The motion of the atmosphere above 50 km (30 mi). The predominant dynamical phenomena of the upper atmosphere are quite different from those encountered in the lower atmosphere. Among those encountered in the lower atmosphere are cyclones, anticyclones, tropical hurricanes, thunderstorms and shower clouds, tornadoes, and dust devils. Even the largest of these phenomena do not penetrate far into the upper atmosphere. Above an altitude of about 50 km (30 mi), the predominant dynamical phenomena are internal gravity waves, tides, sound waves (including infrasonic), turbulence, and large-scale circulation. These are described below.