Voici ce que Wikipedia dit: Occasionally people see a shelf cloud and think they have seen a wall cloud, which is an easy mistake, since an approaching shelf cloud appears to form a wall made of cloud. Generally speaking, a shelf cloud appears on the leading edge of a storm, and a wall cloud will usually be at the rear of the storm, though small wall clouds can occur within the leading edge on rare occasion. La deuxième phrase dit : De manière générale, le shelf cloud se trouve à l'avant de l'orage et le wall cloud est normalement à l'arrière de l'orage, mais de petits wall clouds peuvent être à l'avant de l'orage en de rares occasions.