Ice halos are one of my favorite topics. It's always been very intriguing how certain clouds in the sky could form a distinct ring around the Sun or the Moon. Thanks to explanations by Robert Greenler (Rainbows, Halos, and Glories, Cambridge Univ. Press, 1980) the halo is caused by ice crystals that are found in high-level cirrus clouds.
The ice crystals responsible for ice halos are small
hexagonal prisms (the same shape as an ordinary wooden
hexagonal pencil, but much smaller. These hexagonal
needle-like ice crystals are falling in the upper
atmosphere, but the air friction in falling (like
snowflakes) causes them to fall relatively slowly.
Another major phenomenon of the air friction is that all the
crystals are horizontal. The horizontal orientation is
the most stable when falling through the air. Think of
confetti falling: the confetti tends to fall in a horizontal
orientation. Even though the ice needles are all
horizontal, they are oriented in all horizontal directions,
and at all rotations along the long axis of the
crystal. The hexagonal shape of the crystals refracts
and bends light from the Sun or Moon similar to a triangular
The picture above shows various orientations of the hexagon prisms (rotated about the long axis) and the paths of sunlight affected by the prisms. Sunlight enters from the left in each prism. The various orientations of the prisms cause the light to emerge from another side having been deflected an angle D from the original direction. The paths are calculated from the laws of refraction. In the left orientation, the light is deflected the least. The other two orientations create a larger deflection. The minimum deflection angle creates the halo at an angular size of 22 degrees from the Sun or Moon - the smallest D angle from the left orientation).
Whenever the sky is somewhat hazy, take a look at the
Sun (shield your eyes by standing in the shadow of a tree or
building) and you may see many more of these wonderful
phenomena. This week's photo was taken last Sunday
afternoon on April 1, 2012.
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to firstname.lastname@example.org.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.