Cirrocumulus (Cc) - the name is a composite of the Latin cirrus = wisp or curl and cumulus = small heap. Cirrocumulus is a member of the ten fundamental cloud types (or cloud genera). Cirrocumuli are high-level clouds appearing as a formation of small masses of white or pale blue, fleecy clouds in a globular, wavelike, rippled pattern, usually in regularly arranged groupings. However, unlike cirrus or cirrostratus they are always broken up into tiny elements and ripples. They form at altitudes between about 5 and 10km (about 16,600 and 30,000ft).
Cirrocumuli are often inconspicuous and overlooked, both because it is thin, and also because the individual cloud elements are small. More over they are more distant than the similar lower clouds, altocumulus and stratocumulus. In fact, there is a relationship among these three cloud genera: the thicker the cloud elements and cloud layer, the higher the cloud. Sheets of cirrocumuli are often very thin, always transparent and extremely low in contrast and difficult to see against the sky. Sun or moon are always visible and objects on the ground still cast shadows. Cirrocumulus elements normally show no shading, in contrast do the darker shading thicker, lower altocumulus.
Cirrocumulus are composed of highly
supercooled water droplets (which tend to be converted into ice crystals very rapidly), ice crystals, or a mixture of both. The even-sized cloud particles may produce
coronae and iridescence around the Sun or Moon.
Cirrocumulus form by shallow convection within thin labile layers of air, or by lee-effect lifting. Thickening of cirrus might form cirrocumulus occasionally. More frequently they arise from a layer of cirrostratus, or decaying altocumulus.
Common types and varieties of cirrocumulus:
Cc
stratiformis in the form of an extensive sheet or layer; Cc
floccus and
castellanus as tufted and turreted cloudlets; Cc
undulatus are showing ripples, waves and billows. If these ripples are lense-shaped with sharp edges, they belong the CC
lenticularis species. Cc
lacunosus are characterized by large clear holes in the sheet of cirrocumulus.
What do cirrocumuli tell about the weather?
Fields of Cc castellanus are a good indicator for shallow upper air convection and might be a good indicator for an afternoon or evening thunder. Cc undulatus from west, thickening into altocumulus are heralding an approaching warm front and possibly rain within the next 18 to 36 hours.
