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?

This photo was taken near Portland, Oregon on December 11th, 2005, at around 11AM.

So why would a plane be flying in this unusual pattern? Well, it turns out it’s not really very unusual. It’s called a “holding pattern“, and it’s a very specific pattern that planes fly in when they need to hold their position. Holding patterns have this very distinctive shape because the aircraft have to fly them in a very particular way that takes exactly four minutes to complete, so the ATC can know where they are, and when they will be facing in a particular direction. They fly straight for a minute, then turn 180 degrees over a minute, and then fly straight again. It looks like this:

For passenger jets, holding patterns are most common on the approach to busy airports (the one above is for Martha’s Vineyard), and it’s not too uncommon to make a few turns in a holding pattern shortly before landing. The contrails in the above photo is probably not this, as it’s at too high an altitude. Passenger jets do sometimes enter high altitude holdings (I’ve been in one once as a passenger), which could account for this.

High altitude holds are also used by refueling tankers, as they wait for the planes they are going to refuel. Pilots obviously also have to train to fly in high altitude holds, and given the number of turns above, that’s the most likely explanation for this particular photo.

Remember the winds at altitude are typical around 50-100mph, so after the four minutes loop, the previous contrail could be blown around five miles away, giving the illusion that the plane is now holding over a different area. It’s not. The plane is playing over the same area, but the contrails have moved.

Of course, if you don’t know this, it might look a bit odd to you. Some people work these racetrack contrails into the “chemtrails” conspiracy theory. Look at this poster on the right. There’s a photo of a racetrack contrail with five turns, separated and distorted by the wind. Overlaid is the text “Look up, Phone Radio DJs”.

These racetrack contrails are pretty rare (unless you live near a training area), but hopefully now if you see one, you’ll be able to tell what is going on, and not feel the urge to call a DJ.

[BTW, the plane in the upper right of the poster is a Boeing E-6 Tacamo, dumping fuel]

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.

The earliest report of both a persistent spreading contrail, with gaps, and a distrail comes very shortly after, in 1921, in the US. Air Service Newsletter.

An altitude flight was made in the morning at McCook Field recently by Lieut. J. A. Macready in a La Pere with supercharged Liberty [engine]. When the airplane reached a height of 26,000-27,000 feet at 11:50 a.m., a long feathery white streamer was observed forming behind a rapidly moving dark speck. The cloud was of the cirrus variety, well defined at the edges and apparently 10 to 15 times the width of the plane. The sky behind the first portion was clear blue with no clouds in the near neighborhood. The first streamer seemed perhaps 2 miles long. Then a gap of one-quarter mile. The second streamer formed with a background of light cirrus cloud and after 2 or 3 miles the plane seemed to go into the cirrus background, for the streamer formation ceased while an apparent path of blue continued beyond for a way in the cirrus cloud. The whole streamer may have been 3 miles long. After 20 minutes the streamer had drifted and spread until it merged indistinguishably with the other cirrus clouds visible.

Both these accounts were reprinted in the June 1921 Monthly Weather Review, in the article The Argonne Battle Cloud, which also includes an account from Captain W. H. Nead, that includes an account of iridescence in the trails:

The Rainbow Division, on the morning of October 10, 1918, was lying in what had at one time been a wood just back of Montfaucon. The sky was clear except for a few fleecy clouds to the northwest. Three airmen came from the northwest and passed almost over our regiment, continuing on to the southeast.
Behind each machine was a trail of white, which at first sight appeared to be smoke resulting from poor engine combustion, but which upon more careful observation proved too wide to have been caused by smoke. Perhaps the strangest thing of all was the fact that when the planes reached a certain point in the sky the rainbow (sundog) colors became distinctly visible.

