]> The region of turbulence immediately to the rear of a solid body in motion relative to a fluid The turbulent perturbations of temperature remaining in a stable boundary layer after all turbulent motions have died out. An instability of the basic flow of an incompressible inviscid fluid in two parallel infinite streams of different velocities and densities. A wave disturbance in a two-dimensional flow, the driving mechanism for which lies in the variation of vorticity of the basic current and/or in the variation of the vorticity of the earth about the local vertical. Irregular fluctuations occurring in fluid motions. A hydrodynamic instability arising from the existence of a meridional temperature gradient (and hence vertical shear of the mean flow and a thermal wind) in an atmosphere in quasigeostrophic equilibrium and possessing static stability. Turbulence produced by shear flow. The behavior of a fluid with a Reynolds number typically greater than 1E4 to 1E6, which usually occurs within the atmosphere. The main property of such flows is a constant friction stress within the surface layer that depends only on relative roughness but not on the Reynolds number itself. Thus, molecular viscosity and qualities occurring in flow descriptions that are dependent on the Reynolds number may be totally ignored. An approximation to the gradient Richardson number formed by approximating local gradients by finite difference across layers. The dimensionless ratio of the inertial force (∼U2/L) to the viscous force (∼ νU/L2) in the Navier–Stokes equations, where U is a characteristic velocity, L is a characteristic length, and ν is the kinematic viscosity of the fluid; thus, Re=ULv. The Reynolds number is of great importance in the theory of hydrodynamic stability and the origin of turbulence. The inertia force generates vortex stretching and nonlinear interactions and hence creates randomness. Turbulence occurs when the inertia term dominates the viscous term, that is, when the Reynolds number is large. For many engineering flows, turbulence occurs when Re > Rec, where the critical Reynolds number is roughly Rec = 2100.