Richardson number
The Richardson number, denoted Ri, is named after Lewis Fry Richardson (1881–1953). It is a dimensionless number that expresses the ratio of the buoyancy term to the flow shear term in fluid dynamics: R i = buoyancy flow shear = g ρ ∂ ρ / ∂ z ( ∂ u / ∂ z ) 2 , {\displaystyle \mathrm {Ri} ={\frac {\text{buoyancy}}{\text{flow shear}}}={\frac {g}{\rho }}{\frac {\partial \rho /\partial z}{(\partial u/\partial z)^{2}}},} where g {\displaystyle g} is the local acceleration due to gravity, ρ {\displaystyle \rho } is the mass density, u {\displaystyle u} is a representative flow velocity, and z {\displaystyle z} is depth.