And even back in 1921, the mysteries of contrail formation were not too difficult to discern:

The end products of complete combustion of gasoline are water vapor
and carbon dioxide, and it is found that if the water vapor were condensed,
there would result a little more than 1 gallon of water per
gallon of gasoline consumed. It was found by Wells and Thuras, in
studying the fog off the Newfoundland coast (see U. S. Coast Guard ,
Bull. 5, 1916) that there were 1,200 water droplets of diameter 0.01 mm.
in a cubic centimeter of air in a dense fog. If we assume that an airplane
travels 3 miles on a gallon of gasoline (approximately the figure
given by the Aerial Mail Service) it is possible to show that if only a
small part – a fourth or fifth – of the water vapor were condensed,
there would be abundant cloud to produce the effect observed at the
Argonne Battle. It should be stated, however, that this water vapor
would have to be discharged into air which was very cold and nearly
saturated. This seems to be the correct explanation, and is substantiated
by scientists at the Bureau of Standards, who say that they have
actually observed this cloud behind airplanes and automobiles. The
Bureau of Standards is working on a device for condensing and using
this water aboard dirigibles as ballast.

The weather review goes on to discuss how vapor trails (contrails) can form at low altitudes, even down to the ground, if it is cold and damp enough. They even give examples I’d never heard of, contrails formed by a horse, and a herd of reindeer:

A horse, warm from running over the ice, on a cold day in Greenland, was accompanied by a cloud 50 meters in height formed from its breath. Von Hann gives a similar case regarding a herd of reindeer. The human breath also has been seen to transform itself into small clouds of ice needles.

• 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.

What is Barium?

Barium is a metal, like calcium. You never find it in its metal form (outside of a lab), as it oxidizes rapidly in the air. Instead you’ll find compounds, usually barium sulfate or barium carbonate. Barium compounds are used in the plastics, rubber, electronics and textile industries, in ceramic glazes and enamels, in glass-making, brick-making and paper-making, as a lubricant additive, in pharmaceuticals and cosmetics, in case-hardening of steel and in the oil and gas industry as a wetting agent for drilling mud. Barium in water comes primarily from natural sources as it is present as a trace element in both igneous and sedimentary rocks. Barium is generally present in air in particulate form as a result of industrial emissions, particularly from combustion of coal and diesel oil and waste incineration.

µ and Parts Per …

When you measure the concentration of a substance in water, you can express it in various ways. You have to pay attention to units when converting from one way to another.

A liter of water weighs 1 kilogram, which is 1000 grams.

A milligram is 1/1000th (a thousandth) of a gram. 1mg = 1 milligram = 0.001g

A microgram is 1/1000000 (a millionth) of a gram. 1ug = 1µg = 1 microgram

Note that last line, because it’s important. The symbol µ is the greek letter “mu”. In measuring, it’s used to mean “micro”, or “millionth”. (To type µ, hold down the Alt key, type 230 on the numeric keypad, and then release the Alt key). Since it’s difficult to type, it’s often written using the letter “u”. Make sure you understand the difference between a milligram (mg, 1/1000th or a gram) and a microgram (µg, ug, 1/1000000th of a gram). A milligram is thousandth, not a millionth. It’s a little confusing sometimes.

A microgram is a millionth of a gram, so it’s a billionth of a kilogram. Since there are 1000 grams in a kilogram, and 1,000,000 micrograms in a gram, there are 1,000,000,000 µg in a kilogram. All this is basic high school science.

Concentration in water is measured as ppm, ppb, g/L, mg/L, µg/L. These are parts per million, parts per billion, grams per liter, milligrams per liter and micrograms per liter. We can convert between these easily:
1 ppm = 1 mg/L = 1000 ppb = 1000 µg/L
1 ppb = 1 µg/L = 0.001 ppm = 0.001 mg/L
(remember that 1 Liter is 1000 grams, so 1 mg in one liter is a thousandth of a gram in one thousand grams, or 1 part in a million).

Chemtrail claims

This video is very popular right now. Claiming that water was analyzed and found to have barium in it.

http://www.youtube.com/watch?v=okB-489l6MI

The video was taken in Stamps, Arkansas, which is not entirely surprising as that’s in a region of the US the might be renamed “Contrail Alley”. It’s at the intersection of the cross country routes between the West Coast, and the major airport in Atlanta, Orlando and Jacksonville. Stamps is midway between the two major regional VORs (Texarkana and El Dorado), right next to the major East-West airway Victor V278, and on the edge of a MOA that traffic has to skirt occasionally. It’s also directly below the Atlanta to Dallas, San Antonio to New York and Houston to Chicago flight routes. On just ONE of these routes (Atlanta to Dallas) there is a scheduled commuter flight, directly overhead, at contrail altitude every 15 minutes! The same frequency of flights is found on the Houston-Chicago route, which crosses at right angles almost exactly overhead. Hence, when the weather is right, it is inevitable that you will see contrails in a grid pattern, “a giant checkerboard”. See this Google Earth file: airlines-over-stamps.kmz

