Contrail Observations

[ElMondoHummus]

I'm not sure how low they can start, but were these that you saw coming from the engine exhaust, or from the wingtips? I've seen, especially at lower altitudes and high humidity, visible trails left by water vapor condensing in the wingtip vortices.

I don't remember clearly; it's been a couple of days. But you've hit upon a possibility: It may have indeed been engine exhaust. I don't know why I didn't think of that.

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Two notes on personal research I've done:

1. I found a PBS link - http://www.pbs.org/wgbh/nova/sun/contrail.html - that doesn't answer my query about minimum altitudes, but does suggest it's a higher altitude phenomenon ("...something that typically occurs only in the upper troposphere, the atmospheric layer several miles up where airliners cruise."). So some info is out there; I'll see what I can find on my own.

2. When Googling "contrails form under what conditions", the first 4 links are chemtrails woo. That's discouraging. Thankfully, Googling just "contrails" gives you some rational links before the conspiracy-minded stuff comes up. Anyway, there are days Google discourages me; it's simply too easy for unsupportable drivel to get ranked. But, what can be done about it? People just need to be educated and understand the principles of skeptical thinking to navigate their way through the mush.

But some don't, and that's what's discouraging.

 

[ElMondoHummus]

The USAF Weather folks have a formula for predicting them, but I don't know what it is. Exhaust contrails are generally limited to fairly high altitude(say above 25,000 MSL), but wing tip contrails can easily form at very low altitude with high angles of attack. It is quite common to see them during high humidity conditions.

Here's a descent article from NASA....

http://enso.larc.nasa.gov/sass/contrail_forecast/contrail_prediction.html

Oh! Thank you! That's some good info there.

 

[SpaceMonkeyZero]

Oh there are plenty of conspiracies. They are just never that interesting. Nixon tried to keep a lid on the fact that he had some of his staff members engaged in some illegal activities. Then they caught some burglers and found they had connections to the white house and it fell apart.

The governor of Connecticut (where I live) had to step down and ended up going to prison after he accepted some gifts from contractors, including a hot tub and a new deck for his house and then attempted to hide the fact that they were installed for free by the same contractors who did work for the state.


Representative Dan Rostencowski apparently embezzled a whole bunch of money from the US post office.

Bill Clinton may have told an intern (to paraphrase). "Don't go telling people about those BJ's you gave me" Which is kinda something you could take as asking her to lie to investigators.... kinda.

Yes, Conspiracies exist. CTers try to claim that the definition of a Conspiracy Theory is that it is a Theory of a Conspiracy. In reality a crime of Conspiracy is investigated, with hard facts, not "theories".

While Conspiracy Theories use speculation and conjecture rather than real world facts. Sometimes they'll use stuff just plainly made up. Then they claim that well, "You MUST accept my theory because Conspiracies Do Exist! Therefore my Theory is valid!!!111eleventyones4eva!"

 

[grmcdorman]

Just out of curiosity, at what altitude would contrails generally start? ...

Not contrails, but I've often seen wing-tip condensation trails from aircraft landing at the Toronto [Canada] airport. That depends on the humidity and temperature - if the conditions are right, it gets bigger. I've actually seen a landing (passenger) aircraft with the entire wing obscured by condensation up to the height of the fuselage.

I haven't seen engine trails, although for some aircraft one can occasionally see the exhaust (faint but dark).

 

[Bell]

I've actually seen a landing (passenger) aircraft with the entire wing obscured by condensation up to the height of the fuselage.

The first time I noted that was when a jet on approach for Schiphol flew over my parents house and started to bank. I guess the sun then hit the top of the wing or something, but suddenly there was a huge white cloud coming from the top of the wing. Scared the bejezus out of me, I thought it exploded

 

[Plantfoam]

Well, it's obvious that Microsoft is in on the whole chemtrails thing, because they included them in the last few versions of MS Flight Simulator.

