Published: 2014-08-19 08:57:00
Updated: 2014-08-19 12:51:48
Posted August 19, 2014
By Mike Moss
It's always enjoyable to get photos from viewers that raise questions about what's going on in the sky, and a recent AskGreg submission is a nice example. Ask Greg
Amanda had taken a photo of what seemed an odd inverted triangle in the sky at sunset from Sanibel Island in Florida and later found that about the same time some relatives had shot a photo from Emerald Isle in our state that also struck them as unique. The two photos are shown here, and are both good examples of the fact that once the sun has gone a little below the horizon, clouds that are too far away to see can still have some neat effects on the way the sky looks closer to us.
In each case the sun is below the western horizon but is still sending rays of light across the sky above.
In the case of the Emerald Isle photo, skies overhead are mostly clear, but you can see a kind of shadow cast by the distant cloud in the form of the "fan" across the middle part of the sky that is darker. Outside this area, unblocked sunlight is being scattered by air molecules, dust, haze and any pollutant particles in the sky, and that scattered light makes those portions of the sky look brighter. The name for this effect (streaks of brighter and darker sky created by sunlight being blocked in some areas by clouds, buildings or mountains and shining through gaps in other areas) is "crepuscular rays," something we might more commonly call "sunbeams."
The picture from Sanibel Island has a similar origin in an unseen cloud that is casting a shadow, but it is even more dramatic because outside that shadow, the sun is lighting up the bottom of a layer of high clouds. In effect, here a lower cloud in the distance is casting a shadow upon the closer, higher cloud, creating the dark column. This photo may have one more element to it, though, as it appears there might be either an evaporating rain shaft or a fuzzy lower cloud in the upper foreground that makes the shadow a bit more complicated than the first example.
In each of the photos, the rays of light and the edges of the shadowed regions are parallel to one another in reality, but appear to diverge or spread out due to the perspective of seeing them over a great distance (much as a train track has parallel rails that appear to converge to a vanishing point far away). Because of this, you can sometimes look the opposite direction from the sun in these situations and see the rays appear to converge again toward the opposite horizon, in which case they are referred to as "anti-crepuscular rays."
There are some great examples of these and other related ray and shadow effects, along with some nice explanatory graphics and discussion, at the Atmospheric Optics web site, and I've included a link to that section.
Keep an eye on the sky, and if you see something out of the ordinary, feel free to send us a picture! If it's something that makes you wonder about what's going on, it always helps to include as much info as possible about location, date and time and which direction the camera was pointed.