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I often hear the claim that "chemtrails" have to exist, because normal contrails don't linger very long. Is there a definitive scientific explanation for why normal contrails sometimes do, in fact stick around and sometimes even seed very large clouds?

An ideal explanation would also clearly state why one wouldn't be able to identify the correct conditions from the ground.

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    @Oddthinking: Why did you delete my answer? Chemtrails do not exist, that is a fact. Your question implies the existence of chemtrails, so the only answer to your question can be: There are no chemtrails. No one is poisoning the people on Earth from airplanes. Contrails will stay contrails for a while and then disappear or turn into clouds. You are already presuming there is such a thing as chemtrails, but there isn't. – jjack Sep 18 '15 at 19:10
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    Contrails are "Condensation Trails". They are formed by "billions of liquid droplets or ice crystals", see en.wikipedia.org/wiki/Contrail. – jjack Sep 18 '15 at 19:19
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    You even use the word "chemtrails" in your headline. How could it not be about chemtrails? – jjack Sep 18 '15 at 19:56
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    @jjack: Thank you for illustrating my point. Saying 'chemtrails don't exist' is not an answer to the question. Read it all. – Oddthinking Sep 19 '15 at 1:01
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    Your question - the headline - presupposes the existence of chemtrails, something which doesn't exist. Therefore the only answer to your question can be, that chemtrails do not exist! This reasoning is called "classical logic" (en.wikipedia.org/wiki/Classical_logic). Your question rests on false assumptions. – jjack Sep 19 '15 at 9:01
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The longevity of a contrail is dependent on the weather conditions which indeed might be difficult to spot from the ground. I don't know about "unable" though. As far as I know the 'controversy' about contrails has never had any substantial evidence.

As for the time a contrail is visible, the most described factor is Humidity1:

When the ambient relative humidity is high, the resulting ice-crystal plume may last for several hours.

Here is an article explaining how the trails have been used by sailors for forecasting, and has a couple of links to other articles explaining the phenomenon.

(1) Encyclopedia Britannica

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    Links not working anymore. Was it? – Kangarooo Apr 15 '15 at 13:41
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The conditions required for contrail persistence are ice-supersaturation. This means that the air is cold and humid enough that the water vapor will directly "condense" out of the air in the form of ice (the process is called "deposition"). Ice supersaturation is around 60-70% of the normally reported relative humidity, which applies to water1.

However, and this leads to the reason why you can't tell from the ground, deposition in most conditions only happens on to ice. i.e. it requires some kind of seed, or nuclei, for the ice to form on. And for ice, this requires a specific molecular framework.

Now actual condensation into liquid is less fussy about what types of nuclei are required. So once the relative humidity in the air goes over 100%, the water condenses out as liquid, and you'll get a cloud.

So you can have a cloudless sky that still has regions (usually in layers) that are ice-supersaturated, but not water supersaturated. When a plane flies through those regions, the water in the exhaust bumps up the humidity over 100%, liquid water droplets condense out. If it's below -40°C/-40°F (yes they are the same) then these droplets will freeze and become ice crystal seeds.

So a contrail starts as a bunch of seed crystals. If the air is ice-supersaturated, these crystals will grow (increasing in mass by a factor of 1,000 or more), and hence can persist for hours, and even spread out to cover the sky.

(1) [FAA, Contrail Microphysics, 2010.][1] http://www.faa.gov/about/office_org/headquarters_offices/apl/research/science_integrated_modeling/accri/media/Contrail%20Microphysics.pdf

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