Critical richardson number
From AMS Glossary
critical Richardson number
The value of the gradient Richardson number below which air becomes dynamically unstable and turbulent.
This value is usually taken as Ric = 0.25, although suggestions in the literature range from 0.2 to 1.0. There is also some suggestion of hysteresis, where laminar air flow must drop below Ri = 0.25 to become turbulent, but turbulent flow can exist up to Ri = 1.0 before becoming laminar.