2. Cirro-stratus (Ci-St).—A thin, whitish sheet of cloud,[1] sometimes covering the sky and giving it only a milky appearance; it is then called cirro-nebula—at other times presenting more or less distinctly a formation like a tangled web. This sheet often produces halos around the sun and the moon.
This name is apt to be misleading to observers who have followed the old nomenclature. It applies not so much to the striated or banded cirri as to the whitish, or creamy, haze with banded or feathery edges. Frequently it appears as a whitish bank, with here and there a web of tangled fibers; at times it covers the whole visible sky. The halo produced when a cirro-stratus film is in front of the moon is varied in form. Occasionally mock moons, paraselenae, are formed; so also are the light pillar and the “heavenly cross.”
Cirro-stratus clouds have long been associated with approaching stormy weather, and tradition seems to be borne out by investigation. The name cirrus haze is sometimes applied to cirro-nebula.
3. Cirro-cumulus (Ci-Cu).—Mackerel Sky.—Small globular masses or white flakes without shadows, or showing very light shadows, arranged in groups and often in lines.
Cirro-cumulus clouds are not always distinguishable from alto-cumulus clouds. They are much higher, however, and the arrangement usually possesses a geometric regularity. C. F. Brooks describes them as “small white flakes or tenuous globular masses which produce no diffraction colors when covering the sun or the moon.”
4. Alto-stratus (A-St).—A thick sheet of gray or bluish color, sometimes forming a compact mass of dark gray color and fibrous structure. At other times the sheet is thin, resembling thick Ci-St; and through it the sun or the moon may be seen dimly, gleaming as through ground-glass. This form exhibits all the changes peculiar to Ci-St, but it is about one-half as high.
It is not always easy to distinguish alto-stratus from cirro-stratus clouds. One cannot always estimate its altitude and, if the cloud is thin, it may be about as white as a cirro-stratus formation. The lower edge may be undulate, but it is hardly
- ↑ Not every “thin whitish sheet of cloud” is a cirro-stratus formation. The low, white cloud veil of winter days may produce a halo; but it is not a cirro-stratus cloud.
