illustration. There are multiple contrails, but only the one that intersects the sun creates the edge shadow. Photo from “Col” on usenet

[UPDATE] This video I shot in my kitchen gives an excellent illustration of these types of shadow.

[Math Warning] It’s a bit difficult to explain these edge shadows (also called volumetric shadows). You can think of them in terms of three dimensional geometry. The contrail is a line in three dimensional space. The camera (or observer) is a point in three dimensional space. The sun is essentially infinitely far away, and so is only really relevant as a directional vector (a vector in three dimensions, where the magnitude is unimportant ). These three quantities are what you need to consider to understand the condition for the edge shadow.

Given the line (contrail) and direction vector (sun) we can form a plane that contains the line, and is parallel to the direction vector. This plane cuts through the contrail, the atmosphere, and intersects the ground. Projecting two end points of the contrail along the plane in the direction of the sun’s vector, we get essentially a two dimensional parallelepiped (although the far edge is not very well defined). This parallelepiped is quite thin (it’s as thick as the contrail), so when viewed from the side, you won’t see much. But when the viewer is in the plane of the parallelepiped – specifically anywhere along the line on the ground formed by the intersection with the plane formed by the contrail and the sun – then they will be viewing the parallelepiped from edge-on, and so it will seem to be a dark line that intersects the contrail.

Since only the portion of the contrail that is roughly within this plane is contributing to the shadow, there may be other potions of the contrail that are not in the the plane, and hence do not seem to be casting a shadow. In fact they are, but since it is in a different plane, they are not visible unless the viewer was to move to a new position. This can be seen in the two photos above. In one it looks as if the plane was following a dark line, and then veered off. In the Shuttle photo, the highest portion of the trail, although brightly illuminated, is not contributing to the visible shadow. However a viewer in another location may have been able to see a different edge shadow trailing from this upper portion.

[Update] I found a useful video of an contrail edge shadow (or “volumetric” shadow, described above):

It’s all in one shot, which is great, as you can see how the various misinterpretations can arise. It starts out with a shot of a black line across the sky, with no visible reason for it being there. The sun is obscure by the house on the right.

The cameraman then zooms in in the dark line, and we see a contrail being formed along it. It looks like the plane is following the dark line, or somehow projecting a beam of dark energy in front of it!

Then the cameraman walks around the house, and we see the source of the dark line – the contrail is EXACTLY lined up with the setting sun, and is simply casting a volumetric shadow which the cameraman is lined up with. The shadow is accented as the suns rays are nearly parallel to the contrail. Here I’ve stitched some frames together so you can see this. I encourage you to examine the video to confirm this.

[Update 2]  Check out this video of the Space Shuttle launch, a dark shadow forms in the last 30 seconds:

[Update 3] Crepuscular rays are parallel, but usually don’t look it. This is partly why the contrail edge shadows are not recognized as the same type of thing.

Crepuscular rays are parallel!

view of crepuscular rays from space:

Persisting and Spreading Contrails

Do contrails sometimes persist and spread out?

Yes, see the Encyclopædia Britannica article on vapour trails (contrails):

Contrail, streamer of cloud sometimes observed behind an airplane flying in clear, cold, humid air. It forms upon condensation of the water vapour produced by the combustion of fuel in the airplane engines. When the ambient relative humidity is high, the resulting ice-crystal plume may last for several hours. The trail may be distorted by the winds, and sometimes it spreads outwards to form a layer of cirrus cloud.
vapour trail. (2007). In Encyclopædia Britannica.Retrieved May 4, 2007,from Encyclopædia Britannica Online:

(The above quote is from the current EB. However, a Google books search dates the inclusion on the EB back to 1983)

Also see “A Field Guide to the Atmosphere“, by Schaefer and Day, 1981:

Sometimes [contrails] are ephemeral and dissipate as quickly as they form; other times they persist and grow wide enough to cover a substantial portion of the sky with a sheet of cirrostratus (Page 137)

Are spreading contrails a relatively new thing?

No, it has been exactly the same for decades, the only change has been the size of jet engines (producing bigger contrails), engine technology  (burning fuel more efficiently in high bypass jet engines creates cooler exhaust which is more likely to condense before it mixes with the surrounding air) and the amount of air traffic (producing more contrails). Spreading contrails have been mentioned consistently through the history of aviation, including in the popular press. Like Sports Illustrated , Nov 6th 1989:

Now, late in the afternoon, the hatchery explored and the fishing over for the day, Crooks points to the sky. Blue all day, it has now turned hazy. “Contrails,” he says. “The haze is caused by aircraft contrails that have gotten spread out till they cover the sky. This is a major air route from the East Coast to the West.”

For scientific discussion, see, for example, all these articles on contrails. In particular the one from 1970 titled “Airborne Observations of Contrail Effects on the Thermal Radiation Budget

The spreading of jet contrails into extensive cirrus sheets is a familiar sight. Often, when persistent contrails exist from 25,000 to 40,000 ft, several long contrails increase in number and gradually merge into an almost solid interlaced sheet.
Contrail development and spreading begins in the morning hours with the start of heavy jet traffic and may extend from horizon to horizon as the air traffic peaks. Fig. 1 is a typical example of midmorning contrails that occured on 17 December 1969 northwest of Boulder. By midafternoon, sky conditions had developed into those shown in Fig. 2 an almost solid contrail sheet reported to average 500 m in depth.

Airborne Observations of Contrail Effects on the Thermal Radiation Budget
Peter M. Kuhn
Journal of the Atmospheric Sciences
Volume 27, Issue 6 (September 1970) pp. 937–942

(Click on any of the images in this article for a larger view)

Then a few years later, in 1975, we have the article : Multiple Contrail Streamers Observed by Radar, which again has photos (taken in 1971) of spreading and persisting contrails, as well as extensive discussion of these observations.

Multiple Contrail Streamers Observed by Radar.
Konrad TG, Howard JC (1974)
Journal of Applied Meteorology:
Vol. 13, No. 5 pp. 563–572

Here’s a description from 1970, from a local newspaper, the Arcadia Tribune, April 29, 1970:

Aircraft contrails begin to streak the normally bright Arizona sky at dawn. Through the day, as air traffic peaks, these contrails gradually merge into and almost solid interlaced sheet of cirrus cloud – an artificial cirrus cloud that is frequently as much as 500 meters deep.

One of the earliest reference to contrails covering the sky is from the Mansfield News Journal, August 11, 1957, Page 29:

“Within the past few years, the weather bureau has begun to report the trails as actual cloud layers when there are sufficient trails to cover a portion of the sky.”

Here’s a description from 1955:,1486793

An extremely persistent con trail might stay in the sky all day

But even earlier, and with a perfect description of what “chemtrail” theorist claim cannot happen comes this account from 1944:

The News, Frederick, MD, March 7, 1944

Contrails frequently have a tendency to cause a complete overcast and cause rain. In Idaho I have seen contrails formed in a perfectly clear sky and four hours later a complete overcast resulted

Below is the entire top of the page of that newspaper, in case you want to look it up.

And from the book “Flight To Arras” by Antoine de Saint Exupery, written in 1942 about a military mission in 1940:

The German on the ground knows us by the pearly white scarf which every plane flying at high altitude trails behind like a bridal veil. The disturbance created by our meteoric flight crystallizes the watery vapor in the atmosphere. We unwind behind us a cirrus of icicles. If the atmospheric conditions are favorable to the formation of clouds, our wake will thicken bit by bit and become an evening cloud over the countryside.

Another from 1958

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