March 23 is World Meteorological Day, a celebration of all-things-weather from the United Nation’s World Meteorological Organization (WMO). As a specialized agency of the United Nations, WMO is dedicated to international cooperation and coordination on the state and behavior of the Earth’s atmosphere, its interaction with the land and oceans, the weather and climate it produces, and the resulting distribution of water resources.
WMO programs facilitate and promote:
- the establishment of networks of observational stations to provide weather, climate and water-related data;
- the establishment and maintenance of data management centres and telecommunication systems for the provision and rapid exchange of weather, climate and water-related data;
- the creation of standards for observation and monitoring in order to ensure adequate uniformity in the practices and procedures employed worldwide and, thereby, ascertain the homogeneity of data and statistics;
- the application of science and technology in operational meteorology and hydrology to aviation, transport (air, land and maritime), water resource management, agriculture and other focus areas;
- activities in operational hydrology as well as closer cooperation between National Meteorological and Hydrological Services in states and territories where they are separate; and
- the coordination of research and training in meteorology and related fields.
The theme of this year’s World Meteorological Day is “Understanding Clouds”. Designed to highlight the enormous importance of clouds for weather, climate, and water, the cloud theme of this year’s focus also celebrates the inherent beauty and aesthetic appeal of clouds.
World Meteorological Day also marks the launch of a new edition of the International Cloud Atlas after the most thorough and far-reaching revision in its history. The new WMO Atlas containes hundreds of images of clouds, including a few new cloud type classifications. For the first time ever, the Atlas has been produced in digital format and is accessible through computers and mobile devices. The International Cloud Atlas can be found here.
Classification of Clouds
The present international system of Latin-based cloud classification dates back to 1803, when amateur meteorologist Luke Howard wrote The Essay on the Modifications of Clouds.
There are ten basic cloud “genera,” which are defined according to where in the sky they form and their approximate appearance. The new International 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 usually between 6,500 to 23,000 feet; and low-level clouds usually have their base at a maximum of 6,500 feet.
Most cloud names contain Latin prefixes and suffixes which, when combined, give an indication of the cloud’s character. These include:
- Alto: mid-level (though Latin for high)
- 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 International Cloud Atlas has added a new species: volutus or roll cloud (from the Latin volutus which means rolled), which occurs within the genera Altocumulus and Stratocumulus. It describes a long, typically low, horizontal tube shaped cloud mass that often appeals to roll about a horizontal axis.
Five new supplementary features have been added: asperitas, cavum, cauda (often known as tail cloud), fluctus (widely known as Kelvin-Helmholz wave) and murus (known as wall cloud).
Best known of these is asperitas, from the Latin meaning roughness. The Cloud Appreciation Society argued for a new classification to be used to describe clouds with this appearance. The Atlas includes the winning photograph from a Cloud Appreciations Society competition on Asperitas.
“Asperitas was first identified with the help of citizen science, enabled by modern technology. When Cloud Appreciation Society members send us photographs of dramatic skies from around the world, it is possible to spot patterns. This is how the proposal for a new classification came about, and we are delighted the WMO has chosen to include it in their definitive reference work for cloud classification,” said Gavin Pretor-Pinney, founder of the Cloud Appreciation Society.
A new accessory cloud, flumen, has been included. Commonly known as “beaver’s tail,” it is associated with a supercell severe convective storm.
The International Cloud Atlas also proposes five new “special clouds:” cataractagenitus, flammagenitus, homogenitus, silvagenitus and homomutatus. The suffix genitus indicates localized factors that led to cloud formation or growth, while mutatus is added when these caused the cloud to change from a different form. These special clouds are influenced by large waterfalls, localized heat from wildfires, saturation of air above forests and humans. Thus, a common example of homogenitus is contrails, sometimes seen after aircraft.