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Special clouds |
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Click the small image for full size version.
© 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 CH and C. |
| 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 CH and C. |
© R.M. Blackall |
© Crown copyright |
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 CH 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. |
© C.S. Broomfield |
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