III. Weather Phenomena

5. Halo


When there are cirrostratus clouds in the sky around the sun or the moon, every once in a while there will be one or two colorful circles of light surrounding the sun or the moon. The colors are red on the inside and violet on the outside. Sometimes there will be some colored or white light spots or light arcs. This type of optical phenomena of light circles, light spots and light arcs is called halo phenomena.

There are seven colors in a halo (red, orange, yellow, green, blue, indigo, and violet) because of the refraction and reflection of the light as it passes through the ice crystals, as shown in Fig. 3. When the light enters at point A into the ice crystals it will be refracted twice, and is dispersed to a different spectrum of light. If the light is only reflected by the ice crystals once, then the halo will be white and not colored. When the halo is surrounding the sun, it is called a solar halo; when it is surrounding the moon it is called a lunar halo.

Fig. 3: The formation of halo and 22° halo.

The formation of halo and 22° halo

Due to the complex combination of different types, orientation, movement patterns of the ice crystals and the angle of the sun, there can be many types of halos. Most of them have been observed. The most common one is the 22° inner halo (also known as the small halo), which means that the visual radius (the angle of vision lines between the sun or moon and the solar or lunar halo) is 22°. The small halo is dark red on the inside and blue or slightly violet on the outside. The part of sky surrounded by the halo is darker and greyer than the outside. When the visual radius of a halo is 46° then the halo is called an outer halo (or large halo). The large halo is darker in color and is rarer.