In an intriguing paper, Schultz et al. (2006, hereafter S06) have reviewed observations of mammatus clouds in the scientific literature and offered some candidate mechanisms for the formation of mammatus. This reasonably comprehensive review is both interesting and stimulating. I concur with the authors that although mammatus cloud formations have little or no direct societal impact, they are a fascinating enigma, certainly worthy of careful scientific scrutiny. One candidate mechanism they did not mention, however, is what is known in oceanography as “doublediffusive convection” (Turner 1973, p. 251 ff.). Its most well-known application is to the thermohaline flows known as “salt fingers.” The physical mechanism is that the fluid has two constituents that contribute in opposing ways to buoyancy; increased salinity makes the water more dense, whereas temperature increases make the water less dense. When warm, salty water overlies cold freshwater, the resulting density distribution is nearly neutral and stable to perturbations according to static stability theory, because the two constituents contribute oppositely to buoyancy. However, the rates of diffusion of the two constituents are very different— heat diffuses much more rapidly that salinity, so if a parcel of warm, salty water is perturbed downward, its heat content diffuses more rapidly than the salt. This ultimately causes a negative buoyancy in the descending finger; that is, the perturbation is unstable. The opposite happens in a parcel of cool freshwater perturbed upward. The resulting ascending and descending parcels eventually mix the layers at the interface. Of interest to the observations presented in S06 is that the stratification produced by the action of salt fingers comes to produce a layered, steplike profile of the constituents (Turner 1973, 270–273; Gregg 1973). The detailed view of stepped temperature profiles (producing very shallow superadiabatic lapse rates in the temperature profiles) in Fig. 10 of S06 could be considered compatible with the oceanographic observations. Turner (1973, p. 270) is careful to point out that layering need not be the result of double-diffusive convection, but this observation is at least consistent with the hypothesis of the presence of double-diffusive convection. How might this process be associated with mammatus clouds? The presence of particulates that are heavier than air in a “colloidal” solution would play a role comparable to that of salinity—acting to increase the overall density of the air-particulate mixture. Those particulates could be condensed water or volcanic ash particles. If the anvil (or ash) cloud is relatively warm compared to the air below, then the situation would be analogous to that in oceanic thermohaline flows that produce salt fingering. As is the case for thermohaline flows, the diffusion rate of heat would certainly be considerably more rapid than that of the particulates. In fact, particulates such as ash particles should be diffused quite slowly. I will return to this shortly. For the moment, consider the particulates to be water droplets or ice particles. In the atmosphere, within a descending “finger” initiated by the negative buoyancy produced as a result of double-diffusive convection, the particles would descend with the air, which would be warming as a result of compressional heating, even as their relative warmth as a result of originating in the warm anvil diffuses into the surroundings. If the particles begin to evaporate (or sublimate) as a result, they would contribute their latent heat and the initially unstable descent owing to negative buoyancy would be correspondingly diminished. By the level where all the particles have evaporated, the air would then move verCorresponding author address: Dr. Charles A. Doswell III, Cooperative Institute for Mesoscale Meteorological Studies, 120 David L. Boren Blvd., Suite 2100, Norman, OK 73072-7304. E-mail: cdoswell@gcn.ou.edu MARCH 2008 N O T E S A N D C O R R E S P O N D E N C E 1093