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.
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.
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.
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
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.
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.