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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 2019 was themed on ‘Understanding Clouds’. It marked 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 were very pleased that, almost ten years later, Asperitas was finally accepted as an official classification by the World Meteorological Organisation.

Gavin Pretor-Pinney, Member 0001, gave 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.

  • Margaret Harwood avatar


    March 21, 2017 at 8:44 am

    Well done Gavin, took a long time phew!! Will watch the momentous occasion on line
    Margaret and Richard Harwood

  • Gavin Pretor-Pinney avatar

    Gavin Pretor-Pinney

    March 24, 2017 at 11:16 am

    Thanks Margaret and Richard! It was great fun going over to the WMO in Geneva as guest speaker to give the talk to mark World Meteorological Day.

  • Tamika Blyther avatar


    March 25, 2017 at 10:33 pm

    I love watching the changing cosmic transformation of the clouds.I have always been drawn to the marvelous beauty of the sky.I always say I must have been cloud born.So happy to have found such a great society.

  • Laurence Green avatar

    Laurence Green

    March 27, 2017 at 4:34 pm

    Dear Gavin and all at Cloud Appreciation Society

    Well done in no small way! CONGRATULATIONS!!!!!!!!!

  • Laurence Green avatar

    Laurence Green

    March 27, 2017 at 4:42 pm

    Dear Gavin, Ian, and all at Cloud Appreciation Society

    Well done in no small way! CONGRATULATIONS! Big time!

    Marvellous that, at long, long last, the Asperitus cloud formation has been officially recognised.

    I, with a couple of friends, tonight will be opening and enjoying a bottle wee bit of bubbly tonight to celebrate.

    Great news! Well done!


  • Roberval De Oliveira Santos avatar

    Roberval De Oliveira Santos

    March 28, 2017 at 9:34 pm

    Congratulations, Gavin !
    Very well done ! I hope one day I can also contribute with our Society on discovering new clouds.
    By the way, I’ve just read an article about the WMO event on a brazilian newspaper (Folha de São Paulo) a translation to portuguese published by BBC Brasil. The link is:

    Best Regards


  • Frances Macdonald avatar

    Frances Macdonald

    March 30, 2017 at 5:57 pm

    Wonderful news, well done. I ?am a bit confused as to the name, is it asperatus or asperitas? I am sure it was originally asperatus, but it seems to have changed and I have missed it. When you say it, is it asperAtus or aspEratus? Not that it is really important, however you say it, it is just a stunning cloud.

  • Joan H. Laurino avatar

    Joan H.

    April 7, 2017 at 11:04 pm

    Well, well, well, I had my own Asperatus venture right here at Ocean Beach, San Francisco, CA. USA. We hardly get these clouds, and when watching the skies (storm was coming in) during my power walk, I noticed strange clouds forming. Later, sure enough, there were the Asperatus! Although not as exciting as the other photos; however, for S.F., here, it was a windfall. No champagne here, but a glass of wine did celebrate. :)

  • Frances Macdonald avatar

    Frances Macdonald

    April 10, 2017 at 10:55 am

    asperatus or asperitas ?

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