The halo is caused by the refraction of light passing through ice crystals in the atmosphere. there are two types, a 22 degree halo:
A 22? halo is a rather frequently appearing halo, an optical phenomenon forming a circle 22? around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere. As the light beam passes through two sides of the prism forming a 60? angle, the angle of minimum deviation is almost 22? (e.g. 21,84? in average; 21,54? for red and 22.37 for blue.) This wavelength-dependent variation in refraction causes the inner edge of the circle to be reddish while the outer edge is bluish.[1] A 22? Halo may be visible on as many as 100 days per year.[2]
Pathway of light through a hexagonal prism in the optimal angle resulting in minimum deviation.Light passing through the hexagonal ice prisms is deflected twice which produces deviation angles ranging from 22? to 50?. Lesser deviation results in a brighter halo along the inner edge of the circle, while greater deviation contribute to the weaker outer part of the halo. As no light is refracted at smaller angles than 22? the sky is darker inside the halo. [3]
A 22? halo is a rather frequently appearing halo, an optical phenomenon forming a circle 22? around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere. As the light beam passes through two sides of the prism forming a 60? angle, the angle of minimum deviation is almost 22? (e.g. 21,84? in average; 21,54? for red and 22.37 for blue.) This wavelength-dependent variation in refraction causes the inner edge of the circle to be reddish while the outer edge is bluish.[1] A 22? Halo may be visible on as many as 100 days per year.[2]
A 46? halo is a rare and huge halo, together with the smaller 22? halo a circular optical phenomenon centred on the sun. At sun elevations between 15-27?, it is often confused with the more colourful and frequently observed supralateral and infralateral arcs. It is named for crossing the parhelic circle 46? from the sun.
46? halos are similar to but much broader and much fainter than 22? halos. They form when sunlight enters randomly oriented hexagonal ice crystals through a prism face and exits through a hexagonal base.[1] The 90? inclination between the two faces of the crystals causes the colours of the 46? halo to be more widely dispersed than those of the 22? halo. In addition, as a lot of rays are deflected at larger angles than the angle of minimum deviation, the outer edge of the halo is more diffuse.[2]
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