Look up! There are some “new” clouds in the sky. The World Meteorological Organization (WMO) – the UN’s weather agency – announced twelve new additions to its prestigious International Cloud Atlas on Thursday.
First published in 1896, the atlas is considered the most authoritative and comprehensive reference volume for identifying clouds. This is the first time it has been updated in thirty years.
Clouds, like flora and fauna, have an official classification system. There are ten different genera, which are defined by altitude and appearance. These are then subdivided into species based on a cloud’s shape and structure. Within these, there are varieties that describe the arrangement and transparency of different clouds. Whittling things down even further, there are also supplementary features/accessory clouds that merge with or attach to the main cloud body. In total, there are about one hundred combinations.
This new version of the Cloud Atlas recognizes one new species called volutus, but it is more commonly known as a roll cloud. This tube-shaped cloud appears to roll around a horizontal axis and is typically associated with the leading edge of a thunderstorm. But, on occasion, advancing cold fronts can also trigger their formation.
Six new supplementary/accessory features were also added. For avid sky-watchers, however, they are already widely known by their common names. These include:
- asperitas (wave-like cloud)
- cavum (fall streak cloud)
- cauda (tail cloud)
- fluctus (Kelvin-Helmholtz wave cloud)
- murus (wall cloud)
- flumen (inflow cloud band)
Furthermore, five new “special clouds” were also part of the update. These form because of unique localized factors, including human activity such as exhaust from jet engines.
Of all these new additions, the asperitas (formerly known as undulatus asperatus) has garnered the most attention. These low-level clouds are caused by weather fronts that create rolling waves in the atmosphere and resemble the underside of a turbulent sea. It was first photographed in 2006 by a cloud-watcher in Iowa. Then in 2008, after several other sightings around the world, the Cloud Appreciation Society, an international group of cloud enthusiasts, began to lobby the WMO to acknowledge it as a new cloud type.
Available in digitized form for the first time, the WMO hopes this new edition of the International Cloud Atlas will help to increase public understanding of the critical role clouds play in the atmosphere. “If we want to forecast weather we have to understand clouds. If we want to model the climate system, we have to understand clouds. And if we want to predict the availability of water resources, we have to understand clouds”, says WMO Secretary-General Petteri Taalas.
The WMO cloud classification system can be traced back to Luke Howard, the so-called father of meteorology. In 1803, he published “The Essay on the Modifications of Clouds” which organized its then nebulous subject using a Latin nomenclature.
Asperitas Cloud. Credit: WMO