Space Chronicles on Ice #13
by Don Pettit
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The 22-Degree Halo
Dec 19, 2006:
A few days ago, consumed by the katabatic wind, we found ourselves inside of a suspended collection of ice crystals. This was a rather low lying cloud, perhaps being only 100 meters thick and moving over the ground like the wispy vapors from a pot of dry ice at a Halloween party. At these temperatures, ice crystals take on the form of hexagonal prisms and as such, have rather interesting optical properties. Every second side of a hexagonal prism is at 60 degrees, and this coupled with the refractive index of ice creates a bent ray of light at a 22-degree angle. When you have a large collection of these prisms, as in a cloud, they produce a halo around the sun at a 22-degree (half) angle. If there are a number of short, fat hexagonal prisms, looking like a flat six-sided plate, aerodynamic forces from their suspension tend to make them flutter like little leaves, thus being oriented preferentially parallel to the ground. Having many of these short, flat oriented prisms produces a rather strong refracted intensity at the 22-degree point, but located at the 9 and 3 o'clock positions of the halo. These bright spots are called sundogs.
When inside this cloud, we were able to see the detailed effects of the 22-degree halo, complete with tangent arcs and the fainter but distinct 46-degree outer halo. When the height of our submerged cloud decreased with the ebbing wind tides, only the bottom half of the halo would be seen. Apparently, the cloud height was not sufficient to form the entire halo from our viewing position within the cloud. Cold and tired from a full day’s work, this was a spectacular way to finish up our meteorite hunting.
Nature is filled with beautiful surprises that can distract one from the practicalities of survival. Like Syrians singing occipital songs, nature will fool you into a complacent stupor just before the pending wreck on the icy reef. While we were in awe of this display, we failed to notice that our visibility had dropped to less than 1/2 kilometer. Being several kilometers away from camp, tired and cold, it would not be good if we could no longer see to find our way back to camp. Although for this case, we could see well enough to find our way home, there could be times when the Syrians may not let us. With the modern equivalent of wax ear plugs, we could thus shun the Syrian's sweet songs by tuning in our GPS units, and avoided the pending wreck on the icy reef.