In this laboratory experiment, we release a dense saline fluid down a slope into a reservoir of fresh water. Such "gravity currents" are ubiquitious in the environment, seen as fog creeping down the San Francisco hills or the daily sea breeze invading the streets of Chicago during summer. Kelvin-Helmholtz instabilities form in the shear layer at the edge of the gravity current; these billows pair, grow, and collapse under their own weight, in turn mixing the two fluids.
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