Thanks, Fubaya. Until I read your post, I was planning to link that same page, which I discovered several months ago after a friend told me about contrail shadows.
Might explain it? That’s open and shut, thanks!
The sky would be pitch black in broad daylight if the atmosphere didn’t absorb and scatter light (or if we didn’t have one).
This is a common volumetric effect, and what we are seeing is the occlusion of sunrays through the thickness of the atmosphere. What we think of as typical shadows is only the planar projection/occlusion of this light onto another surface; say, your body casting a shadow on the sidewalk.
However, if you were to fill the area with fog, you would see a similar occlusion of rays projecting from your body to form the cast shadow.
In this picture, we’re seeing only the occlusion as projected through something opaque in the atmosphere—not what’s casting it, or any planar projection we commonly think of as a shadow.
Would a shadow of a contrail appear vertical, reaching the horizon? A contrail is horizontal, and would have to be long enough to reach the horizon in order to cast a shadow at the horizon, and the contrail would be visible there, too.
A vertical shadow would require a vertical planar surface upon which to cast the shadow.
The first rule of crepuscular rays is we don’t talk about crepuscular rays. Sheesh.
This all depends on angles and alignments. At just the right angle everything could alight to cast a column that appeared as a vertical shaft, keeping in mind these are the shafts of light occluded, and not the shadow projecton cast as if on some wall in the sky.