Smoke produced by smoldering material. The flux of smoke originating well after the active flaming combustion period with little or no vertical buoyancy and, therefore, most susceptible to subsidence inversions and down-valley flows.
Source: National Wildfire Coordinating Group. 2012. Glossary of Wildland Fire Terminology. PMS 205. Accessed on 9 September 2013. http://www.nwcg.gov/