Cirrus Clouds

Cirrus clouds are white or light grey thin, wispy tails of cloud. They occur when water vapour undergoes deposition above 5,000 metres in temperate regions or 6,100m in tropical regions. They often appear before a frontal system or tropical cyclone. These types of cloud can also produce several optical effects.

Cirrus Clouds

Glories

Glories are concentric, faintly coloured rings that appear around the shadow of the observer. For this to happen, the ice crystals in the clouds need to be aspherical.

Circumhorizontal Arc

These are similar and just as common as a rainbow, but in an upside down curve. This is when the ice crystals are horizontal, flat and hexagonal. Light from the sun passes through the circumhorizontal arc causing the sunlight to refract and invert, creating a colourful, upside down arc. The sun must be at a lower altitude than 32 degrees and an angle of 20 degrees above the horizon causes the brightest of arcs. They go from blue on the inside via green, to yellow and then to red on the outside. Circumhorizontal arcs have purer colours which are more clearly separated then a rainbow as the sunlight that is refracted is almost parallel.

Circumhorizontal Arcs