Claudia Hinz observed a strange ‘rainbow’ on 5 May 2005 from Mt. Wendelstein (alt. 1835m). During a strong ice pellet shower from collapsing cumulonimbus there appeared an almost 20° long faint, but coloured piece of bow. It was rather diffuse, ragged and very broad. The sketch shows the bow. The sun was 50° high and the bow was approx 8° below the horizon.
From meteorological experience, a dying cumulonimbus can only produce ice when it collapses from the top and in the lower levels the temperature is low enough that the ice does not melt. That was the case here, the cloud collapsed and decayed fully into graupel. There was definitely no rain, which I had also not experienced previously with a ice pellet showers.
I do not want to commit myself whether there can be an ice pellet bow. However, the ice grains may occasionally be covered with a water layer, giving a smooth outer surface. Of course, the refraction in the ice-water interface must also be considered. If that interface was uneven it could explain some of the bow width.
On 11th of January 2006 Christian Fenn reported crystal-clear ice balls (photo 1, 2), in sleet falling at an air temperature of -4°C. He searched for an ice bow formed from the headlights of his car. In this picture the possible ice bow is compared to a (terrestrial) rainbow. There are also images by Christan Fenn of the ice balls.
A primary bow from ice spheres would have a radius of 45.5° compared to the 42° of a water bow – perhaps noticeably greater. One could imagine in that, in theory, concentric spheres of ice with an outer layer of water might produce a broad bow of (for yellow sodium light) 42 to 45.5 degrees radius.
ReplyDeleteHowever, ray tracing calculations
http://www.sundog.clara.co.uk/rainbows/twin1.htm
show that even small departures from sphericity (2%) are enough to blur or destroy a rainbow. In small raindrops, surface tension forces are relatively strong and keep the drops closely spherical. There are no such forces in ice unless we suppose that the ice balls are formed by the very slow freezing (from the centre) of water drops. It is therefore very difficult to see how ice particles could be sufficiently optically smooth and homogeneous (plus concentric with any surrounding water) to meet the stringent requirements of bow formation. At most
one might expect a brightness in the area of an expected bow. But then Nature has a habit of creeping up and surprising us!