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Asperitas Appearing in Hong Kong

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Educational Resources

Asperitas Appearing in Hong Kong

November 2015

figure 1

Figure 1   Photo by Leung Ping Wai

A spectacular cloud feature appeared in the sky over Hong Kong around noon on 23 April 2015. It is a rather rare cloud type known as "asperitas". Although asperitas had been observed in various forms at other places in the world, the above photo was very likely the first one captured in Hong Kong. In 2009, the Cloud Appreciation Society of the United Kingdom proposed that asperitas should be added as a separate new cloud type. While the formation mechanism of asperitas clouds has not yet been fully known, the World Meteorological Organization (WMO) plans to include it in the forthcoming update of the International Cloud Atlas. This will be the first new cloud type to be officially recognized by WMO since 1951.

In general, asperitas has the following basic characteristics:

  1. It has well-defined, wave-like structures in the underside of the cloud. It is characterized by localized waves in the cloud base, either smooth or dappled with smaller features, sometimes descending into sharp points, as if viewing a roughened sea surface from below.
  2. It is more chaotic and with less horizontal organization than undulatus.
  3. Varying levels of illumination and thickness of cloud can lead to dramatic visual effects.
  4. Asperitas occurs mostly with stratocumulus or altocumulus.


  1. Some typical photographs of asperitas clouds can be accessed from the following webpage: