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| CH1 |  |
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Cirrus uncinus (Ci unc) – known as ‘mares tails’ these clouds are composed of ice crystals above 20,000ft drawn out by the wind, forming filaments or hooks that do not progressively invade the sky. They are usually formed in settled, often windy conditions, from rising dry air. Thickening cirrus (see CH4) indicates an approaching warm front and an imminent deterioration in the weather. |
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Cirrus fibratus (Ci fib) – cirrus arranged into parallel bands by the wind. Cirrus clouds can sometimes form from the spreading out of aircraft contrails and persist for several hours. |
| CH2 |  |
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Cirrus spissatus (Ci spi) – thick, dense cirrus often covering large areas of the sky. It indicates an approaching warm front and wet or stormy weather within 48 hours. |
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Cirrus floccus (Ci flo) and Cirrus castellanus (Ci cas) – these species exhibit cumuliform turrets (castellanus) or occur in ragged patches (floccus), often featuring trailing filaments or virga. |
| CH3 |  |
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Cirrus spissatus cumulonimbogenitus (Ci spi cbgen) – often difficult to distinguish from cirrus spissatus, this cirrus comprises the remnants of decayed cumulonimbus capillatus anvils. Unlike other cirrus clouds, they can be thick enough to obscure the sun, and can therefore be grey in colour. |
| CH4 |  |
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Cirrus uncinus or cirrus fibratus progressively invading the sky – ‘Mares tails and mackerel scales make tall ships carry low sails’. The same species as CH1, these clouds progressively invade the sky ahead of an approaching warm front, and are sometimes accompanied by optical phenomena such as haloes. In time they thin out into uniform cirrostratus. |
| Other forms include cirrus intortus (twisted), radiatus (parallel bands), vertebratus (appearing like ribs or fishbones) and duplicatus (in more than one layer). | |