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The edge of a veil of cirrostratus
may be straight and clear-cut, but more often it is irregular and fringed
with cirrus. Some of the ice crystals which comprise the cloud are large
enough to fall, and thereby produce a fibrous aspect. Cirrostratus occasionally
may be so thin and transparent as to render it nearly indiscernible, especially
through haze or at night. At such times, the existence of a halo may be
the only revealing feature. The angle of incidence of illumination upon
a cirrostratus layer is an important consideration in evaluating the identifying
characteristics. When the sun is high (generally above 50° altitude),
cirrostratus never prevents the casting of shadows by terrestrial objects,
and a halo might be completely circular. At progressively lower altitudes
of the sun, halos become fragmentary and light intensity noticeably decreases.
Cirrostratus may be produced by the merging of elements of cirrus; from
cirrocumulus; from the thinning of altostratus; or from the anvil of cumulonimbus.
Since cirrostratus and altostratus form from each other, it frequently
is difficult to delineate between the two. In general, altostratus does
not cause halo phenomena, is thicker than cirrostratus, appears to move
more rapidly, and has a more even optical thickness. When near the horizon,
cirrostratus may be impossible to distinguish from cirrus. |
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