A flow of turbulence kinetic energy from larger eddies to smaller eddies. In the atmospheric boundary layer, turbulence is usually produced at scales roughly equal to the boundary layer depth (order of 1 km) by buoyancy or wind shear, and is dissipated by viscosity into heat at the smallest scales (order of 1 mm). Richardson's (1922) poem eloquently describes this cascade: Big whorls have little whorls, which feed on their velocity / And little whorls have lesser whorls, and so on to viscosity.