Unstable atmospheric conditions easily allow for vertical motion in the atmosphere where parcels of air maintain a greater temperature than their surroundings, allowing for continual rise. These conditions allow for good smoke dispersion, but also thunderstorms and increased active fire conditions.
Source:
Hardy, Colin C., Ottmar, Roger D., Peterson, Janice L., Core, John E., and Seamon, Paula. 2001. Smoke Management Guide for Prescribed and Wildland Fire 2001 Edition. Produced by the National Wildfire coordinating Group Fire Use Working Team.