 

[Thunder]

Now, are contrails formed by exhaust from the engines condensing or is the heat from the engines causing ice crystals in the air to condense?

 

[ktesibios]

As I understand it, if the humidity at the plane's altitude is high enough and the temperature is low enough, adding the water vapor in the exhaust (water is a combustion product of hydrocarbons) creates a mixture which, as it cools, becomes supersaturated for water. Water droplets form and freeze into ice crystals. That's why the contrails form a little behind the plane- the air/exhaust mixture has to cool down enough for the water to precipitate.

Another mechanism is when the change in pressure produced by the passage of the plane cools the air enough that the water vapor already present preciptates into droplets, which freeze into ice.

There are mathematical treatments on the University of Alaska site I linked to somewhere above which will explain it a lot better than I can.

 

[CurtC]

Now, are contrails formed by exhaust from the engines condensing or is the heat from the engines causing ice crystals in the air to condense?

Parky, jet fuel is mostly molecules of carbon and hydrogen. When it burns, you add a bunch of oxygen from the air, to make carbon dioxide and water. See how that happens? Each carbon atom in the fuel gets two oxygen atoms, and every two hydrogen atoms in the fuel get one oxygen atom (H₂O). The point of this is that there is a lot of water produced when you burn fuel - each gallon of jet fuel results in about one gallon of water, if you were to catch it and condense it to liquid.

Ever notice on cold mornings how there's white wispy stuff coming out of your car's tailpipe? That's the water vapor condensing into small droplets. It only does this while the engine is cold, because after the exhaust pipe heats up, the exhaust disperses too much in the air before it cools off enough to condense.

So this very humid exhaust gas comes out the back of a jet engine, and if the air is cold enough, it drops down in temperature so that the water vapor in the exhaust condenses into droplets.

If the surrounding air is very humid, those droplets stay as droplets. If the surrounding air is dry, they evaporate back to water vapor.

 

[defaultdotxbe]

Parky, jet fuel is mostly molecules of carbon and hydrogen. When it burns, you add a bunch of oxygen from the air, to make carbon dioxide and water. See how that happens? Each carbon atom in the fuel gets two oxygen atoms, and every two hydrogen atoms in the fuel get one oxygen atom (H₂O). The point of this is that there is a lot of water produced when you burn fuel - each gallon of jet fuel results in about one gallon of water, if you were to catch it and condense it to liquid.

the equation for combustion of kerosene is:
2 C₁₄H₃₀ + 29 O₂ = 28 CO₂ + 30 H₂O

note 2 molecules of kerosene make 30 molecules of water (its too late for me to bother with all the molar mass conversions to give a volume:volume ratio) but suffice to say theres plenty of water coming out of jet engines

 

[DRBUZZ0]

Yes, Conspiracies exist. CTers try to claim that the definition of a Conspiracy Theory is that it is a Theory of a Conspiracy. In reality a crime of Conspiracy is investigated, with hard facts, not "theories".

While Conspiracy Theories use speculation and conjecture rather than real world facts. Sometimes they'll use stuff just plainly made up. Then they claim that well, "You MUST accept my theory because Conspiracies Do Exist! Therefore my Theory is valid!!!111eleventyones4eva!"

Well there are some credable "Theories" which cannot be proven. For example, the tape gap in the Nixon tapes is believed by most historians to have been intentional and probably either done by Nixon or ordered to have been erased.

Analysis of the tape showed that it had been erased and re-erased by recording silence onto it multiple times. But there's no hard evidence that proves Nixon did it himself or who did it.


Also there are theories about what happened to Jimmy Hoffa. Clearly his disaperance had hallmarks of a mob hit and his body was never found. Some theories of what happened include that it was disposed of in a mafia-controlled trash incinerator. That's not really that far fetched. The mob did own some garbage companies and it would have been a good way of getting rid of it. Other theories are that it was dumped in a ladnfill or disposed of in a rendering plant. This isn't really that far fetched either. And of course, who exactly was behind it hasn't been proven either.


