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You occasionally see very pretty photos of contrails, like this one:

Very pretty.  But what is it?  It’s clearly not a regular exhaust contrail, as the trail seems to start actually ON the wing, and it has a weird rainbow effect you don’t find in exhaust contrails.

It’s actually an aerodynamic contrail.  It’s formed by the reduction of pressure in the air as it moves over the wing.  When the pressure of a gas falls, then its temperature also falls (the same principle as is used by your refrigerator).  The reduced temperature cause small drops of water to condense, which then may freeze.  The drops get larger as more water condenses on them.  The different sized drops refract water by different amounts, which accounts for the “rainbow” effect.

A rather more scientific explanation (also explaining the exact colors) can be found here:

http://contrailscience.com/files/Gierens_Aerodynamic_poster_060625.pdf

The interesting thing about this type of contrail is that they are actually more common in warm weather.   They need a lot of moisture, and cold air is generally dry, so they are more common in the summer months, and in warmer climes. See:

As aerodynamic contrails are independent of the formation conditions of jet contrails, they form an additional class of contrails which might be complementary because they form in predominantly in layers that are too warm for jet contrail formation.

There is actually surprisingly little work being done on the formation of aerodynamic contrails.   A lot of the time they only show up as wingtip contrails (which you can see are very dense in the above photo).  Conditions need to be just right for the full wing to generate a contrail, and it typically does not last very long at all.

You’ve probably seen aerodynamic contrails on landing planes, like:

Here you can see the wing contrails vanish almost immediately.  The long persisting contrails (which won’t persist very long) are from the outer end of the lowered flaps - where air is compressed and then expanded very rapidly, causing a lot of moisture to condense.   You can also see there’s a lot of moisture in the air, it’s quite misty looking.

This fantastic photo set on Flickr has some nice photos of both exhaust and aerodynamic contrails, you can clearly see the difference:

http://flickr.com/photos/jpro747/sets/72157594548787911/with/511377939/

In this shot, you can see the jet seems to be underneath a layer of clouds, suggesting it’s at a low altitude, where exhaust contrails would not normally form.  Although with this image, it’s little hard to see what is in front of what.  I suspect that the shadow you see near the tail is actually the shadow of the plane on the clouds, meaning the plane is just above them, or in them - in a region of high humidity, either way.

From the same set, we can see that the two types of contrail are not mutually exclusive.  You can have both at the same time:

Note you have the thick white contrails coming from the engine, and the aerodynamic contrails coming from the wings.

(side note here: most of the photos in jpro747’s set were taken FROM THE GROUND with a Canon 350D ($400) attached to a 1200mm 6-inch Dobsonian telescope ($300-$1000, depending on quality).   In most of the shots, you can clearly see the type of plane, and usually the airline markings.   Now, why has NOBODY in the supposed 2 Million “chemtrail” community managed to take a closeup photo of a jet spraying chemicals.  Considering you can do it with $700 worth of equipment, it seems rather odd).

This video shows an aerodynamic contrail forming in patchy air:

You can see the trail looks very similar to the photos, especially in the final segment of the video.  There are few wide shots, so it’s hard to tell how long the trails is lasting for, but at 0:50, the camera pulls back, and the provious trail is either off screen, or has already evaporated.    Note this video was shot in Torino (Turin), Italy,on August 16th 2008 - during the summer.  That’s when these type of contrails are more likely, as they need very humid air.

The book “Cloud Studies” was published in 1905, over a hundred years ago.  It was written by Arthur W. Clayden, M.A., Formery principal of University College, Exeter, UK. The book is available in PDF form since it’s out of copyright:

http://contrailscience.com/files/Cloud_Studies.pdf

But the images are rather low quality, so I scanned them in from my copy of the 1925 edition (mostly the same photos), and I present them here:

http://picasaweb.google.com/Uncinus/CloudStudies1905And1925

It’s interesting that there are some clouds there that you might think look unnatural, or man made.  But these photos were taken before powered flight was invented.

So if you are ever looking up at the sky, and you see a cloud that looks a little odd, and you are wondering if this is a recent phenomenon, then have a look back at what clouds were like in 1905.  You’ll probably find your cloud here.

