Squaw Valley, California, 12 May 2000

Photographs and observations by Martha Dawson.

The display occured at Squaw Valley, CA (east of Fresno, west of Kings Canyon/Sequoia National Park) at about 1 pm and lasted close to 2 hours. Several photographs were taken and used as input for the simulations.

The halo was distinguished by:

  1. The circumhorizon arc - which only appears when the Sun has an altitude greater than about 58 degrees. This is produced by refraction through 90 degree prism faces of ice crystals in the form of hexagonal plates floating with their end faces approximately horizontal (symmetry axes vertical, ie, like dinner plates on a table)
  2. A combination of the 22 degree halo (circular) - produced by randomly oriented hexagonal crystals - and the circumscribed halo (oval) - produced by rod-like hexagons floating with their long axes approximately horizontal (like horizontal pencils). This combination makes a halo which is slightly wider than it is tall and has fuzzy sides and a sharper top and bottom. It is coloured - but not as strongly as (1).
  3. A trace of the parhelic circle - produced by a wide variety of ray paths in hexagonal crystals, all of which are effectively reflections. These are either single reflections from hexagon faces or internal reflections of rays which enter and leave the crystal through faces which are parallel to one another.

Simulations were made with a beta(pre)-release version 3 of HALO which computes directly in colour. A mixture of randomly oriented hexagons, horizontal rods and Parry-oriented crystals was used.

The position and size of the parhelic circle is very sensitive to the solar altitude and observations of it can be used as a clock accurate to about 10 or 15 minutes. I have made a movie of a series of short simulations. It runs from 66 to 74 degrees in steps of 1 degree and finishes with 72 degrees (the best fit) and then the overlay of the photograph.

On the 12 May, the solar altitudes at 119.5W, 36.5N were:

Local time              solar altitude
12h                             68d 16m
13h                             71d 50m
14h                             66d 50m
15h                             57d 00m

Last update: 27 February 2001

Go to Bob Fosbury's homepage