Special clouds

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© Joyce Imrie

NACREOUS CLOUDS  resemble pale cirrus or lenticular altocumulus and show very marked irisation, the most brilliant colours occurring when the sun is just below the horizon. They are sometimes called mother-of-pearl clouds. They can still be distinguished up to about two hours after sunset as thin grey clouds standing out against the starry sky. On moonlit nights they may be visible throughout the night. They have been observed mainly from Norway and Scotland, at altitudes between 21 and 30 km. They are ignored when assessing C_H and C.

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© R.M. Blackall

NOCTILUCENT CLOUDS  resemble thin cirrus, but are usually bluish or silvery, sometimes orange to red, or reddish when on the horizon. They are extremely rare, being most commonly observed on clear midsummer nights between latitudes 55 to 65 north. They become visible at the same time as the brightest stars and appear to be more brilliant after midnight. Their altitude is between 76 and 90 km. It is believed these clouds consist of ice crystals. They are ignored when assessing C_H and C.

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© Crown copyright
© C.S. Broomfield

CONDENSATION TRAILS  (contrails) form in the wake of aircraft when the air is sufficiently cold and humid. They are often short-lived (top photograph), but, especially when cirrus and cirrostratus are present, they may spread out and persist for several hours (bottom photograph). Persistent trails are reported by using the C_H code figure most appropriate; sometimes it is impossible to distinguish between old trails and cloud. They may produce halo phenomena with exceptionally pure colours. Over Great Britain they rarely form below 28,000 ft in summer and 20,000 ft in winter. They may cast shadows on thin clouds beneath them. A series of such shadows may be the only indication that there is more than one layer of cloud present.