But back to the video. It shows a jar of dirty water (collected 9/1/2007), which was collected by Bill Nichols. He’s posted some comments on the YouTube video describing how he collected the water:

it was rainwater. i collected it in two separate bowls on the hood of a pickup truck in my backyard. we are 25 miles from the nearest interstate. this is a very poor county, the only industry is chickens, logging , farming, a little oil—no coal burners or anything like that. i wasn’t looking for attention. i was looking for answers, ksla said they would pay to get it tested. i dropped it off, and they asked my opinion

i put 2 clean bowls there specifically because i wanted to catch what was falling. i don’t recall exactly when i put the bowls there, but they were there for about a month before i contacted ksla. the goo that i caught was full of barium. have a cool day!

Pause for a second, and consider if you left a bowl out for the month of August in rural Arkansas, what would you expect to find in it after a month? Some dirty water? Perhaps a little dust? What’s dust made of outdoor? Dirt, dried topsoil. What would you expect to find in the dirt in Arkansas – one of the richest sources of barium in the US? You’d expect a bit of Barium – but did they actually find any more than you’d get in tap water?

This dirty water was tested, the test results are available in full here. You can also see the results in the video, at around 00:55 to 00:59. Here they are pieced together.

And just to be clear, here’s a closeup of the results, and the units:

That’s quite straightforward right? Barium found at 68.8 µg/L. That’s 68.8 parts per billion. Now listen to the audio at that precise point (also transcribed on the KSLA web site):

“The results: a high level of barium, 6.8 parts per million (ppm), more than three times the toxic level set by the EPA”.

Immediately you can see something is wrong here. it’s 68.8, not 6.8, and it’s not parts per million, it’s parts per billion. So it’s actually 0.0688 parts per million.

And what of “three times the toxic level set by the EPA”? They are referring to the EPA Limits, as quoted by the CDC:

“The EPA has set a limit of 2.0 milligrams of barium per liter of drinking water (2.0 mg/L), which is the same as 2 ppm [parts per million].”

So the EPA limit is 2 ppm (2000 µg/L), and the tests actually found 0.0688 ppm (68.8 µg/L), just 3.4% of the allowable limit.

That limit’s not really a “toxic level” either. There’s no evidence that it would be toxic even at that level (which, remember, is 29 times higher than what was actually found). The world health organization has set a drinking water level of 7 ppm after doing studies into the health effects of barium.

Barium has always been in water

The WHO also reported on the barium levels in drinking water (meaning, from a tap, not some dirty puddle) and they found:

“In a study of water supplies of cities in the USA, a median value of 43 μg/litre was reported; in 94% of all determinations, the concentrations found were below 100µg/litre (IPCS, 1990)“

So the average was 43 µg/L, but most were below 100µg/L. This means the amount of Barium found in this supposed chemtrail residue was about the same as was found in the municipal water supplies in the US, back in 1990. This is pretty low, it varies with geography based on the type of rocks in the aquifer. In Tuscany, Italy, the Barium in drinking water was around 1000µg/L (1ppm), high, but still within safe limits.

The amount of barium will also vary based on the weather. Very heavy rains will leach more barium out into the groundwater. So you’d expect more barium after very rainy seasons. This is actually what you find if you look at the historical records in California (which has very uneven annual rainfall). You see spikes in barium whenever there is a wet year after a dry year. Recent years like these are 1991, 1995, 1998 and 2004 (2001 and 2003 also spiked to a lesser extent). The expected peaks were confirmed by the results of Rosalind Peterson at California Skywatch.

So what’s going on here? Chemtrail theorists are constantly claiming that “chemtrails” are made of barium, and that it’s affecting our health. But whenever water is tested, it is found to have perfectly normal levels of barium, which vary as expected based on the rainfall. In the cases where they claim it’s got an unusual amount, this is just a misunderstanding of the units and limits involved.

