The new International Cloud Atlas

By Stephen Burt

Automatic weather stations (AWS) are increasingly commonplace in meteorological reporting systems: today more than half of all surface observations come from sites that are partly or fully automated. AWS have many advantages, particularly in remote sites, or providing observations outside of normal working hours (for instance, almost all UK night-time synoptic observations are now made by AWSs). Despite advances in computer technology and image recognition systems, however, automatic observations of cloud amount and types are still pretty much hopeless, and are best left to manual observing sites. So how do we ensure global consistency in cloud reporting? Is cumulus humilis reported in the observation from Shanghai the same type of cloud reported from Los Angeles International Airport?

While cumulus, stratus and cirrus are very easy to distinguish, how are observers trained to tell the difference between cumulus fractus, cumulus humilis, cumulus mediocris and cumulus congestus? The differences are very important – one can be a sign of fair weather and very limited convective activity, another in this series denotes deeply unstable conditions, with showers or thunderstorms likely within the next hour or two. Of course, the answer has to be some kind of visual reference aid for observers – and the latest WMO version of the International Cloud Atlas has just been published on the web. It includes hundreds of photographs of clouds and related phenomena in full colour, including for the first time entries for lightning, contrails, rainbows, solar haloes, rime ice and noctilucent clouds.

The first published classification of clouds only dates back to the beginning of the 19th century and was the work of Lamarck (1802). This celebrated French naturalist did not set out to classify all possible clouds; he confined himself to distinguishing certain forms which seemed to him to be the manifestation of general causes which it would be useful to recognize. But this work, in spite of its real value, did not make any impression even in France and his nomenclature does not seem to have been used by anyone. Perhaps this was due to his choice of somewhat peculiar French names which would not readily be adopted in other countries or perhaps the paper was discredited through appearing in the same publication (Annuaire Meteorologique), as forecasts based upon astrological data.

The real breakthrough came about later that same year, when London chemist and amateur meteorologist Luke Howard published a cloud classification entirely independently of Lamarck’s which remains very largely the one we use today. Whereas Lamarck contented himself with defining and naming a certain number of interesting forms, Howard set out to establish a complete classification covering all possible cases and, importantly, began to suggest the physical mechanisms behind their formation – heap clouds (cumulus), layer clouds (stratus), and fibrous (ice) clouds (cirrus). From these three basic types, all others were derived by transition or association. His original paper On the modification of clouds was published early in 1803, and influenced artists (John Constable owned a copy), poets (Goethe was a great admirer) and the general public as well as the embryonic science of meteorology; it included exquisite watercolours of each of his main cloud types. His original artwork is held in the Science Museum on behalf of the Royal Meteorological Society.

The first photographic Cloud Atlas was published in 1896. It was based upon the work of a Swede (Hildebrand Hildebrandsson) and a titled English aristocrat, the Hon. Ralph Abercromby, who published a classification of clouds reconciling existing customs and including later acquisitions, such as altocumulus, altostratus and cumulonimbus. The super-rich Abercromby had previously made two journeys round the world in order to assure himself that the cloud forms were the same in all parts – well, that was his excuse, anyway, and why not indeed? One of the principal characteristics of this classification was the importance attached to height as a criterion, and they grouped the clouds in four levels, the mean heights of which they fixed provisionally from measurements made in Sweden (of course, this was long before radiosondes and aircraft measurements). The International Meteorological Conference, held at Munich in 1891, expressly recommended these authors’ classification and gave its sanction to the appointment of a special committee entrusted with its final consideration and publication with illustrations in atlas form. This committee met at Uppsala in August 1894 and proceeded to choose the illustrations to be reproduced. With this object in view, an exhibition of more than three hundred cloud photographs or sketches had been arranged (remember photography was still a fairly specialised and expensive occupation, and of course monochrome only). The publication commission, consisting of Hildebrandsson, Riggenbach and Teisserenc de Bort, had to contend with great technical and more particularly financial difficulties. In the end, Teisserenc de Bort took upon himself the sole responsibility for the production of the atlas. This work contained 28 hand-coloured plates accompanied by a text in three languages (French, German, English) giving definitions and descriptions of the clouds together with instructions on how to observe them. This first International Cloud Atlas constituted a great advance by making cloud observations throughout the world truly comparable with one another.

Following a 1910 reprint with only slight modifications, the increasing importance of aviation led to the setting up of a new International Commission for the Study of Clouds in London in 1921. This eventually led to a new and expanded Cloud Atlas in 1930, reprinted again with minor modifications in 1939. A completely new edition was agreed in 1947 but was not published until 1956, while minor changes were made in a 1975 edition which, until now, stood as the meteorological community’s definitive cloud classification tool – despite many of the photographs being in black-and-white, mostly 60 or 70 years old, and almost all from temperate latitudes within the northern hemisphere.

This new Atlas has been in preparation for 5 years, and came about partly from pressure from Gavin Pretor-Pinney of the Cloud Appreciation Society to include a ‘new’ type of cloud – originally proposed as asperatus (wave-cloud), now recognised in the new publication as asperitas. Work on asperitas clouds was also undertaken within the Department of Meteorology. The opportunity was also taken to update the photographs used, to expand their coverage from all parts of the globe, and to include much more detail on their synoptic environment (including, in many cases, near-simultaneous satellite imagery). The result is a collection of images which, while meeting is desired training and reference objectives, is also a work of art, and an inspiration to all of us to look up at the sky … the world’s greatest free art gallery!

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