Cool Stars 14 - Submitted Abstract # 167 This version created on 05 October 2006 Atmospheric dynamics and the mass loss process in red supergiants Eric Josselin, GRAAL-Universite Montpellier II Bertrand Plez, GRAAL-Universite Montpellier II We present high-resolution visible spectroscopy of a sample of red supergiants. We observe large velocity gradients, characterizing both upward and downward supersonic motions, which are time variable, on time scales of a few hundreds days. These convective motions must generate turbulent pressure, which will strongly decrease the effective gravity. We suggest that this decrease, combined with radiative pressure on molecular lines, initiate the mass loss in red supergiant stars. ----------------------------------