24 days of kitchen oceanography

Experiment 15: 

We layer warm, salty water above colder, fresher water in a stable stratification by pouring cold water in a container that is already half full of hot water that is dyed red. This stratification leads to double-diffusive mixing. Molecular diffusion of heat, which equilibrates temperatures, happens by molecules colliding and exchanging energy. Molecular diffusion of salt requires molecules to change position which is much slower. Temperature differences equilibrate 100 times faster than differences in salinity. This can lead to unstable density stratifications when water has the same temperature but a higher salinity in layers resting on layers with lower salinities. This causes instabilities and vertical movements, so-called “salt fingers”, that transport the saltier water down. These salt fingers mix water masses more effectively than molecular diffusion could.