1. Buoyant jet in which the buoyancy is supplied from a point source; the buoyant region is continuous. See thermal. 2. A mostly horizontal (sometimes initially vertical) stream of air pollutant that is being blown downwind from a smokestack. Typical smoke-plume diameters are of order 1–10 m initially, gradually expanding to 100 m or more, while lengths can be order of 1–100 km. The path and shape of the smoke plume can indicate the nature of turbulence in the atmospheric boundary layer, such as looping plumes, fanning plumes, and coning plumes.