Photographs of a volcanic column in a recent eruption of Reventador show a prominently scalloped umbrella that is unlike any umbrella previously documented on a volcanic column. We propose that the scallops in this umbrella are the result of a turbulent Rayleigh--Taylor (RT) instability, a type of fluid instability with no precedents in volcanology. Negative buoyancy drives this instability, and we ascribe the unusual negative buoyancy of the Reventador umbrella to the fact that the Reventador column fed on a cool co-ignimbrite cloud. From the wavelength of the scallops, we estimate a value for the eddy viscosity of the umbrella, νd ≈ 4,000 m2/s, the first such value to be inferred directly from an observation in the field. Collapse of the umbrella back to the ground could result in a previously unrecognized hazardous flow. We hope this work will elicit new reports on scalloped umbrellas and further study of the characteristics and evolution of such umbrellas.