Clouds in the sky change their shape constantly. Sometimes this is too slow to be noticeable, at others a cloud can morph twice within a few lines of Shakespearean dialogue. Due to this ephemeral character clouds had not been formally categorised before the beginning of the nineteenth century. Nowadays there is an internationally recognised nomenclature used by meteorologists. This system was devised by a man fascinated with weather: Luke Howard.

What's in a name?

Prior to the systems described below clouds had not been ignored for their weather predicting abilities, but the wisdom was not scientific. With few exceptions, no cloud types were even named; they were just described by their colour and form: white or black, mare's tails or mackerel skies. Clouds are used in a few instances as forecast tools in weather proverbs, but the folklore is often related to the place where the observer lives and works. So, while proverbs (often summarising observations over many years) may be locally true, they are not always universally applicable.

'Red sky at night sailor's delight, but red sky in the morning sailor's warning.'¹

This appears to be true for all of the British Isles, reflecting whether a weather front is coming from the Atlantic or the Continent.

Soon after 1800 two natural philosophers turned their minds to defining what types of cloud there were.

It is generally accepted that both were inspired by the Linnean system of classification of living things devised by the Swedish taxonomist Linnaeus². One was the French naturalist Jean Baptiste Lamarck (1744-1829), more famous for his attempt to describe a mechanism for evolution. The other was Luke Howard (1772 - 1864) a Quaker businessman and one of the founder members of a philosophical group, the Askesian³ Society. The 'Essay on Modifications of Clouds' he wrote for the society named the cloud types still used today.

Lamarck's System

'It is not in the least amiss for those who are involved in meteorological research to give some attention to the form of clouds; for, besides the individual and accidental forms of each cloud, it is clear that clouds have certain general forms which are not all dependent on chance but on a state of affairs which it would be useful to recognize and determine.'⁴

Lamarck in his Annuaire Méteorologique of 1802 initially proposed five main types of clouds 'related to general causes which are easily ascertained.' They were:

• Hazy clouds (en forme de voile)
• Massed clouds (attroupés)
• Dappled clouds (pommelés)
• Broom-like clouds (en balayeurs)
• Grouped clouds (groupés)

Three years later, Lamarck devised a more detailed classification scheme, which comprised twelve forms. Unfortunately his classification system did not make an impression on the scientists and naturalists of the day, not even his own countrymen, and does not seem to have been used by anyone except himself. This was possibly due to his use of French names for his cloud types (which would not be easily adopted in other countries), or by publishing his essay in the same publication as forecasts based on astrological data (the Annuaire Méteorologique), which may have cast some doubts on its credibility. These are suggestions made in the International Cloud Atlas⁵. Nevertheless, it goes on to say:

'Four out of Lamarck's five principal types appear under different names in Howard's nomenclature. It is remarkable that these two men, of different scientific cultures, and never having come in contact with each other, should have arrived independently at such compatible results.'

Luke Howard

Luke was the born in London in November 1778, first child of Quaker parents, and was sent to a Quaker school near Burford in Oxfordshire. He was a lifelong member of the Society of Friends (the more formal name of the Quakers) and devoted much energy and time to their good works. As an adult he became a businessman, developing a firm that manufactured pharmaceutical chemicals: Howards and Sons Ltd. It has been suggested that his lifelong fascination with weather was heavily influenced by the unusual weather of 1783.

In this year there was the 'Great Fogg', due to the masses of debris in the sky from two huge volcanic eruptions, one in Iceland, the other in Japan, the combined effect of which was felt world-wide. In Europe the particles high in the atmosphere caused months of cold dull haze, and a series of spectacular sunsets.

Trying to understand what was happening in the sky was a lifelong interest of Howard from then on. This research led to the lectures Howard gave at the Askesian Society. Howard's interest in atmospheric phenomena was first established by a paper to the Askesians on Causes of Rain (23 February 1802). He wrote of the society that it:

'...met every fortnight during the winter, each member being required by the rules to bring in an essay, in turn, for discussion, or pay a fine. It was the obligation thus contracted, which occasioned me to present to that society the Essay on Clouds.'

He was a prolific writer and editor. His seven lectures in Meteorology comprised the first textbook on weather. His Climate of London was the first book on urban climatology. He noted in this the effect that large urban areas can create, where the temperature is notably (and consistently) warmer than in adjacent rural areas. His many contributions to the emerging science of meteorology resulted in him being made a Fellow of the Royal Society in 1821.

The key idea set forth by Howard in the Essay on Clouds is that it is possible to identify, from within the complexity of changing skies, a number of simple categories. The basic categories he described are:

1. Cumulus (Latin for 'heap') 'Convex or conical heaps, increasing upward from a horizontal base'.
2. Stratus (Latin for 'layer') 'Widely extended horizontal sheets'.
3. Cirrus (Latin for 'curl') 'Flexuous fibres extensible by increase in any or all directions' - could also be described as 'wispy'.
4. Nimbus (Latin for 'rain') 'Systems of clouds from which rain falls' (usually appear dark grey).

There are also intermediate categories: Cirro-cumulus, Cirro-stratus, Cumulo-stratus. In all there are about ten categories generally used nowadays. Not all clouds bring rain, some are signs of fine weather, which explains why the fourth type is needed. 'Nimbus' can be added to the beginning or end of a cloud type, eg Cumulo-nimbus or Nimbo-stratus. Generally Cumulo-nimbus clouds are the most dangerous, associated with hail, lightning, tornadoes, downdraughts, downbursts and flashflooding.

In his paper, Howard presents a lengthy description of, and rationale for, these various forms of cloud.

There are many unusual types of clouds. Some of the less common descriptive terms are as follows:

• Altus (alto) meaning 'high'
• Capillatus meaning 'having hair'
• Castellanus meaning 'castle-like battlements'
• Congestus meaning 'piled up'
• Floccus meaning 'tufted'
• Fractus meaning 'broken, irregular or ragged'
• Humilis meaning 'flattened'
• Lenticularis meaning 'lens or almond shaped'
• Mamma meaning 'udder or protuberance'
• Pileus meaning 'cap or hood'

References and Further Information

BBC Weather will provide much general information about the causes of weather, and history of forecasting, as well as up-to-date forecasts.