Friday, September 11, 2009

Lunar Fog Bow

When I went to bed last night I didn’t think there would be much of a chance for sky watching during the night as we were shrouded in thick wet fog.

But as luck would have it when I checked weather at 0130 hr, a large hole had opened in the fog and one could see high broken clouds and a bright moon overhead.  Also  a few weak aurora were moving about, so I decided to go out on the odd chance I might catch them in an active period.  After about ten minutes of walking about the yard looking for a good spot to try for a picture if the aurora flared up, I noticed a pale white patch off to the west.  It took me a bit to realize that I was looking at a rare Lunar Fog Bow.  I took a few pictures of the fog bow, and in one there is even a hint of aurora in part of the frame. 










Slight reddish cast to top of the lunar fog bow can be seen.

While taking pictures of the fog bow the opening overhead closed up and as the moon disappeared so did the bow. So while I didn’t get any aurora pictures, it was worth while just to see the stars and see the lunar fog bow.   Compared to a rainbow this fog bow was quite broad and it did have a slight reddish cast to the top of the arc.


In this photo the fog bow is starting to fade at the top, but you can see a hit of green aurora in the top left.


The reason that the fog bow is so pale is the fog droplets are so small, usually less than 0.1mm.* 

*more information is taken from “Out of the Blue” by John Naylor. Drops as small as this are found in fogs and clouds, and they are also known as white bows.

The absence of color in a fog bow is due to an overlap of the red and blue bands, which occurs when the drops are very small. The overlap is not always complete, and sometimes a fog bow sports a reddish fringe.  Although the diameter of a fog bow is always less than that of a rainbow, its arc can be up to three times as broad.  Lunar fog bows have been seen, though rarely.


  1. This is very cool. I saw a fog bow I believe when I was working over at West Dock a month ago. It was during the day though, but just like a pure white rainbow in the fog. I had never seen anything like it before.

  2. Yes, fog bows can be white only, and are sometimes called White Bows


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Arctic Smoke Signals by James W. Helmericks is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License.