A 22° halo is an optical phenomenon that belongs to the family of ice crystal halos , in the form of a ring with a radius of approximately 22° around the Sun or Moon . When visible around the moon it is called a moon ring or winter halo. It forms as the sun- or moonlight is refracted in millions of hexagonal ice crystals suspended in the atmosphere. [1] The halo is large; the radius is roughly the size of an outstretched hand at arm's length. [2] A 22° halo may be visible on as many as 100 days per year—much more frequently than rainbows. [3]

Even though it is one of the most common types of halo, the exact shape and orientation of the ice crystals responsible for the 22° halo are still the topic of debate. Hexagonal, randomly oriented columns are usually put forward as the most likely candidate, but this explanation presents problems, such as the fact that the aerodynamic properties of such crystals leads them to be oriented horizontally rather than randomly. Alternative explanations include the involvement of clusters of bullet-shaped ice columns. [4] [5]

As light passes through the 60° apex angle of the hexagonal ice prisms it is deflected twice resulting in deviation angles ranging from 22° to 50°. The angle of minimum deviation is almost 22° (or more specifically 21.84° on average; 21.54° for red light and 22.37° for blue light). This wavelength-dependent variation in refraction causes the inner edge of the circle to be reddish while the outer edge is bluish.

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