Clusters of Cumulonimbi can be found in an unstable environment where dynamical forcing in the atmosphere plays a role as an additional triggering effect. Cumulonimbus clusters can occur in any unstable air mass; in a cold air mass behind a polar Cold Front (see
), in the frontal zone of a polar front (see Conceptual Model
Front Intensification By Jet Crossing
or the Conceptual Models
Occlusion: Warm Comveyor Belt Type
) and in warm air of a thermal ridge as dealt with in this chapter.