by RJN » Wed May 05, 2010 11:08 pm
After revisiting Les Cowley's impressive website, I now believe that the most likely explanation of the pictured phenomenon is a fogbow with an unusual amount of bright internal supernumeraries. In retrospect, I should have done more of my background research on his site and less on wikipedia. I would advise Les or others to update the wikipedia pages on glories and fogbows.
Although an unfortunate oversight, this was a good opportunity for me to learn more about an interesting area. Scientists in my view are ordinary people trying to make sense of extraordinary events, and like everyone we make mistakes. This is not the first APOD that I have erred, and surely it will not be the last.
Here is the science as I currently understand it. First, fogbows and glories are different physical phenomena. Fogbows are extensions of rainbows to fogs and clouds and are relatively well understood in terms of reflection and refraction. (The supernumeraries are perhaps less well understood.) Glories are less well understood but are generally thought to also include a significant amount of diffraction. Fogbows and glories can be individually very different because of different droplet size distributions.
Observationally, fogbows are almost always angularly larger than glories. Although both depend on water droplet size, fogbows, which are similar to common rainbows, may vary from 20 to 40 degrees in angular radius, while glories typically range from the central shadow to as much as 20 degrees. Now the angular size of the arcs on this APOD were not reported and hard to estimate from visual cues. Still, I will try to email the photographer and obtain this information. (If someone wants to do this first, please have at it.)
Next, it appears that glory arcs are brightest near the central shadow and the surrounding arcs are progressively less bright. In contrast (ha!), fogbows have their brightest arcs far from the center and have more faint arcs, called "supernumerary arcs", inside. An exception to this is a full "secondary fogbow" (caused by an additional internal reflection inside the drops, I think) which are well outside the primary fogbow and are more faint. (This was another reason why I thought this APOD featured a glory.) This, currently, is the main reason why I now feel that this APOD features a fogbow and not glories -- the internal arcs appear fainter like supernumerary arcs of a fogbow.
Color progression: this was not as clear to me as indicative to fogbows as it was to Alter-Ego (but thanks AE!). I found the colors too hard to track to be definitive. Perhaps if I expanded the image and went through it more carefully the color progression would more clearly indicate a fogbow.
I apologize for this mistake. I will fix the APOD probably sometime this weekend when I return home. I also apologize for the slow progress of my understanding, which I might blame on being on travel and so busy. I thank everyone for their points in the discussion, and would welcome even more comments. Putting my scientist hat back on, this and other unusual arc cases might make good "mini-papers" that would help future photographers (and scientists and science writers) better label and understand unusual atmospheric optical phenomena.
- RJN
After revisiting Les Cowley's impressive website, I now believe that the most likely explanation of the pictured phenomenon is a fogbow with an unusual amount of bright internal supernumeraries. In retrospect, I should have done more of my background research on his site and less on wikipedia. I would advise Les or others to update the wikipedia pages on glories and fogbows.
Although an unfortunate oversight, this was a good opportunity for me to learn more about an interesting area. Scientists in my view are ordinary people trying to make sense of extraordinary events, and like everyone we make mistakes. This is not the first APOD that I have erred, and surely it will not be the last.
Here is the science as I currently understand it. First, fogbows and glories are different physical phenomena. Fogbows are extensions of rainbows to fogs and clouds and are relatively well understood in terms of reflection and refraction. (The supernumeraries are perhaps less well understood.) Glories are less well understood but are generally thought to also include a significant amount of diffraction. Fogbows and glories can be individually very different because of different droplet size distributions.
Observationally, fogbows are almost always angularly larger than glories. Although both depend on water droplet size, fogbows, which are similar to common rainbows, may vary from 20 to 40 degrees in angular radius, while glories typically range from the central shadow to as much as 20 degrees. Now the angular size of the arcs on this APOD were not reported and hard to estimate from visual cues. Still, I will try to email the photographer and obtain this information. (If someone wants to do this first, please have at it.)
Next, it appears that glory arcs are brightest near the central shadow and the surrounding arcs are progressively less bright. In contrast (ha!), fogbows have their brightest arcs far from the center and have more faint arcs, called "supernumerary arcs", inside. An exception to this is a full "secondary fogbow" (caused by an additional internal reflection inside the drops, I think) which are well outside the primary fogbow and are more faint. (This was another reason why I thought this APOD featured a glory.) This, currently, is the main reason why I now feel that this APOD features a fogbow and not glories -- the internal arcs appear fainter like supernumerary arcs of a fogbow.
Color progression: this was not as clear to me as indicative to fogbows as it was to Alter-Ego (but thanks AE!). I found the colors too hard to track to be definitive. Perhaps if I expanded the image and went through it more carefully the color progression would more clearly indicate a fogbow.
I apologize for this mistake. I will fix the APOD probably sometime this weekend when I return home. I also apologize for the slow progress of my understanding, which I might blame on being on travel and so busy. I thank everyone for their points in the discussion, and would welcome even more comments. Putting my scientist hat back on, this and other unusual arc cases might make good "mini-papers" that would help future photographers (and scientists and science writers) better label and understand unusual atmospheric optical phenomena.
- RJN