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When the sun is sufficiently high above the horizon, clouds
in direct sunlight are white or grey. However those which
receive light from the blue sky are bluish-grey. Some clouds,
which appear brilliant white in reflected light, show marked
contrasts in brilliance when illuminated from behind. The
colour of the sun may change as it approaches the horizon
and clouds in the vicinity may show a corresponding colouration.
The under surface of a cloud may redden when the sun is on
the horizon. Pictured opposite is stratocumulus.
Haze may make distant clouds appear yellow, orange or red.
Dust particles introduce a white tinge to the blue of the
sky; thus the sky is of a deeper blue when the air has its
origins in polar regions.
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