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
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.
When the sun is low on the horizon,
cirrostratus clouds can appear in a magnificent array
of colors as longer wavelengths of
sunlight (red, yellow, and orange) are
reflected off of the clouds.
The cirrus streaks in this photograph
in a southwest to northeast direction, indicative of warmer air
advancing at higher levels. Lower on the horizon, thickening cirrostratus
clouds effectively hide the sun, signifying changing weather ahead.
As the warm front approaches, these clouds
will thicken and be replaced
lower and more dense cloud types.