|High Sun Ice-Halo imaged in Bolivia, October '08 by Steve Hunt.
Image ©Steve Hunt, shown with permission.
Halos are visible world-wide regardless of the ground level temperature because cirrus clouds are always cold. This is a 22° halo or circumscribed halo. The two are hard to tell apart at high sun.
Both halos leave a dark 'hole in the sky' because hexagonal crystals closer than ~22° to the sun cannot refract light towards the eye. The halo rays pass between faces inclined 60° to each other and are deviated by a minimum of ~22°. However, some rays are deviated as far out as 50° from the sun. The halo has a red edge because red rays are deviated least.