Since those "theories" are based on limited evidence and some assumptions they're not really "proven" but they're also not completely beyond the realm of possibility.

 

[Alferd_Packer]

Parky, google the term Ice Supersaturation Or just click on the link.

This helps explain why contrails can persist even in areas with no visible clouds.

another interesting term is sub-visual cirrus

 

[DRBUZZ0]

Coincidentally, the other day I was driving when I noticed an aircraft making some very dramatic turns. It looked like it was at a relatively high altitude, but that's hard to estimate. I couldn't make out the details very well, even when it was almost directly above, which means it was higher than the general aviation aircraft that generally only go up to a few thousand feet or less. It was relatively small - either that or it was at a very high altitude, but I think it was small - smaller than a commercial jet. Possibly military or learjet-type aircraft.

Anyways, it was going at a decent clip and making some rather dramatic S-turns, going back and forth but headed generally in a southern direction, toward long Island Sound.

It may have been training or something or perhaps it was a Sunday pilot with privately owned jet. There are a few T-38's and a few soviet airframes which ended up in the hands of rich folks. I believe there are even one or two F-4's being flown by enthusiasts with deep pockets.

Too bad the contrail faded pretty fast. Otherwise you'd see a big line snaking back and forth across the sky

 

[TjW]

The first time I noted that was when a jet on approach for Schiphol flew over my parents house and started to bank. I guess the sun then hit the top of the wing or something, but suddenly there was a huge white cloud coming from the top of the wing. Scared the bejezus out of me, I thought it exploded

No, this is quite common. Airplanes have lower-than-average pressure on the top surface of the wing. When the pressure is lowered, air expands, and as it expands, it cools. If the cooled air drops below the dewpoint temperature, the water in the air condenses, making a localized cloud over the wing.
The more lift that needs to be generated, the lower the pressure. So a rotation on takeoff, or banking, may trigger the effect.
You'll sometimes see contrails from the vortices on wingtips and even on flap panels. Same general idea, the pressure in the center of the vortex is lower.

 

[jsiv]

I WAS ONCE ABDUCTED AND PROBED BY AN UNMARKED WHITE CHEMTRAIL.
...

 

[Thunder]

The clouds from the Space Shuttle are giant contrails right?

 

[Bell]

No, this is quite common. Airplanes have lower-than-average pressure on the top surface of the wing. When the pressure is lowered, air expands, and as it expands, it cools. If the cooled air drops below the dewpoint temperature, the water in the air condenses, making a localized cloud over the wing.
The more lift that needs to be generated, the lower the pressure. So a rotation on takeoff, or banking, may trigger the effect.
You'll sometimes see contrails from the vortices on wingtips and even on flap panels. Same general idea, the pressure in the center of the vortex is lower.

Interesting. Thanks for the explaining.

 

[Elizabeth I]

Here is an awesome website about atmospheric optics.

Especially check out airplane condensation cloud.

 

[CurtC]

The clouds from the Space Shuttle are giant contrails right?

You mean the white clouds you see at take-off?

No, those clouds are mostly the chemical from the solid rocket boosters.

 

[BenBurch]

You mean the white clouds you see at take-off?

No, those clouds are mostly the chemical from the solid rocket boosters.

According to Wikipedia;

The propellant mixture in each SRB motor consists of ammonium perchlorate (oxidizer, 69.6% by weight), aluminum (fuel, 16%), iron oxide (a catalyst, 0.4%), a polymer (such as PBAN or HTPB, a binder that holds the mixture together, also acting as secondary fuel, 12.04%), and an epoxy curing agent (1.96%). This propellant is commonly referred to as Ammonium Perchlorate Composite Propellant, or simply APCP. This mixture develops a specific impulse of 242 seconds at sea level or 268 seconds in a vacuum.