If trying to identify a lowered portion of the rain free cloud base as a wall cloud, the most significant characteristic to check is persistent rotation.
What is a wall cloud?A wall cloud is a huge, persistent, localized, and frequently abrupt lowering of cloud which develops beneath the surrounding base of a cumulonimbus cloud and eventually from tornadoes form.
The wall cloud is quite smaller and more dense than a shelf cloud and is generally under a rain free cloud base. Scud clouds are frequently mistakenly called funnel clouds or wall clouds. In the real event, these are just ascending clouds because of the raised low level relative humidity. They are not going to rotate and will rise steadily.
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