Observations from a Doppler lidar, which are enhanced by stratospheric aerosols from Mount Pinatubo's eruption, provide unique measurements of mountain waves over the Rocky Mountains, including nearly instantaneous observations of wave breaking in the lower stratosphere. The wave breaking is revealed by a flow-reversal aloft that marks a localized critical layer created when the wave amplitude was great enough to cause overturning. The altitude, spatial scale, and duration of this wave-induced critical layer are documented. The mountain waves led to turbulence affecting commercial aircraft, including one accident in which a cargo jet lost an engine and part of a wing in extreme turbulence at the same time the lidar was making observations nearby. Although earlier observations of mountain-wave drag assumed wave stationarity over 3--5 h, significant changes over less than 2 h are documented here. Such nonstationarity could contribute to discrepancies remaining between observations and simulations of mountain-wave drag.¿ 1997 American Geophysical Union