Near the interaction zone at the surface, there will be a dry slot caused by the updraft on one side and the cloudy area below the rear flank downdraft on the other side. This is the source of the hook echo seen on radar near the surface. Hook echoes are thus a relatively reliable indicator of tornadic activity; however, they merely indicate the presence of a larger mesocyclone structure in the tornadic storm rather than directly detecting a tornado. During some destructive tornadoes, debris lofted from the surface may be detected as a "debris ball" on the end of the hook structure. Not all thunderstorms exhibiting hook echoes produce tornadoes, and not all tornado-producing supercells contain hook echoes.