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Thermodynamics

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Haloes

When the sun or moon is seen through a thin veil of high cloud, haloes are formed. The common halo is the ring known as 22˚ halo. The radius of the ring subtends an angle of 22˚ at the eye, with the sun or moon at the centre. The haloes are due to the refraction of light through a cloud of icy crystals which are in the shape of hexagonal prisms. Large number of these prisms are present in a cloud and the haloes are formed. For ice, = 1.31 and taking A = 60˚ and applying the relation

The value of δ = 22˚ which is the angle of minimum deviation and also the angular radius of the halo. The inner edge is red and the outer edge is blue but the colours are fainter than the colours of a rainbow. As the prisms are oriented in all possible directions in a cloud, the light rays are incident in all possible directions but the emergent rays are more concentrated near the minimum deviation position. This is due to the fact that the angle of deviation changes very slowly with the angle of incidence near the position of minimum deviation.

The 46˚ halo or larger ring is also sometimes observed but it is fainter than 22˚ halo. It is formed due to refraction of light rays in ice prisms with refracting angles 90˚. Taking the angle of the prism = 90˚ and for ice = 1.31, the angle of minimum deviation is calculated by applying the relation

The value of δ = 46˚ and this is the angular radius of larger halo.