On World Meteorological Day (23rd March 2017), the World Meteorological Organization (WMO) released its new, online, digitised International Cloud Atlas, which is the global reference for observing and identifying clouds. It contains a number of new cloud classifications, including the eagerly-awaited asperitas, a dramatic undulated cloud which has captured media interest and public imagination. The RMetS originally reviewed examples of asperitas provided by Gavin Pretor-Pinney of the Cloud Appreciation Society.
The present international system of Latin-based cloud classification dates back to 1802, when amateur meteorologist Luke Howard wrote his essay on the Modification of Clouds. There are ten basic cloud ‘genera’ which are defined according to where they form in the sky and their appearance. The new Cloud Atlas has made no additions to these 10 genera. High-level clouds typically have a base above about 16,500 feet; middle-level clouds have a base that is between 6,500 to 23,000 feet; and low-level clouds usually have their base at a maximum of 6,500 feet.
- Alto: mid-level
- Cirrus/cirro: feathers, wispy
- Cumulus/cumulo: heaped up/puffy
- Nimbus/nimbo: rain-bearing
- Stratus/strato: flat/layered and smooth
The 10 genera are subdivided into ‘species’, which describe shape and internal structure, and ‘varieties’, which describe the transparency and arrangement of the clouds. In total there are about 100 combinations.
The new cloud descriptions
- Volutus: A long horizontal cloud shaped like a tube. It is typically low in the sky, detached from other clouds and often appears to roll.
- Asperitas: A well-defined wave-like structure in the underside of a cloud with the look of a roughened sea surface viewed from below.
- Fluctus: A relatively short-lived wave formation, usually on the top surface of a cloud, in the form of curls or breaking waves (Kelvin-Helmholtz waves).
- Cavum: Commonly known as a ‘fallstreak-hole’, or ‘hole-punch cloud’ (see tWC Issue 8).
- Murus: Commonly known as a ‘wall cloud’, this a localised, often abrupt lowering of cloud from the base of a Cumulonimbus from which vortices such as funnels and tornadoes sometimes form.
- Cauda: A horizontal, tail-shaped cloud that extends at low level from the main precipitating part of a super-cell Cumulonimbus. It is typically attached to the ‘wall cloud’, murus.
- Flumen: Bands of low cloud that move towards the updraft of large severe storm clouds. They are organised along the air inflow into the super-cell storm cloud and are not attached to murus.
- Flammagenitus: Cloud formed as a result of rising air currents initiated by the heat from forest fires, wildfires or volcanic activity.
- Homogenitus: Cloud formed as a consequence of human activity; for example aircraft condensation trails (contrails), or clouds generated by rising thermals above power stations.
- Homomutatus: Cirri-form clouds, resulting from the spreading out of aircraft contrails (Cirrus homogenitus) which, over time transform into larger areas of more natural-looking cirri-form cloud.
- Cataractagenitus: Clouds developed locally in the vicinity of large waterfalls due to rising air.
- Silvagenitus: Cloud developed locally over forests as a result of increased humidity due to evaporation from the tree canopy.