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sheet-like and nearly transparent
Cirrostratus are sheet-like, high-level clouds composed of ice crystals.
Though cirrostratus can cover the entire sky and be up
to several thousand feet thick, they are relatively transparent, as the
sun or the moon can easily be seen through them.
These high-level clouds typically form when
a broad layer of air is lifted
[Image: halo through the cirrostratus clouds (58K)]
Sometimes the only
indication of their presence is given by an observed
halo around the sun or moon.
Halos result from
of light by the cloud's ice crystals.
Cirrostratus clouds, however, tend to thicken as a
warm front approaches,
signifying an increased production of ice crystals. As a result, the
halo gradually disappears and the sun (or moon) becomes less visible.