Turbulence is important for its e}ects on atmospheric chemistry\ mixing air of di}erent composition[ For over 39 years\ it has been known that wind shear associated with the jet stream is one important cause[ However\ atmospheric radar shows that other processes are also very signi_cant*speci_cally\ the breaking of internal gravity!waves generated by ~ow of air over mountains[ These mountain waves propagate upward from their source near the ground\ and can generate turbulence many kilometres higher in the atmosphere[ Turbulence near the tropopause level has been invoked as a mechanism for exchange of air between stratosphere and troposphere\ including constituents such as ozone and water vapour[ VHF radar data here show direct\ high!resolution evidence of this turbulence\ persisting for two days[ Breaking of mountain waves is known to be an essential input for improved global circulation models and weather models^also\ on a more general note\ atmospheric turbulence at these altitudes is not only an abstract problem\ but experienced at _rst hand through its dangerous e}ect on jet aircraft travel[ Calculations of the intensity of turbulence\ using energy dissipation rates derived from broadening of Doppler echo spectra\ show that mountain wave breaking is sometimes the single most important source of turbulence in the lower atmosphere[ ร 0887 Elsevier Science Ltd[ All rights reserved[