There are lots of photos of contrails from World War II. I’ve collected a few of them here:

http://picasaweb.google.com/Uncinus/WWIIContrails

The collection shows all types of contrail formation - from the very short ones, to long persistent trails that spread out like cloud cover.

Why so many photos of contrails in WWII, and not so many from the 50’s and 60’s?  The simple reason is that contrails only form at very low temperatures, which are normally found at high altitude, and in peacetime there was NO REASON TO FLY THAT HIGH until the advent of commercial jet travel a few decades later.

The only reason these planes are flying that high is so they can avoid anti-aircraft fire.  The bombers fly as high as they can, and then their fighter escorts fly even higher, so they can see incoming aircraft targeting the bombers, and swoop down to attack.  This type of escorting is called “Top Cover”.  The most classic example of this is the famous photo “Top cover over J-Group”:

This photo was also taken over Emden, on September 27th, 1943, by Stanley M. Smith.

There’s a great new series on the BBC in the UK: Britain From Above, that shows how various things look from a high perspective, using very interesting visualization techniques.

One of the most interesting (for me) was an episode that showed all the air traffic in UK airspace in a 24 hour period. That involved mapping the positions of 7,500 aircraft, showing how they crisscross the UK:

This is just part of them, captured midway through the day.  But it’s interesting to see the width of the paths they fly along - particulalrly those that fly over London to Manchester and Scotland.   It’s also very interesting to see the area where paths cross.  These would very obviously give rise to the to contrail “grids” that some people feel are so suspicious.

Britain has nothing like the amount of traffic that the US has. According to the National Air Traffic Controllers Association:

On any given day, more than 87,000 flights are in the skies in the United States. Only one-third are commercial carriers, like American, United or Southwest. On an average day, air traffic controllers handle 28,537 commercial flights (major and regional airlines), 27,178 general aviation flights (private planes), 24,548 air taxi flights (planes for hire), 5,260 military flights and 2,148 air cargo flights (Federal Express, UPS, etc.). At any given moment, roughly 5,000 planes are in the skies above the United States. In one year, controllers handle an average of 64 million takeoffs and landings.

Here you can make out the shape of the US purely from the flight paths.  You can see the major cities, and the air links between them.  You can also see that there is really nowhere in the US where you don’t have commercial flights flying over you.  Even more, there’s hardly anywhere where you don’t have two or more flight routes intersecting near you.

So it’s no surprise that in regions where the weather is right for contrails to persist, then you’ll see some kind of “grid”, or intersecting contrails in the sky, like this:

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

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.

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I’m very interested in contrails, but when you look for information about them on the internet, half the sites that turn up are about a conspiracy theory which claims that any trail that last more than a few minutes is actually a “chemtrail”, comprised of dangerous chemicals, particularly barium.

There are even web sites, where people who are convinced this theory is correct are taking the highly laudable step of attempting to verify their hypotheses with scientific experiments.

Unfortunately, they get the science terribly and inexcusably wrong. Take, for example, this graph found on the Arizona Skywatch site:

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This photo has the largest number of contrails I’ve seen in a single photo:

There seem to be at least 30, possibly more (click the photo for a larger verision).  What is even more remarkable is that it was taken sometime before 1967.  That’s over forty years ago.

The photo is plate 113 of the book Cloud Studies in Colour, by Richard Scorer and Harry Wexler, published in 1967 by Pergamon Press.  The photo was taken by Richard Scorer, probably in England.  The accompanying text reads:

Condensation trails are left by aircraft when the air is sufficiently cold for the mixture of air and exhaust to be saturated.  This does not usually happen except when the temperature is close to or below -40C, in which case the cloud freezes almost instantaneously and does not readily evaporate.  The cloud is then spread out by any wind sheer which may be present.

Another bit of “Chemtrail” mythology was born with this YouTube video:

http://youtube.com/watch?v=MVc9GX5K_As

This video has been seized upon by the Chemtrail community as evidence that “chemtrails” are real, and that the German government has admitted they are spraying “chemtrails”.

Unfortunately the entire video is a very bad translation of a story about one Meterologist, Karsten Brand, who is was concerned that the military was using too much chaff (anti-radar powder). The military admitted that they do use chaff, but not too much, and it’s not harmful. Brandt contend they use so much it can affect the weather, and might be harmful to people. He’s also annoyed because it interferes with his weather radar.