Yes, there is barium in the drinking water, there always has been, and always will be. Trace amounts, mostly from the environment and some industrial pollution. It’s a very small amount, and not dangerous. There is no evidence to suggest it has anything to do with “chemtrails.”

Update #1: 5/2/2009

Jeff Ferrall, the reporter in the story now says:

http://contrailscience.com/barium-chemtrails/comment-page-8/#comment-23164

Yes, I did make corrections to my first report, which originally aired almost 2-years ago now… after quickly realizing my very embarrassing mistake. I was not happy with myself. Unfortunately, the first version of my report got out to the internet before I could make the correction(s), and the wrong version is shown repeatedly.
…
My feeling is, and maybe you’d agree, that if such aerosol mixes were created and loaded into jets with either a separate/independent dispersal method other than the exhaust, or actually in the fuel itself… somewhere, somehow, you’d expect someone to talk. I have not heard that yet.
…
I also interviewed the scientist who originally patented what some believe was a precursor to so-called chemtrail technology. He’s a very kind, helpful man who could not have been more helpful. He says he knows nothing about any such conspiracy.

There’s also a mention of this story In Skeptical Enquirer magazine:
http://www.csicop.org/si/show/curious_contrails_death_from_the_sky/

Update#2: 3/14/2010

More people make the same mistake.  This time someone in Austrailia, and the story was picked up by a Los Angeles environmentalist.   Again mg is confused with µg, making the results 1000 times as high:

http://www.examiner.com/x-10438-Human-Rights-Examiner~y2010m3d13-video-White-Clouds-of-Death-Aussie-exposes-geoengineered-chemtrail-contents?#comments

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.

I think this is simply a case of observer bias.

It’s easy to amass a large collection of photos of persistent contrails. I’ve got a lot myself, and I live in Los Angeles, where we don’t really get very many days when the conditions are right. All you have to do is only take photos on days when there are contrails persisting, and it will seem like there are “chemtrails” every day.

To get a real picture of what is going on, you need to take photos every day, and then see how many have persistent contrails in them. To be really accurate, you should take photos all across the country, and even across the globe, and see how many have contrails in them. What we need is a huge database of photos of the sky taken at random places and times.

Luckily, with the advent of digital photography, we have several such databases in the form of Flickr.com and picasaweb.google.com, and several other sites, which contain millions of photos that have at least some sky in them.

How many contain contrails (persistent or otherwise?) well, it turns out that practically none of them do.

Of course if you search for “contrail” or “chemtrail”, you’ll find a bunch. But that’s just you selecting them. How do we get random photos of the sky?

A good way it to search for things where the photographer is looking up. Like, “skyscraper“, “radio tower“, “kites“, or “skyline“. This gives you a very nice random sampling of millions of photos of the sky in all different weather condition, in all different locations, over the past decade or so. Some of these tend to have a narrow field of view, which you can expand with the keyword “fisheye”, like “fisheye sky“.

And what do we find? ALMOST NO CONTRAILS!!! I had to look at hundreds of photos before I finally found ONE that had a contrail in it. Even more rare was finding a photo with several persisting contrails, and I never found one with anything like a “grid”.

Several things can be deduced from this. Firstly contrails are actually pretty rare across the country. Of course this depends on where you live, but basically on average, there are not that many visible contrails in the sky.

Secondly, there’s still lots of clear blue sky and fluffy white clouds. All kinds of skies actually. Nothing has changed.

Thirdly, “chemtrail” theorists often say “why are there no photo of contrails before 1999/1990/1980/whenever”, seeing as they can’t find any in their family photo albums. The answer is of course that there ARE photos of contrails, all through the history of aviation. You don’t see them in the old photo albums for the same reason you don’t see them on Flickr.

But it’s actually very simply. The amount of water in the exhaust is pretty constant, but the amount of moisture in the air is not. The humidity varies with altitude, and a layer of low humidity can be sandwiched between two layers of high humidity. As a plane climbs or descends through this layer, then the trail will only form in the areas of high humidity, and so look like it was “switched off” in the area of low humidity.

