1. santamonica

    santamonica New Member

    Hello,

    A very simple question from a layman, can a contrail form shadows? I am asking because someone just made a statement on FB saying " If two shadows are formed from the trail of a plane it has to be a chemtrail because if it was only water vapor shadows can't form.

    Is that correct?
     
  2. Mick West

    Mick West Administrator Staff Member

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  3. Mick West

    Mick West Administrator Staff Member

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  4. santamonica

    santamonica New Member

    What would you say about this, does this picture illustrate what you wrote above? ContrailShadows.
     
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  5. Spectrar Ghost

    Spectrar Ghost Senior Member

    It's true shadows can't form with only water vapor, which is optically transparent. However, contrails are made of ice crystals, not vapor, and ice crystals are not transparent. Anyone who is claiming contrails cast no shadow is de facto claiming clouds cast no shadow, since contrails are a form of cirrus cloud. Of course this is absurd.
     
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  6. Spectrar Ghost

    Spectrar Ghost Senior Member

    That's a good shot of multiple shadows. Nice job.
     
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  7. santamonica

    santamonica New Member

    At what altitude do ice crystals form?
     
  8. Spectrar Ghost

    Spectrar Ghost Senior Member

    Any altitude with sufficient moisture and below freezing temperatures. They can self-nucleate at -40deg. Dry adiabatic lapse rate is ~10deg Celsius per km, so if it's 68F/20C on the ground, they could be as low as 6km/20k ft. Usually a bit higher, but well within cruising altitude.

    Edit: given sufficient moisture, of course. That's the main variable.
     
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  9. santamonica

    santamonica New Member

    Thank you Mick West and Spectrar Ghost for the clarifications! :)
     
  10. Rico

    Rico Active Member

    As a note of clarification, ice crystals can form all the way down to the surface if it is cold enough where moisture cannot remain in vapor form. Remember that temperature varies from place to place, from time to time, and season to season.

    That's why we have weather phenomenon like Ice Fog
    [​IMG]

    Or even low-level contrails
    [​IMG]
     
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  11. mrfintoil

    mrfintoil Active Member

    In a recent discussion about "dark trails" I got presented with this classic image with the diverging trail. I decided to create a simulated scene to see what exactly is going on, and curiously enough, using a low sun angle, and recreating the path of the contrail, I got the exact same angle of the volumetric shadow through the cloud layer. Not because it was highly necessary to debunk this particular photo, but it is always fun to test optical illusions by emulating them.

    contrail_shadow_simulated.
     
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  12. Jay Reynolds

    Jay Reynolds Senior Member

    Which program did you use for the simulation?
     
  13. mrfintoil

    mrfintoil Active Member

    3dsmax, rendered with vray. Very simple set up.
     
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  14. mrfintoil

    mrfintoil Active Member

    I made another illustration concerning volumetric shadows. This time in relation to this image COPP_gqWIAABAVV.
    (source)

    planeGif.

    Approximative matching view:
    plane_view.
     
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  15. JRBids

    JRBids Senior Member

    shadow.PNG
     
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  16. mrfintoil

    mrfintoil Active Member

    Funny, @JRBids. That was exactly the same confused question asked when I first encountered this image. Why do these people believe the shadow must be above the plane when simple logic tells us that shadows are cast away from light sources?

    I guess the confusion comes from the expectations of clouds being opaque, and as the plane is visible to them it must indicate the plane is below the clouds. I guess that's where the confusion begins.

    The only difference between my illustration and the real image is that the plane is much higher above the cloud layer, hence the larger shadow. But for the sake of not spreading things too much apart in my illustration I put them closer together. Principle remains the same though.
     
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