The Meterologist behind all this, Karsen Brandt (show above, supposedly blaming it on “chemtrails”), actually goes out of his way to distinguish these odd cloud images from “chemtrails”, in this article, translated,

http://www.worldweather.net/news/specials.mv?action=show&id=5947

Following the publication of pictures of non-existent clouds on the radar we have many questions. Here are the answers to the most frequently asked questions:

Are they “Chem Trails?
On various Internet sites will be discussed passionately that aircraft chemical balance to either make the ozone hole or act for the United States (who else also ..) changing the weather to world domination. This is - according to the supporters of this conspiracy theory - not just a trial from time to time, but regular, worldwide and especially in Germany. The sprayed chemicals leave greyish-white trails in the sky That is not a normal contrail, “Chem Trails” would quite different and would also behave differently.

The beauty of this conspiracy theory: Everyone can see the trails, everyone may feel threatened, but no (normal) citizens can touch or examine them. On the Web pages are shown photos of various trails, the “strange” pattern in the sky behind.
For the layman, it is at first glance strange that one day contrail stay in the sky for hours, but the next day dissolve within minutes . But this “phenomenon” varies with the humidity and the change is very easy and simple to explain.
Of course, we also can not be excluded with absolute certainty that a plane sprayed chemicals in the atmosphere. But as regularly and to the extent claimed by the conspiracy theorists, you can use common sense to exclude. For such a comprehensive worldwide conspiracy, should not only hundreds, thousands of US pilots be involved, but also scientists, German authorities, etc., etc. . How high is the probability that with such a number of people, there is no leak?

The big problem with the video is the translation. In the shot above, the german word “duppel” is translated as “chemical trails (chemtrails)”, when the actual translation is “chaff”. The German word for “chemtrail” is “chemtrail“.

Another rather obvious distinction is that Brandt is talking about invisible clouds, whereas “chemtrails” are very visible (since they are aircraft contrails).

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Q) How long do contrails last?
A) Anywhere from less than a second, up to several hours.

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:

Q) 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?

I took this photo on December 16, 2007, at around noon. It’s facing south from Los Angeles, California.

It was a particularly good day for contrails. You can see about seven in this photo. There’s also a nice contrail shadow, and a halo around the sun.

I was looking at the MODIS archive of satellite photos, and realized they had one for that date, and rather luckily it seems to have been taken within an hour of the photo I took, so it shows the exact same contrails from space. LA is just a bit to the right of the center of this photo.

As the photos are about an hour apart, the trails have shifted and distorted a little. But you can still match them up. Here I’ve color coded them.

It’s interesting also to look at the full size satellite photo of the area, so you can see where trails form, and where they do not form.

Los Angeles is in the middle of the photo. You can see the trails forming over Los Angeles, and it’s quite clear from looking the the cloud bands that they are forming in an area of high humidity.

Lookin up the coast, there is a gap in the clouds, and a corresponding lack of contrails. Then in the top left corner the clouds resume, and so do the contrails. In fact you can see several much thinner trails there. Here’s a close up:

These are probably the same planes that created the trails over Los Angles, just about an hour later, as they continue to fly north. The abrupt start of the trail as it enters the moist air is very obvious here.

This is all interesting in the context of the “chemtrail” theory. You often hear the chemtrailers posting things like “heavy spraying over Los Angeles today!!!” Then there are days with “no spraying”. MODIS is a wonderful resource for explaining why this perception happens. Sometimes there is a moist region of air at the right level over a particular place, and that place gets a lot of persistent contrails.

Looking at contrails from the ground only gives you a little bit of the picture. The satellite photos tell the whole story.

For example, here’s a “Massive Chemtrail Assault” Youtube video, from Austin, TX, 11/15/07

http://www.youtube.com/watch?v=GXJRNnmx0h8

And here is the satellite image of the greater area from that day. Austin is on the right, towards the end of the huge sweep of clouds. It’s clear that the contrails are just forming around the edges of a region of high humidity.

http://rapidfire.sci.gsfc.nasa.gov/subsets/?USA6/2007319/USA6.2007319.aqua.1km.jpg

Full Size original images here, about 1MB each:
My Photo: img_3175a.jpg
MODIS photo: aeronet_la_jolla2007350terra250m.jpg