You can get the same effect with temperature. A warm layer of air can actually lay on top of a colder layer in what is called an “inversion” (you’ll hear this on the weather sometimes, referred to as an “inversion layer”). When a plane flies through this inversion layer, the trail can be “broken”.

It’s not just climbing or descending flights either. The boundaries between these layers is not flat, and gravity waves or rising convection currents of air can create large volume of air that differ in temperature and/or humidity from the neighboring air, and so can break (or make) the contrail when then plane flies through them.

Don’t take my word for it though. Do a Google search for “broken contrail” and you’ll find lots of examples, and similar explanations (Click the photos above for two).

NOAA:

Occasionally a jet plane, especially if ascending or descending, will pass through a much drier or more moist layer of atmosphere which may result in a broken pattern to the contrail, with it appearing in segments rather than in one continuous plume.

AirSpace Magazine

If the contrail stops, then starts up again, creating a broken line, chances are the airplane flew through a dry patch.

Doc Weather

What is happening here is that warm air rising from the ground carries vast amounts of water vapor upwards into the highest levels of the atmosphere. This water vapor exists as massive, plumes of warm, moist air ascending to very high levels. When a rising plume reaches the upper atmosphere it condenses into high ice clouds known as cirrus or feather clouds. In the case of the jet contrail (condensation trail) in the first image, the condensing cloud formed where the aircraft passed through a rising plume of moist warm air. The air on either side of the plume was not sufficiently moist to support the formation of an enduring cloud. As a result the contrail only remained visible in the warmth plume.

Here’s one making the rounds, scary looking barrels, and a sign on the wall that possibly says “Hazmat inside”

What is it? It’s a Boeing 777-200LR Worldliner, specifically it’s WD001, a plane that was used for flight testing. The original photo can be found here – note the “Hazmat” text was added later. The barrels contain water, which is pumped around to shift the center of gravity to test various flight characteristics.

Here’s a description of a similar setup from the 2002 book, “Inside Boeing, Building the 777″, page 76., describing tests done in 1994.

From Boeing’s blog:

Remember, we test at the extremes of the weight/CG envelope. This requires us to control the CG during ground and flight conditions. We can move weight, in the form of water, forward or aft with the use of the water ballast system. This system is comprised of 48 barrels, each capable of carrying 460 pounds, connected by tubing to a pump. A computerized system tracks fuel placement, fuel burn, people placement, ballast, flap setting, landing gear position and water barrel quantity. The information is processed to display the airplane’s current CG. We move water or specify fuel tank usage to configure the CG within the specified test requirements.

Why are there overhead luggage compartments? It’s a test plane, and for FAA certification they have to demonstrate that everything works. That includes stuff like the emergency oxygen system, and more minor things like the luggage compartments. It’s a requirement that they don’t pop open in flight – so that needs to be tested. They are also handy for stowing the engineers’ stuff.

Here’s some pictures from Boeing:

And a lot more photos can be found on Boeing’s site.

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This one gets a lot of use in the “chemtrail” forums:

Particularly because of the unusual collections of pipes sticking out in various places. There’s those two at the front, and then there is a group over the wing. Here’s some close ups

:

Very sinister looking tubes, but why are half of them facing the wrong way?

The plane is not for spraying the atmosphere, it’s for sampling the atmosphere. It’s a research aircraft, registration N701BN, operated by th e department of energy’s national labs. It’s pretty much one of a kind, so it’s hardly likely to be responsible for all the persistent contrails we see every day. The research is mostly on pollutants in the atmosphere, particularly from coal and oil burning power plants. But they also investigate the properties of clouds, which includes contrails.

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Here’s another photo you see in “chemtrail” videos, with the implied suggestion that it’s some kind of evil spraying device:

Actually it IS a spraying device, but quite innocuous. It’s on an NKC-135A (55-3128) with the refueling boom modified to spray water. This used by the air force to test icing of planes in flight.

Here’s the original photo:

See also: https://www.safaq.hq.af.mil/news/march04/raptor.html

Here’s some more details:

nkc-135-icing-attachmentpv1983_2688.pdf

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This plane is quite interesting:

It’s an E-6B “Tacamo”. This photo shows it dumping fuel (photo from tacamo.org). The E-6B is used by the United States Strategic Command as an airborne communication center. You can see the navy logo on the right wing. The E-6B is a modified version of the Boeing 707-320, and the fuel vents have been moved from the wing tips to between the fuselage and the engines in order to separate it from the communication equipment in the wing tips. This is what the wing-tip ESM/SATCOM pod looks like:

It looks like this odd assemblage is also creating some wingtip vortex contrails as well. The plane is pretty much all white, which is something you hear mentioned from time to time in “chemtrail” conspiracy theories.

Here’s another photo of the same plane, taken from a “chemtrail” YouTube video:

It shows the opening for the ELF trailing wire antenna. This is a very long wire antenna that is extended behind the plane for several hundred feet and used for communications with submarines.

————————————————————————————–

This plane also looks at first glance like it might be dumping fuel (click image for full sized photo):

But the trails are actually coming from six smoke generators. It was part of a NASA test to study wake vortices, you can read about it here:

http://www.dfrc.nasa.gov/gallery/photo/B-747/HTML/ECN-4242.html

Six smoke generators were installed under the wings of the 747 to provide a visual image of the trailing vortices. The object of the experiments was to test different configurations and mechanical devices on the747 that could be used to break up or lessen the strength of the vortices. The results of the tests could lead to shorter spacing between landings and takeoffs, which, in turn, could alleviate air-traffic congestion.

Here’s another image of the same plane:

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This plane also occasionally get brought up in chemtrail conspiracy groups:

This is obviously not a contrail, it’s far too low and the trail is dropping too rapidly.

It’s a Boeing 747-100 “Supertanker”, modified by Evergreen Aviation, the only one of its kind. Specifically designed for fire fighthing. That’s it dumping water.   Here’s some more recent photos.

Here’s a video of it in action, titled “B747 chemtrails”. It’s interesting reading the comments, as the first comment correctly identifies what it is, and then everyone else just ignores that and starts speculating.

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This one looks like a plane spraying stuff. But again it’s rather close to the ground. It’s actually taking off with the assistance of rockets. It’s not spraying, that’s just rocket exhaust.

This particular plane is a Boeing B-47B, rocket assisted take off, April 15, 1954. An no, that’s not a contrail in the sky behind it – it’s rip in the photo. Click on it for a large version from Wikipedia.

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This one is used for cloud seeding. It does not actually spray anything but uses silver iodine flares that are either ejected, or burn in place.

It’s operated by the Sandy land Underground Water Conservation district of Plains, Texas, as part of their SOAR program. They have some more photos of similar equipment on their site. They are all small aircraft not capable of getting to the above 30,000 feet where contrails normally form.

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This next photo is also of silver iodine flares, fixed underneath at large plane.

These also show up in “chemtrail” literature. They are sold by Weather Modification Inc, they make a range of weather modification equipment. About this one they say:

WMI racks for ejectable flares are mounted on the belly of the aircraft fuselage. Each rack holds 102 cartridges. When fired, the pyrotechnic is ignited and ejected from the aircraft. In this configuration, the WMI Lear 35A is equipped with four 102-count racks for ejectable glaciogenic pyrotechnics, a total of 408 flares.

Here’s another, this time from North American Weather Consultants, Inc.

About which they say:

This aircraft-mounted cloud seeding generator is fixed in place, and can burn a silver iodide solution during flight.

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This one is the “Mk.32 drogue-type underwing pod on the Armée de l’Air Boeing C-135FR Stratotanker” (“93-CC”- s/n 63-8472 of GRV 93). It’s an in-flight refueling system on a French Boeing C-135FR Stratotanker, photographed in Canada, Feb 2005.

See: http://www.baha.be/Webpages/Navigator/News/tanker_flight_240205.htm

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: http://www.britannica.com/eb/article-9074829

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 efficiency (burning fuel more efficiently creates more water vapor) 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:

http://sportsillustrated.cnn.com/vault/article/magazine/MAG1068997/4/index.htm

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 titled “Airborne Observations of Contrail Effects on the Thermal Radiation Budget“, from 1970:

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:
http://news.google.com/newspapers?id=SosSAAAAIBAJ&sjid=pvYDAAAAIBAJ&pg=851,1486793

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.