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.