Turbulence in which the products and squares of the velocity components and their derivatives are independent of direction, or, more precisely, invariant with respect to rotation and reflection of the coordinate axes in a coordinate system moving with the mean motion of the fluid. Then all the normal stresses are equal and the tangential stresses are zero. Atmospheric turbulence is generally nonisotropic, although isotropic turbulence is that most easily produced in wind tunnel experiments and forms the basis of much of the theoretical analysis of turbulent flow. A related but less restricted type of turbulence is known as homologous turbulence, in which the fluctuations differ only in scale at every point in the flow. See stress tensor, Reynolds stresses.