October Ice Halo

Photo by Donald F. Collins

Ever wonder what causes the halo around the sun or moon? This phenomenon is fairly common and is caused by the prism effects of millions of ice crystals that compose the cirrus clouds. This event, however, is highly unusual in that the cirrus clouds caused three types of effects: 1) the circular halo - it looks like a rainbow ; 2) a sun dog - the colored image of the sun as a bright spot in the lower left; and 3) a tangential arc - a second curve near the top of the circular halo. The sun was deliberately blocked by the trees to prevent over-exposure in the camera. The image was also contrast and color enhanced to make the features more visible.

The circular halo is formed by tiny, pencil-like ice crystals as shown in the drawing to the left (Diagram from the University of Illinois WW2010 Project.) The needle-like ice crystals are all falling in the sky and oriented horizontally - because that is how things fall due to air friction. The horizontal orientation, however, is random, so any crystal that forms the proper angle relative to the observer and the sun bends the sunlight at a 22 degree angle, thus forming a circle 22 degrees from the sun.

The sundog or bright image of the sun in the lower left of the photo (also called a "parhelion") is caused by refraction by hexagonal platelets of ice as opposed to hexagonal needles. See the diagram at right. Notice that the sundog is not quite coincident with the 22 degree sunhalo. This is a 3-D geometry effect in that the hexagonal plates at the elevation angle in the sky relative to the observer are inclined to the observer - they are still parallel to the ground, however. Sun dogs are most frequently seen near sunset, the sun dogs on either side of the setting sun. The hexagonal ice crystal plates are then parallel to the observer's line of sight. A further complication is caused by internal reflection of the rays of light in the ice crystals. (Diagram by Donald Collins).

The drawing below shows the hexagonal cross section for an ice crystal. It behaves optically very similarly to a common triangle crystal or prism. The vertices of the triangle have been "Cleaved" off to form the hexagon. The heavy arrow shows the optical path of a light ray through the prism-like crystal. The colors also become separated when the light traverses a prism (not shown here) but quite apparent in the photograph. (Diagram by Donald Collins).

For ice crystals in the atmosphere, the orientation of the hexagons is random. The flat plate crystals tend to fall through the air keeping the flat sides horizontal (the way leaves fall), or the hexagon rods fall with the long axes horizontal, but the rotational orientation of the hexagons is random and the direction of the long axes of the hex rods is in any horizontal dirrection. One would think that this would smear out any patterns or arcs. However, in the drawing below, if the orientation of the hexagon is any but the symmetrical case, the deflection of the light ray is greater. This angle of deflection is a minimum (22 degrees). For crystals too close to the sun, they bend the light a greater amount, hence the sky appears dark for the crystals too close to the sun. To the observer, only the crystals that are oriented at the proper angle for the minimum deflection (shown above) contribute to the halo or sundog.

An excellent source on the Internet for explanations of ice haloes may be found in this page by by Les Cowley. An excellent textbook is Rainbows, Haloes, and Glories by Robert Greenler, Cambridge University Press, 1980.

Physics Photo of the Week

Warren Wilson College

March 25, 2005