
.topic 20
Greyish or bluish cloud sheet or layer of striated, fibrous or uniform appearance, totally or partly covering the sky, and having parts thin enough to reveal the sun at least vaguely, as through ground glass. Altostratus does not show halo phenomena.


.topic 30
An optical phenomenon in the form of a ring, arc, pillar or bright spot, produced by the refraction of light by ice crystals suspened in the atmosphere (cirriform clouds, ice fog, etc.). This phenomenon, when formed by refraction of the light of the sun, may show colors, while a halo phenomenon produced by the light of the moon is always white.


.topic 40
Grey or whitish, or both grey and whitish, patch, sheet or layer of cloud which almost always has dark parts, composed of tessellations, rounded masses, rolls, etc., which are non-fibrous (except for virga) and which may or may not be merged; most of the regularly arranged small elements have an apparent width of more than five degrees.


