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The Asperitas Cloud and World Meteorological Day (23 March)

The newly classified Asperitas cloud spotted over Burnie, Tasmania, Australia, by Gary McArthur (Member 5353)

The Asperitas Cloud and World Meteorological Day (23 March)

World Meteorological Day on Thursday 23 March is themed, this year, on ‘Understanding Clouds’. It marks the publication of the latest edition of the World Meteorological Organisation’s definitive reference work on cloud classification: the International Cloud Atlas. This official resource for cloudspotters includes, for the first time, the ‘Asperitas’ cloud. It is a new classification of cloud, with a chaotic, turbulent appearance, that was proposed by the Cloud Appreciation Society back in 2008, based on photographs sent to us from members all around the world. It is a classic example of citizen science, in which observations by the general public, enabled by the technology of smartphones and the Internet, have influenced the development this most official of classification systems.

Asperatus clouds over the Lofoten Islands, Norway. © Ragnhild M Hansen
Asperatus spotted over the Lofoten Islands, Norway, by Ragnhild M Hansen (Member 27985)

First published in 1896, the International Cloud Atlas was based on the Latin naming system for clouds that was proposed in 1802 by Luke Howard. Howard gave definitions for cloud types such as Cumulus, Stratus and Cirrus. By the time the first edition of the Atlas came out, the naming system had expanded to ten main types, or ‘genera’, of clouds. And it has been revised and added to each time a new edition was launched every few decades or so.

The 2017 edition on the International Cloud Atlas is being published online. Besides our new Asperitas classification, it includes a number of new Latin terms for cloud formations that were hitherto known just colloquial names such as the breaking-wave shaped ‘Kelvin-Helmholtz cloud’ and the hole-punch shaped ‘fallstreak hole’. These will now become known by the Latin terms ‘fluctus’ and ‘cavum’ respectively.

Asperitas clouds over north Dorset, UK.
The chaotic waves of Asperitas are sometimes interspersed with what look like peaks of meringue. Spotted north Dorset, England, by Jo Adams.

Ever since we first noticed distinctive turbulent waves of cloud back in 2006 in images sent from Cedar Rapids, Iowa, US, we have argued that this formation did not easily fit within the existing naming system. So we are very pleased that now, almost ten years later, Asperitas is finally being accepted as an official classification by the World Meteorological Organisation.

Gavin Pretor-Pinney, Member 0001, is giving a talk to the WMO in Geneva, Switzerland, on World Meteorological Day to mark the launch of the new online edition of the International Cloud Atlas. The whole event is being streamed live on the WMO Facebook page and the WMO website. The event begins at 1.30pm (UTC) on 23 March, with Gavin’s talk happening at 2.30pm (UTC). We will post a link here to the online resource when it becomes available to the public.


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