Correct Option
A solar halo is an optical phenomenon primarily caused by the refraction and reflection of sunlight through hexagonal ice crystals. These ice crystals are typically found in high-altitude cirrus clouds, which form above 6,000 meters and are composed entirely of ice. The specific orientation and crystalline structure of these ice particles bend the light at a 22-degree angle, creating the characteristic ring around the Sun or Moon.
Incorrect Options
(a) Water vapour in Stratus clouds: Stratus clouds are low-level clouds composed mainly of water droplets, not ice crystals. Halos require the specific refractive properties of ice crystals, which are absent in water droplets at these altitudes.
(b) Ice crystals in Cirro-cumulus clouds: While cirro-cumulus clouds are high-level and can contain ice crystals, their broken, patchy structure, consisting of small, rounded masses, is generally not conducive to forming the distinct and consistent halo phenomenon. Cirrus clouds, with their more uniform distribution of ice crystals, are the primary cause.
(d) Dust particles in Stratus clouds: Dust particles primarily cause light scattering phenomena, such as haze or the reddish appearance of sunsets, rather than the refractive effects that produce halos. Furthermore, stratus clouds are low-level and do not contain the ice crystals necessary for halo formation.