Contrail Science

I find clouds fascinating.   If I see a lenticular cloud while driving I might nearly crash the car while straining for a better view.   I’ve only seen mammatus clouds once in my life, and stopped to try to take a photo.  I’m quite fascinated by contrails and contrail shadows, especially the “black beam” shadows that seem to extend in front of the contrail.   

Yet my fascination is not shared by the general population.  When I saw the mammatus there was nobody pointing up at the sky.    Other people on the freeway were not swerving their cars, despite the impressive black beam in front of them.   Why not?

Unfortunately, the answer is rather simple: they’re just not that into clouds.  

Yes, my cloud obsession is simply not shared by other people, much in the same way that I don’t understand my friends interests in certain sports, or cars, or the contestents on American Idol.   They might be similarly bemused by my lack of interest in such things.  But the point here is that most people’s interest in clouds is limited to A) is it sunny? and B) might it rain?

So it’s hardly surprising that the gradual increase in contrails over the last 30 years has gone much without comment.   Each year the sky looks pretty much the same as last year.    But then, you get some people who, for some reason, started looking at the sky with more interest, and they noticed, for the first time, that the sky sometimes gets covered by contrails that spread out into a layer of cloud.

They then sometimes slip down a slipperly illogical slope – if they had not noticed this before, then that means it was not there before, hence it just started, and hence again it must be something deliberate.   So the “chemtrail” theory is born – these persistent contrails are actually some kind of nefarious deliberate spraying by the powers that be.

You’ll note that this theory is based on one fact in the theorist’s mind: “I did not notice this before”.

So, if you did not notice something, and hence it did not exist until you noticed it, then what about all those people who have NOT yet noticed that contrails spread?  Do the contrails not spread for them?   

The reality is that the vast majority of people simply don’t pay much attention to clouds, or planes in the sky. The contrails have been there all their lives, and they might as well be noticing the type of covering used on the road outside their house, or what hairstyles the newsreaders have   For most people it’s stuff that is simply there, and does not impact their lives in one way or the other.  It changes slowly over time, but day to day you don’t notice any change.

Sometimes though some people notice things that others have not, and then you can get into an argument about how things are, or were.  One such argument took place in 1971 between celebrity activist Arthur Godfrey and Senator Gordon Allott, R-Colorado,  over possible effects of the then proposed supersonic transport (the SST, a high speed plane like the Concord).   

One of the concerns Godfrey raised about the SST was the possibility of excessive contrail coverage.  The problem of contrails creating clouds was well known in 1971, and Godfrey was well aware of this from personal experience.  But Senator Allott was simply a person who had never noticed these spreading contrails, and hence we get the following newspaper clipping from the March 11, 1971 Missouri Mexico Ledger: (See the 4th story across at the top: “Godfrey calls SST ‘Nonsense’“)

Godfrey tangled with Sen. Gordon Allott, R-Colo., when he said present jetliners are environment hazard enough without adding SSTs to the skies. Godfrey said he’d seen, on hunting trips to the Colorado Rocky Mountains, blue skies ”clouded over” by jet contrails by 9:30 in the morning.

Allott said he’d lived in Colorado all his life without ever seeing jet contrails form clouds—and said he has too much respect for the environment go go on hunting trips and shoot animals.

If a US Senator failed to notice these spreading contails spreading in 1971, is it really that surprising that people still don’t notice them now?  Or that people who believe in “chemtrails” are simple people who have only now, for whatever reason, taken an interest in the skies.

This is an excerpt from the 1944 film “The Memphis Belle: A Story of a Flying Fortress”. The full original film is public domain, and can be found here:

http://www.archive.org/details/TheMem…

The film shows contrail formation, including showing “broken” contrails.

http://www.youtube.com/watch?v=wfOrez6q7WM

Make sure you listent to the audio, as it explains how high the planes are, how cold it is, and why contrails form.

Contrails are a common sight now, but prior the the advent of commercial jet travel in the late 50s, there was little reason for planes to fly high enough to form contrails.  However, during the Second World War, high altitude airial dogfights over the UK were quite common.  Contrails in the clear blue sky were a common part of wartime Britain.

This excerpt from a Pathe newsreel shows this quite clearly.   It’s a piece about a nice little english village called Meopham – which is in Kent, between London and the bombers.   The shots of the contrails are between pretty pastoral scenes, and puppies playing with kittens.

http://www.youtube.com/watch?v=JuDN4WNwmJ4

(Click on “Watch in high quality” on the Youtube version)

There are two shots of the same contrails. Interesting because you can see that the trails have persisted and drifted to the right. Judging by the amount of change, probably over 20 minutes. You can also see a new contrail being formed.

These contrails look a bit odd, fragmented. We are used to seeing contrails that are long and straight. But the contrails in the film were formed by actual fighter planes, where the pilots were trying to kill each other. Hence they would be climbing and diving, twisting and turning in an attempt to out-maneuver each other. The contrails would start and stop as they climbed in and out of contrail altitude.

While these few contrails are interesting enough, it was just a skirmish on the way to where the real action was taking place:  London, where the dogfights must have been incredible, leaving a criss-crossing web of trails:

The above photos show dogfights during the Battle of Britain in 1940.  The German bombers were accompanied by fighter planes which would try to defend the bombers from the British fighters.   They were at a high altitude to avoid anti-aircraft fire.  Hence you get these amazing patterns of contrails which tell a tale of life and death at 28,000 feet above London.

The British won the Battle of Britain by 1941, establishing control of the airspace, moving the battle to Europe.  This did not end the contrails, but brought a certain order to them.   Bombers departing for Europe would climb to cruising altitude before crossing the channel, and this created another kind of story in the sky:

The above photo shows St Paul’s Catherdral in London, with departing bomber contrails behind it.  This is probably the best photo of a persistent WWII contrail, showing the trail spreading and dropping “mares tails”, exactly like modern contrails sometimes do.  It’s such a good photo you might think it’s a fake, but the original came from the Hulton-Deutsch Collection and can be found in the Corbis photo library as part a retrospective titled “V-1 “Buzz Bombs“.

Here’s another book with contrail photos from 1945:

(High definition version here)

Here’s some American newsreel footage of contrails at the Battle of the Bulge, 1945:

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 (frozen) drops get larger as more water condenses on them.  The different sized drops (or ice crystals) have different optical properties, which affect different wavelengths of light, which accounts for the “rainbow” effect.

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The book “Cloud Studies” was published in 1905, over a hundred years ago.  It was written by Arthur W. Clayden, M.A., Former 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 taken over Emden, Germany, 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:

Here’s a video:

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.