1. Altitude at which the relative horizontal phase speed of internal gravity waves equals the wind speed. As waves approach this altitude from above or below, the vertical component of group velocity approaches zero, causing elimination of the wave as its energy is absorbed and transferred to the mean wind. This causes changes in the wind-speed profile, which in turn can raise or lower the altitude of the critical level. 2. Altitude at which the wind shear is sufficiently strong to cause the gradient Richardson number to drop below its critical value, allowing Kelvin–Helmholtz waves to form. These breaking waves can generate propagating internal gravity waves in the adjacent air above and below. See buoyancy wave. 3. The level at which the ocean surface wave phase speed is equal to the wind (or current) speed. Considerable interaction between the wave field and the mean flow field can be associated with processes at this level. 4. See critical level of escape.