The plume towered to about 100,000 feet, roughly three times the altitude at which commercial aircraft fly. Thunderstorms flatten out at the tropopause, or top of the troposphere, the lowest level of Earth’s atmosphere, since a lid of warm air suppresses continued upward development. Hunga Tonga’s plume, however, was so buoyant that it was able to penetrate this layer and continue into the stratosphere before pockets of air and ash subsided once again. The bulge in the middle of the cloud mass where this occurs is known as the “overshooting top.”