Luke Howard gave clouds their names, but a global language needed global agreement. In 1887 the Swedish meteorologist Hugo Hildebrandsson and the British meteorologist Ralph Abercromby proposed a system with ten basic cloud types, based on Howard's forms and on years of observation around the world.
The first International Cloud Atlas followed in 1896, with photographs and colour plates, so that observers everywhere could match what they saw to the same pictures and words. A sailor in the Pacific and a station keeper in Norway could now report the same cloud in the same terms.
Those basic types, now called genera, still number ten: cirrus, cirrocumulus, cirrostratus, altocumulus, altostratus, nimbostratus, stratocumulus, stratus, cumulus and cumulonimbus. Add species, varieties and supplementary features, and the vocabulary grows much richer.
The atlas is now maintained by the World Meteorological Organization, and in 2017 a new digital edition was published. It added several new features, among them one called asperitas, a name for the rolling, wave-like undersides that can make a sky look like a stormy sea seen from below. It was a rare moment when a new cloud name entered the official book, and the story of how it got there is the next stop on our tour.
