Results from a statistical analysis of backscattered signal power measurements at 4–18 km by the clear-air Doppler radars (MST radars, or wind profilers ) in the Colorado Profiler Network and at Poker Flat, Alaska, show a systematic diurnal variation during the summer months. These diurnal variations have a high level of statistical significance. In the midtroposphere the diurnal range is near 7 dB with largest values in the afternoon, probably associated with the enhanced turbulence and moisture levels due to afternoon convection. In the stratosphere the diurnal range is about 5 dB over Colorado, with smallest values in the afternoon. The stratospheric cycle is interpreted as a response to modulation of the amplitude of gravity waves propagating upward from the troposphere. The diurnal cycle at all heights becomes indistinct during the winter months.