The wall cloud is located in the updraft region of a supercell. Rising air cools and condenses out moisture once it is saturated. Due to the rapidly rising air and the verticality of the rising air, the cloud base is close to the ground within the wall cloud. The wall cloud will often be witnessed as rotating since directional wind shear acts on the updraft as it rises. Tornadoes can occur under the wall cloud. The diagram below shows the wall cloud at "F". 
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