Subsun (or undersun)

(Section 3.2.3.1.8)

This is produced by the reflection of sunlight on plate-shaped ice crystals in clouds. It appears vertically below the Sun as a brilliant white spot, similar to the image of the Sun on a calm water surface. The subsun can only be viewed when looking downward, and so is only observed from aircraft (in Cirrostratus) or in the winter in the mountains (diamond dust).