Volcanic eruption plumes are not generally treated as CbFg, although they are convectively driven to a large extent and for weaker eruptions may be significantly enhanced in height in convectively unstable environments. However, for some months after the climactic eruption of Mt Pinatubo in the Philippines in 1991, meteorological observers from the US military observed what they termed 'volcanic thunderstorms' forming near the summit: cumulus cloud complexes formed near the top of the buoyant ash plume, and frequently developed into cumulonimbus clouds (thunderstorms). The thunderstorms often drifted away from their source region at the top of the plume, producing sometimes significant amounts of localized rainfall, mudfall, and ash fall. They also noted that thunderstorms formed over hot flows and secondary explosions even in the absence of any eruption. Further investigations confirmed that the volcano had clearly enhanced the convective environment, causing thunderstorms to form on average earlier in the day and more reliably than in surrounding areas, and that the presence of volcanic ash in cloud tops in the upper troposphere could be inferred from satellite imagery in at least one case.