International Cloud Atlas

The World Meteorological Organization in March released a new edition of its International Cloud Atlas, the authoritative worldwide reference on cloud classifications. It is the first update in 30 years and the first to be available online. Hundreds of cloud photos submitted by meteorologists, professional photographers, and cloud lovers worldwide have been selected for inclusion, with many supplemented by large-scale analyses, satellite or radar imagery, and atmospheric soundings.

Clouds are classified according to general appearance, internal shape and structure, transparency and arrangement, supplemental features, and special nature. To the previous total of 42, the updated cloud atlas adds 12 new classifications, including volutus (roll cloud), cataractagenitus (formed near waterfalls), and asperitas (from the Latin for roughness).

This photo, by Gary McArthur, shows altocumulus stratiformis opacus asperitas clouds near Burnie, a port on the northwest coast of Tasmania. The winner of the Cloud Appreciation Society asperitas photo contest, it clearly captures the feature’s defining aspects: Well-defined, localized waves in the cloud base that resemble a roughened sea surface viewed from below. Varying levels of illumination and thickness of asperitas clouds can lead to dramatic visual effects.

The cloud atlas is available at http://www.wmocloudatlas.org .

To submit candidate images for Back Scatter visit http://contact.physicstoday.org .