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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?

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. The first powered plane had only flown in 1903, and the very earliest report of a contrail is just 15 years later, 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.

Continue Reading »

Executive 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

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. Continue Reading »

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.

Continue Reading »

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. Continue Reading »

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” are simply the visual effects of shadows. There are actually three main kinds of dark line that are formed by contrail shadows.

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. The photo on the right here shows some thick contrails (or possibly just row clouds) that are self shadowing. The sun has just set in front and to the right of the contrails.

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.

Edge 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 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.

[Math Warning] It’s a bit difficult to explain these edge shadows (also called volumetric shadows, or anticrepuscular 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.

Several planes look a little odd, or have attachments that look odd, and so some people feel they must be part of a decades long conspiracy to spray stuff into the atmosphere to alter the weather or reduce the population. That’s obviously nonsense, but what are these strange planes?

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.

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 used by the air force to test icing of planes in flight. See:

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 form 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.

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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 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.

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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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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: 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).

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.”

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.