(measure of fluid stability consisting of ratio of buoyancy and shear)
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.