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Cloud from jet engines

  1. Aug 15, 2009 #1
    At higher altitudes in the atmosphere, why do aircrafts (particularly fighter aircrafts) with jet engines leaves some kind of white cloud trailing behind?

    It doesn't disappear for a reasonable amount of time.
    What is this?

    Thanks!
     
  2. jcsd
  3. Aug 15, 2009 #2

    Danger

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    There are aeronautical experts here who can give you more information, but the basic idea is that they leave a 'cloud' of water vapour as part of their exhaust. Unless significant winds are involved, the vapour trail persists for quite some time.
     
  4. Aug 15, 2009 #3

    negitron

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    The above is basically correct; however, conditions have to be right for cloud formation in order for the vapor trail to become a visible cloud--you don't see these contrails, as they are called, behind every plane. The tiny, tiny droplets which condense out of the jet's exhaust form a tenuous mist but if the conditions are right (basically air saturated with water vapor and a temperature below the dew point), these tiny, tiny droplets become nucleation sites for even more atmospheric water to condense upon then, POOF, you've got a visible cloud trail.
     
  5. Aug 15, 2009 #4
    You can likely get some good analogous insights from reading about fog formation here on earth. Fog is bascially clouds at ground level. Forecasting when it will appear or not is tenuous at best: we know the conditions, but guessing when the right conditions will be present is a crapshoot.

    Until recently I used to boat summers in Maine; the one time I could be almost positive of fog all day was when the national weather service would declare "Fog will disspate by 10AM". As soon as I heard that, I changed any plans and would plan to remain at anchor all day.
     
  6. Aug 15, 2009 #5

    DaveC426913

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    'Boat' is a verb?
    Despite what it may seem to readers, that doesn't necessarily mean they got it wrong. It simply mean they were predicting for the general (mostly land) area. You, being on the water, would have to factor that in.

    Unless you were specifically listening for the marine forecast...
     
  7. Aug 15, 2009 #6

    negitron

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    http://www.answers.com/boat [Broken] This is not a new usage by any stretch.
     
    Last edited by a moderator: May 4, 2017
  8. Aug 15, 2009 #7

    Danger

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    Actually, yes. 'Boating' is an accepted verb to describe the action of messing about with boats. To take part in such activity is to 'boat'.

    edit: I see that you sneaked in ahead of me, Negitron. Nice link.
     
  9. Aug 15, 2009 #8

    DaveC426913

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    Yeah. I'm familiar with it as an adverb at least. "I went boating." It just sounds funny to say "I like to boat."
     
  10. Aug 15, 2009 #9

    Hurkyl

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    If you want to do some searching on your own, those are called contrails.
     
  11. Aug 15, 2009 #10

    negitron

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    But for God's sake, steer clear of any sites that talk about "chemtrails."
     
  12. Aug 16, 2009 #11
    Thanks! now that i know what the proper name is, i can do some search and reading myself
     
  13. Aug 16, 2009 #12

    negitron

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    Incidentally, in case it wasn't immediately evident, "contrail" is a contraction of "condensation trail."
     
  14. Aug 16, 2009 #13

    Born2bwire

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    Or Woodpecker Grids. Good Lord... :rolleyes:
     
  15. Aug 16, 2009 #14

    russ_watters

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    It is worth pointing out that jet plane exhaust is mostly water....
     
  16. Aug 16, 2009 #15

    mgb_phys

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    But the visible contrail is mostly from atmospheric water, the exhaust water just provides a nucleation source.
     
  17. Aug 16, 2009 #16

    russ_watters

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    I think the first part is right, the second part isn't. The added moisture pushes the air further above saturation. I think this may be a nitpicky point, though and I'm not too concerned about it either way.
     
    Last edited: Aug 16, 2009
  18. Aug 16, 2009 #17
    Air (the gas) contains water vapor. When this rapidly gas travels from high to low pressure it expands which condenses out the water vapor. The high pressure is ahead of the airfoil (wing or rear stabilizer) - the low pressure is after. When atmospheric conditions are correct, this difference in pressure allows the water vapor to condense after the airfoil.
     
  19. Aug 16, 2009 #18

    negitron

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    This is not the usual mechanism for contrail formation, which is as previously noted. The particular phenomenon you refer to is called a wingtip vortex (even thought they often form behind the control surfaces of airfoils, not just at the tips).

    http://en.wikipedia.org/wiki/Wingti...rtices_due_to_water_condensation_and_freezing
     
  20. Aug 16, 2009 #19

    mgb_phys

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    I thought you also got contrails from wingtip vortices and so on

    Who are you and what have you done with the real Russ?
     
  21. Aug 16, 2009 #20

    russ_watters

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    In the wiki, it says that contrails from the wingtip vortices are not the most common type.
    If you reread the post that you were first responding to, you'll see you were responding to something I didn't say. I'm not interested in arguing about something I didn't say and the particulars of the mechanism of contrail creation just don't interest me that much.
     
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