When Wordsworth wandered lonely as a cloud, what sort of cloud was he contemplating? Since the early 19th century, clouds have been classified under 10 categories devised by a London pharmacist named Luke Howard.
His system was based on two principal criteria: the height of the cloud, and its general shape and structure.
At the lowest level, with its base below 500 metres, we have stratus, the sort of drab, shallow and featureless cloud that greets us in the morning, when early fog has lifted and resulted in low cloud. Stratus is also often caused when a breeze cools low-level moist air below its condensation point.
The most common type of cloud, stratocumulus, also occurs at a low level. This is the layered cloud that seems to roll across the sky. (What in fact happens is that it slowly dissolves on its downwind side, while forming upwind.)
Next comes cumulus, from the Latin for a heap, which forms when plumes of warm air rise above their condensation level and form fluffy, rising cloud, heaped upon itself. These are probably the lonely clouds that Wordsworth was talking about.
Cumulonimbus are, like cumulus, convective clouds, but bigger, higher and more rainy. Heat from below and cold winds above may help create the characteristic flat, anvil-like tops, formed mainly of ice crystals. The strong, vertical currents within cumulonimbus clouds are what pilots warn about when advising passengers to keep their seatbelts fastened because "we are about to enter a spot of turbulence".
At a higher level, clouds are formed of a mixture of water droplets and ice crystals. Between 2.5km and 6km above the ground, we meet altostratus, the layered cloud that may start thin and wispy, but thickens to a grey sheet. This may produce light rain, but not as bad as nimbostratus, the deep, layered, black cloud from which rain or snow is already falling. The third of the middle-level clouds is altocumulus, , which is essentially a stratocumulus rolling cloud appearing at a higher level.
The highest clouds, above 6km, are all formed of ice crystals. Cirrus is sometimes thin and wispy, sometimes straggly and twisted. It is sometimes caused by the condensation trails of aircraft. When Luke Howard introduced the term, he described cirrus cloud as comprising "parallel, flexuous, or diverging fibres, extensible by increase in any or all directions".
Cirrostratus is a high, uniform sheet of cloud produced by the slow ascent of air, condensing at a great height. It is often an early sign of rain. Finally, cirrocumulus is the unusual and attractive rippling cloud produced by wave motion through high-level moist air.
But why all these silly Latinate names when all we want to know is whether it is going to rain? Perhaps the real blame lies with Linnaeus, the 18th- century Swedish botanist who devised a precise system for naming plants and animals. It was his work that inspired Luke Howard, also a botanist, but an obsessive weather-watcher as well, to devise a classification system for clouds. Between 1818 and 1820, he published the two-volume Climate of London, based on his meteorological observations of weather in the capital. The book included an earlier essay, "On the Modification of Clouds", in which he gave the cloud formations their names. And we have been stuck with them ever since.
Whatever we may think of this nomenclature, it made a strong impression on Johann Wolfgang von Goethe, who, after reading Howard's meteorological works, wrote to ask about his personal history.
Howard replied with a brief autobiography (including the line: "From the first, my real penchant was towards meteorology"), which so pleased Goethe that he sent a short poem entitled: "Howard`s Ehrengedachtniss". He also gave a description in verse of the principal cloud forms according to Howard's classification. Sadly, it has never been set to music.Reuse content