Naming the clouds

Clouds are displays of water vapour in the sky, sculpted by flowing air. People have been observing them for centuries. The names by which we know them today were thought up by Luke Howard, an English scientist who lived from 1772 to 1864. Today we puzzle at terms such as cumulonimbus or cirrus, but for a gentleman of his era, Latin terminology was a sine qua non.

His cloud names were first set out in a lecture, “On The Modification Of Clouds” probably in 1802. As historians of science have pointed out, he really meant something close to 'classification''. His scheme was the rival to one thought up by a far more distinguished scholar, Jean-Baptiste Lamarck, best known for his pre-Darwinian work on evolution. One reason Lamarck's scheme failed was that the names he proposed were derived from French, but looked odd even to French eyes.

The Howard system needs only three terms to describe all clouds. Stratus clouds are layered or flat, as with a stratum in geology. Cumulus are heaped-up, the term best remembered by its relation to “accumulate”. And cirrus clouds appear like strands in the sky. This term comes from the Latin for a curl of hair.

In addition, Howard described a type of cloud he termed “nimbus” from the Latin for rain.These are clouds that are producing rain or snow, or threatening to. He then added that there could be intermediate types such as cirrostratus, cumulostratus or cirrocumulus. Cumulostratus is essentially cumulus which develops to fill the whole sky.

Clouds more than 6000m above sea level are called “high” and are normally cirrus or close to it. Those from about 2000 to 6000m are called mid-latitude clouds and a new term, alto, has been added to Howard's nomenclature to describe them. They can be cumulus, stratus or a mix of the two. Below 2000m, clouds are “low”.

Of these types, cirrus clouds are the least menacing. They are so high that they often contain ice as well as water vapour. Cumulus is the least agreeable as it may turn into cumulonimbus which, in addition to rain, brings the possibility of a thunderstorm. These are the deepest and highest-energy clouds, sometimes many kilometres tall and containing huge air currents. Nimbostratus clouds also produce rain but usually in a dull and lightning-free fashion. The purists regard fog as stratus that has reached ground level.