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The Richardson number (Ri) is a dimensionless number offering a measure of a fluid's (e.g., atmosphere's) chances of the Kelvin-Helmholtz instability. It is a ratio of buoyancy (in terms of density and gravity) with the component of the flow velocity that is normal to the gravitational force. The Richardson number is used in meteorology and oceanography and is applicable to some other situations where fluids might mix.