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| CH5 |  |
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Cirrostratus (Cs) below 45° – composed of ice crystals above 20,000ft, this advancing cloud (below 45°) often has a defined leading edge. If it develops from spreading cirrus a warm front may be approaching indicating wet weather within 48 hours. However if gaps begin to appear, and the cirrostratus develops into cirrocumulus, a few more days of dry weather are likely. |
| CH6 |  |
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Cirrostratus (Cs) above 45° – the cirrostratus advances above 45° but without covering the whole sky. It loses its defined leading edge and is often accompanied by other forms of cirrus. |
| CH7 |  |
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Cirrostratus nebulosus (Cs neb) or Cirrostratus fibratus (Cs fib) invading the whole sky – this cloud can be fairly dense and fibrous (fibratus), or so thin as to be barely noticeable (nebulosus). In contrast to its lower cousin altostratus, the sun still casts shadows, and sometimes a halo around the sun or moon is the only clue that is is there at all. This cloud often thickens and descends into altostratus and nimbostratus on the approach of a warm front. |
| CH8 |  |
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Cirrostratus (Cs) not progressively invading the sky – a veil of cirrostratus not entirely covering or progressively invading the sky, often in broken patches of cirrostratus fibratus, and sometimes accompanied by cirrus or cirrocumulus. It often gives rise to spectacular sunsets. |
| Other forms include cirrostratus undulatus (undulating waves) and duplicatus (in more than one layer). | |