The Edge of (relative) Darkness!
Posted August 11, 2008
Sounds a bit like an old soap opera, I guess, but this post was prompted by a question from viewer/web visitor Steven Casey, who noticed an interesting phenomenon in the sky while driving north on T W Alexander drive in RTP around 5:30 pm last Wednesday afternoon. The apparition in the sky was rather striking, it seems, as Steven reported that several people were exiting their cars while stopped at a red light to get a better look.
He took it a step further and pulled out a camera, snapped a couple of quick shots, and sent them in to ask how what appeared to him a dark "ripple" in the sky (running upper right to lower left across the center of attached images 1 and 2 ) could be formed.
My initial impression was that there may have been a thin, nearly transparent layer of cirrus clouds in the sky, and that a jet flying considerably above those clouds might be leaving a long contrail behind that was casting a shadow on the clouds, creating the darker line. I've seen this a few times myself, and occasionally there is enough separation between the contrail and the shadow that it takes a moment to make the connection. However, they are usually closer together than could be the case in these photos, and once you make the association between the two, there is an obvious connection as they are clearly parallel to one another. For an example of this, see photo number three, from the "Atmospheric Optics" web site.
However, there is another, less frequently noticed shadow effect associated with contrails, cloud lines, and rocket exhaust plumes that is a little more esoteric in its structure and explanation, and it depends on being in the correct relative position to see it. It is called an "edge shadow," in which no sub-contrail cloud layer is required, and the shadow can appear to extend across nearly clear sky well beyond the location of the contrail itself. In effect, the contrail casts a shadow that is in the form of a thin "sheet" that is only visible if you happen to be viewing that sheet edge-on, so that you are looking at a narrow but deep layer of unlit air bounded on either side by air that is more fully sunlit. Someone on the web made a useful analogy of looking through a piece of window glass - look straight through broadside as you normally would and it is transparent, but look through it edgewise or at a very shallow angle and it is nearly opaque.
Another way to mentally visualize this (or maybe a neat home experiment) is to imagine a fish tank filled with water that has been mixed with just enough milk to make it appear hazy - in a darkened room, you place a sharpened pencil (the tip representing the airplane, the rest its contrail) in the water two thirds of the way up toward the surface, then place a strong light bulb above and "behind" the pencil. This would cast an obvious shadow on the bottom of the tank, but there would also be a "sheet" of unlit milk "haze" extending from the pencil to the bottom. Viewed from either side of the tank, you would only see the shadow on the "ground," but if you looked through the tank from below and behind the pencil, you would see this sheet as a darkened line or band extending downward from the pencil. If the light was directly above and behind the pencil, the line would appear to extend forward well in advance of the sharpened tip, while moving the light to the right of the pencil would cause this dark line to angle down toward the left, and vice versa.
In the case of the shadow in Steve's picture, I'm guessing that had he taken additional shots overhead and behind there would be a well-formed contrail that happened to be lined up almost along the direction from which the sunlight was coming (likely flying from S/SW toward N/NE), and that cast a shadow such that he happened to be located in an area where his view was edge-on to the sheet of darkened air. Though it's a little subtle in his images, this can produce rather dramatic effects in some cases. I found a few photos (4, 5 and 6 above) and video examples on the web that help to illustrate the possibilities. The video can be seen at
where the edge shadow is nicely visible toward the left early on.
Thanks again to Steve for taking the trouble to capture and send in the pix - I know I'll be watching more closely for this when the sun's a little low in the sky and there are contrails about!