Welcome to my new blog :) For my first post, I’d like to talk a little about ice halos. Here’s a paraphrased description from Wikipedia’s article:
An ice halo is an optical phenomenon consisting of colored or white arcs and spots in the sky. They are produced by the ice crystals in cirrus clouds high in the upper troposphere. The crystals behave like prisms and mirrors, refracting and reflecting sunlight between their faces. The particular shape and orientation of the crystals determines the type of halo observed.
Here is a beautiful example of multiple ice halos (photo by solarthermienator, Creative Commons license):
I first become interested in ice halos in August 2010, when I saw a beautiful one just before sunset. I now actively look for ice halos in the sky, and take photos of them whenever I can.
The most common one is the 22° halo, which is a perfect circle surrounding the sun. Here’s one I photographed yesterday (I’ve increased the contrast because the halo was rather faint):
Here is a much brighter example, and you can clearly see the rainbow colours (photo by Graken, Creative Commons license):
The parhelic circle is quite faint, so I’ve marked its path to make it clearer:
The words “parhelion/parhelic” are Greek for “beside the sun”.
A parhelion is a rainbow-coloured spot to one side of the sun, and the same height above the horizon as the sun. They can form on both sides of the sun, and they can be extremely bright (photo by Sami Köykkä, Creative Commons license):
A parhelic circle is a white circle which travels around the sky, at the same height above the horizon as the sun. Its path crosses the sun, but it usually gets fainter once its within 22° of the sun. Usually only part of the circle is seen, but occasionally the full circle is visible – see the beautiful photos here and here.
Here is another photo I took of the same halos:
and labelled to show the path of the parhelic circle:
Finally, here are a few interesting links:
- Spectacular photo of multiple halos
- Diagram of common halos
- Diagram of rare halos
- Directory of halo types
- HaloSim – software for simulating halos