22°-halo (EE01)

The 22° ring is a light ring with a radius of 22° around the sun. Its reddish-brown inner edge is relatively sharply defined. The outer edge, on the other hand, is white and diffuse. The 22° ring is also called a small ring. For someone not familiar with halo phenomena, however, it already appears enormously large. As a guide, you can say that 22° roughly corresponds to the angle formed by the thumb and little finger when the hand is extended and spread. Often the ring is not completely visible. Frequently only the upper part appears. To avoid being blinded by the sun while observing, one should position oneself so that the sun is obscured by an object (e.g., streetlamp).

The small ring is the most common halo phenomenon. It can almost always be seen when, after several days of sunny weather, a low-pressure area approaches from the west. As a precursor to the low, cirrostratus clouds and cirrus clouds cover the sky. These allow sunlight to pass through and act like a kind of white misty veil. The cirrostratus clouds are located at a very high altitude. There, the temperature is so low that water crystallizes into ice on dust particles. The ice crystals usually have the shape of hexagonal prisms. In them, sunlight is refracted like in a prism at 60°. Many millions of these ice prisms are in the cirrostratus layer. If the ice prisms are about as long as they are wide, or the atmosphere is very turbulent, the crystals take on an arbitrary position in space. With randomly oriented crystals, sunlight is refracted in all possible directions. However, at the angle of 22° around the sun, there is strong brightening because, at this angle, the direction of the refracted light beam in the rotating prism changes more slowly. This angle is also called the minimum deviation. The light enters one prism face of the crystal and exits at the next but one prism face. Sunlight is refracted both upon entry and exit. For red light, the refractive index is lower, resulting in the red inner edge of the 22° halo. The formation of the 22° ring is not dependent on the sun's height. On some days, it can be visible for several hours.