File:2020-12-31_16_06_58_Altostratus_with_a_wavy_undulating_base_above_the_Dulles_section_of_Sterling,_Loudoun_County,_Virginia.jpg · Wikimedia Commons · See Wikimedia Commons
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Encyclopedic overview
Altostratus radiatus cloud showing distinctive parallel bands Altostratus are middle-altitude clouds that develop horizontally and have a flat and uniform texture in the mid levels. As a middle-altitude cloud type, they can be made up of water droplets (including supercooled droplets), ice crystals, or a mixture of the two.
Altostratus clouds usually appear as gray or blueish featureless sheets, although some variants have wavy or banded bases. The sun can be seen through thinner altostratus clouds, but thicker layers can be quite opaque.
Excerpted from Wikipedia’s “altostratus” article, available under the CC BY-SA 4.0 licence.