On this page
Look up NCERT's Class 11 chapter on water in the atmosphere and you meet four words: cirrus, cumulus, stratus and nimbus. They sound ancient, but they were invented by one person a little over two hundred years ago. Luke Howard gave clouds names that people in every country could share, and the system is still used by weather services today. This post covers who he was, why his naming worked, how to decode any cloud name, and a little cloud physics with numbers.
Who was Luke Howard?
The UK Met Office describes Howard as "a London pharmacist and amateur meteorologist" who first proposed classifying clouds into types in 1802. Weather was not his job. He studied it alongside his work and shared it with a small science club.
That club was the Askesian Society. The third edition of his Essay on the Modifications of Clouds (1865) reprints the editor's note: a "select number of gentlemen, associated for their mutual improvement" in natural philosophy, meeting every week in winter. The essay's heading says it is "the Substance of an Essay read before the Askesian Society in the Session 1802—3", and the title page gives first publication as 1803. The 1865 title page lists him as F.R.S., a Fellow of the Royal Society.
Why did Howard think clouds could be named at all?
Many people thought clouds changed too fast to classify. Howard disagreed. In the essay he argues that clouds "are subject to certain distinct modifications", produced by the same general causes that drive the weather, and that they are as good a sign of those causes "as is the countenance of the state of a person's mind or body".
He also explains why he chose Latin. Words from a "dead language" could be adopted by "the learned of different nations". He preferred Latin to Greek because, as in natural history, the name should describe what you see, and more readers would understand the literal meaning in Latin.
What do the names actually mean?
Howard defined three simple forms:
Cirrus: "Parallel, flexuous, or diverging fibres, extensible by increase in any or in all directions." Think thin, wispy streaks.
Cumulus: "Convex or conical heaps, increasing upward from a horizontal base." Heaps with flat bottoms.
Stratus: "A widely extended, continuous, horizontal sheet, increasing from below upward." A layer.
He added two intermediate forms, cirro-cumulus and cirro-stratus, and two compounds, cumulo-stratus and cumulo-cirro-stratus, which he also called nimbus, "The Rain cloud. A cloud, or system of clouds from which rain is falling."
Today the WMO International Cloud Atlas recognises ten genera, including cirrocumulus, altostratus, nimbostratus and cumulonimbus. The WMO's principles of cloud classification use genera, species and varieties, "similar to the systems used in the classification of plants or animals", with Latin names, exactly the analogy Howard drew.
Worked example: decode and calculate
Decoding. Take "cirrocumulus". Cirro- means high and fibrous or icy; cumulus means heaped. So it is a high cloud made of small heaps. "Nimbostratus" is a rain-bearing layer. "Cumulonimbus" is a heaped cloud that rains. NCERT's Water in the Atmosphere chapter groups them the same way: high clouds such as cirrus and cirrocumulus, middle clouds such as altostratus, low clouds such as stratocumulus and nimbostratus, and tall clouds such as cumulus and cumulonimbus.
A little cloud physics. NOAA's JetStream guide explains that rising air cools at about 9.8 °C per kilometre until it is saturated. Air that rises 1.5 km without saturating therefore cools by 9.8 × 1.5 ≈ 14.7 °C.
The US National Weather Service training notes on cloud development give an average cloud droplet a fall speed of 1.3 cm per second in still air. From a low cloud base of 500 m, the fall would take 500 ÷ 0.013 ≈ 38,500 s, or about 10.7 hours. That is why cloud droplets hang in the air, and why rain needs much bigger drops.
What mistakes do students make with clouds?
Treating "nimbus" as a cloud on its own in modern lists. In the WMO genera it appears as part of nimbostratus and cumulonimbus.
Thinking clouds are water vapour. The Met Office points out that water vapour is invisible; what you see are tiny liquid droplets, and in high clouds such as cirrus, ice crystals.
Unit slips in the fall-time sum. 1.3 cm/s is 0.013 m/s. Mixing cm and m makes the answer 100 times wrong.
For the bigger picture of how air pressure and winds shape weather, see our post on pressure, winds and cyclones. If the physics of rising, cooling air is hard to picture, Fermi's Simulation Lab has interactive simulations for ideas that are easier to see than read.
What can students learn from Howard's way of working?
Howard observed for years, then designed a system that others could use, not just a description for himself. He chose names that explain themselves, and he allowed for in-between and combined forms instead of forcing every cloud into one box.
You can use the same idea for revision: build a few root words for a topic and combine them, instead of memorising long lists.
Tomorrow, look at the sky once in the morning and once in the evening, and name each cloud using Howard's three roots.
