The most ethereal looking of all the main types, Cirrus clouds are also the highest – composed entirely of ice crystals. These typically fall through the high winds of the upper troposphere to appear as delicate, celestial brush strokes, known as ‘fallstreaks’.
Cirrus often look like white locks of hair (from which the Latin name is derived). Cirrus clouds thickening and spreading across the blue can be the first signs of moisture developing at high altitudes, indicating the start of a common cloud progression that leads to Nimbostratus and rain or snow in a day or so.
Apart from when it is very thick, and known as Cirrus spissatus, Cirrus can sometimes refract and reflect the sunlight to produce coloured arcs and rings known as halo phenomena.
Image: Spotted over The Rocks, City of Sydney, New South Wales, Australia by Tania.