Ever wonder why some planes leave trails of smoke and others don’t? Is it just that certain commercial jetliners are trying out some geometric skywriting while others can’t be bothered?
Actually the answer is much more complicated. First of all, those long trails you see sometimes coming off the back of planes, or from their wings, isn’t smoke at all. It’s more like what you see when you exhale on a cold winter day and see your breath.
Contrails, as the lines are called, are formed when the hot, humid air coming out of the jet exhaust mixes with low vapor pressure in cold temperatures. I’m talking cold like the kind of cold that usually exists 26,000 feet up or higher. This is completely different from the real smoke planes use when they sky-write.
As you’ve probably noticed, some contrails will remain in the air for quite some time, while others dissipate more quickly. It all depends on how humid the air is and how windy it is up there. If the atmosphere is near saturation, the contrail may hang around for hours. But if the atmosphere is dry, they usually only last a few minutes, spreading out, thinning, and then, poof! They’re gone. Unless, of course, they’re captured by a talented photographer…
Photo credits (all used under Creative Commons license.):
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Look up chemtrails…
posted by Nancy on 1-6-2010 at 8:50 am
Younger folks may not realize that piston engines left the same contrails back in their day.
We were told they were ice crystals formed when the heated exhaust gases were suddenly cooled upon emerging from the pipes. Once cooled, they could no longer carry the moisture generated by the combustion process.
That way our teachers didn’t have to explain vapor pressure to 6 year-olds.
posted by Tom on 1-6-2010 at 9:44 am
Chemtrails? Sheesh!! Yet another wacko conspiracy theory.
Don’t you people ever run out of things to cry “Wolf” about?
-”BB”-
posted by Bicycle Bill on 1-6-2010 at 10:19 am
For some reason the second-to-last photo doesn’t process right in my head. If the sun is behind the trail, how is there a shadow behind the trail? Even if the bright light is glare, the area in the sky near where the sun should be is much darker than the rest…
posted by Nicole on 1-6-2010 at 11:40 am
@Nicole
I think the contrail is between the clouds and the sun.
If the shadow was caused by sunlight reflected up to the bottom of the cloud, the shadow would be above the contrail (draw a mental V from the sun down to the ground and back up to the contrail).
posted by Jonny on 1-6-2010 at 12:18 pm
God…, Chemtrails? Really?
This is Mental_Floss right?
@Nicole
Visualize the contrail above the cloud and then you would get that kind of shadow on it, since the trail is between the sun and the cloud. It’s hard to tell in that picture since depth is almost nonexistant.
posted by Allen on 1-6-2010 at 12:29 pm
i’ve seen way too many of these appear to make X’s over crowds of people to believe that its ‘natural’.
I counted 30+ planes with trails in the span of a couple hours during London’s Live 8. Next day, same park: no planes.
Chemtrails (run a google search on that)
I’m a skeptic, but this I’ve seen myself.
file this under: “conspiracy theories that actually came true”- like mccarthy, mk ultra, tonkin, northwoods, and iran-contra.
posted by briiii on 1-6-2010 at 12:42 pm
one of my friends recently found out, in her mid twenties, that those are indeed smoke trails. She had previously thought that they were making lines in the sky for the other planes to follow.
I laughed so hard boxed wine came out my nose
posted by jodie on 1-6-2010 at 2:00 pm
@jodie that’s just too cute.
posted by mattkendrick.com on 1-6-2010 at 2:06 pm
I have a science-type question.. how do your eyes adjust or compensate for tinted glasses like sun glasses or polarized glasses that are yellow or green?
I have yellow-tinted ones and I really noticed it when I first got them but, of course, stopped noticing it after a few weeks.
posted by Sarah in CA on 1-6-2010 at 3:48 pm
I was surprised that David didn’t explain the word contrail is a portmanteau of the two words “condensation trail”. That in itself tells most of the story of what it is. It certainly ISN’T a “chemical trail”!
posted by Gordon Daily on 1-6-2010 at 4:44 pm
that bottom picture is pretty old. he jet engine on it looks like it may be from a Boeing 707 or something.
posted by Ian on 1-6-2010 at 6:42 pm
then why do people get severe colds every time these lines form over our neighborhood
posted by robert on 12-4-2010 at 9:15 pm
i get sick every time thos planes pass by back and forth specialy when the sky is clear and windy why the hell dont do that somewhere else??/
posted by robert on 2-3-2012 at 6:46 pm