Task: Class Definition Understanding (ontology: Earth_Environment/SWEET)

Which of the following definitions best describes 'wall cloud'?

A. The leeward edge of the orographic stratiform cap cloud as seen from the lee side of a mountain barrier, preceding or during a foehn or chinook event. The edge is generally abrupt and resembles a wall of cloud (?foehnwand? in German). This cloud often signifies the occurrence of orographic precipitation, especially snowfall in the cold season, over the peaks. See foehn cloud.
B. A cloud plume often observed to extend downwind from isolated, sharp, often pyramid-shaped mountain peaks, even on otherwise cloud-free days.
C. A principal cloud type (cloud genus), gray colored and often dark, rendered diffuse by more or less continuously falling rain, snow, sleet, etc., of the ordinary varieties and not accompanied by lightning, thunder, or hail.
D. A local, often abrupt lowering from a cumulonimbus cloud base into a low-hanging accessory cloud, normally a kilometer or more in diameter.

Answer: A local, often abrupt lowering from a cumulonimbus cloud base into a low-hanging accessory cloud, normally a kilometer or more in diameter.