On Tuesday, there was overrunning of cirrus over the Lake. On Wednesday, there was rain. Lorraine Symmes took a picture to the west and sent it to me. It showed a halo complex, something that often precedes rain.
All of the optical phenomena in the picture are the result of tiny ice crystals in the sky bending the sunlight. While popular consensus is that ice crystals falling from the sky are uniformly the beautiful stellar crystals, most are not. Most crystals are either simple hexagonal plates or columns. And these fall differently. The small hexagonal plates fall nearly horizontal rather like dinner plates spread haphazardly on a table. Small columnar crystals fall at right angles to the plates, rather like endless pencils spread on the table. This halo complex is made of both types of crystals: plates and columns.
We start at the top of the picture.
Circumzenithal arc: This is caused by the refraction of the sunlight through the 90° edges of the oriented hexagonal plates. This one is a tad faint, but is the portion of the circle with the zenith (off the frame) at its centre.
Supralateral arc: This arc is tangent to the 46° halo with which it is often confused. It touches the circumzenithal arc but is curved down. This is caused by the refraction of the sunlight through the 90° edges of the oriented columns. It is quite bright in this picture and its centre approximately on the sun.
Upper tangential arc: The optical phenomena in the lower portion of the picture appears separate, but is caused by the same types of crystals. The upper one appearing like a graceful bird’s wings is the upper tangential arc. This is caused by the refraction of the sunlight through the 120° edges of the oriented columns. The 120° is accomplished by light passing through alternate sides of those separated by 60°. This arc evolves quickly depending upon the sun’s height.
22° halo: This is the most common of all the phenomena. It is located at an angle of 22° from the sun. Like the others, it is the result of refraction but of randomly-oriented small plates or columns. It is caused by refraction though 120° prisms.
A halo complex usually presages rain.
Photograph courtesy Lorraine Symmes.
Decorative or functional?
Almost all sites that treat the Great Blue Heron give a passing reference to the plumes that extend from the back of the head. When mentioning them, one site explicitly says they are decorative.
Here is a picture of the Great Blue Heron that was taken in November and probably hatched that year. Consequently the dome of the head had not gone completely white and the upper mandible is blue. Nevertheless, the feather plumes have already grown down the back of the head.
This is yesterday’s heron. The adult’s white cap is visible and the upper mandible has become the breeding colour of yellow. The feather plumes extend further down the back of the neck. The question is: Are they functional or decorative? Certainly they look decorative, but then the bird is not flying.
This bird then flew off and the function of the plumes became evident. They provided a smooth flow of air from the head across the back. Recall, that the Great Blue Heron flies with its neck tucked against its back. Not every large bird has such a flexible neck. The crane must fly with its neck extended. Now, look at the plumes coming off the back of the head. The air flows smoothly from the head and along the back making it one continuous surface and thus minimizes drag. The plumes are not decorative, they are functional.
