Physical constitution

(Section 2.3.5.7)

Altostratus is generally a layer of great horizontal extent (several tens or hundreds of kilometres) and fairly considerable vertical extent (several hundreds or thousands of metres). 

Altostratus is composed of water droplets and ice crystals. In the most complete case, three superposed parts can be distinguished:

(a) An upper part, composed wholly or mainly of ice crystals

(b) A middle part, composed of a mixture of ice crystals, snow crystals or snowflakes and supercooled water droplets

(c) A lower part, composed wholly or mainly of ordinary or supercooled water droplets or drops

Sometimes the cloud may consist of only two parts, either: 

• An upper part like (a) above and a lower part like (c) or
• An upper part like (b) and a lower part like (c).

Less frequently, the entire cloud may also be like (a) or like (b) alone.

The constituent particles in the lower part of Altostratus are so numerous that the outlines of the Sun or Moon are always blurred. Surface observers will never see halo phenomena. In the thickest parts, the positions of the Sun or Moon may be completely concealed.

Raindrops or snowflakes are often present in Altostratus and below its base. When precipitation reaches the ground, it is generally of the “continuous” type and in the form of rain, snow or ice pellets.