A debris cloud is a rotating cloud of dust or debris at or near the ground, lofted by a vortex in contact with the surface, and it is direct confirmation of a tornado even when no condensation funnel is visible. NWS Norman describes it as a rotating cloud of dust or debris, near or on the ground, often appearing beneath a condensation funnel and surrounding the base of a tornado.
How it develops
When a tornadic vortex reaches the ground, the strong inflow at the surface and the sharp pressure drop inside the vortex lift loose material from the surface. Fine dust, soil, vegetation, and, in populated areas, roofing and structural fragments spiral upward around the core and outline the circulation that is otherwise invisible.
The visible size of the debris cloud usually exceeds the width of any condensation funnel because material is lofted by the broader inflow, not only by the core pressure drop. In dry environments, a debris cloud may be the only visible signature of the tornado from the ground.
How spotters and forecasters use it
A rotating debris cloud beneath a thunderstorm cloud base confirms a tornado. NWS spotter training treats this equally with a condensation funnel touching down for the purpose of tornado identification and reporting.
Debris also drives radar confirmation. The tornadic debris signature on dual-polarization radar shows a region of low correlation coefficient collocated with a velocity couplet, which the NSSL and WDTD document as strong evidence of a tornado on the ground even if visibility is poor from the surface.
What can fool the eye
A straight-line outflow surge can loft a wide plume of dust that looks alarming but does not rotate. Blowing dust on the leading edge of a strong gust front, or a haboob associated with a collapsing storm, moves as a translating wall rather than a spinning column.
The distinction is rotation. If the dust column is spinning around a vertical axis, it is a debris cloud tied to a vortex; if it is moving forward with no rotation, it is outflow-driven dust and no tornado is implied.