  Red supergiant stars represent a key phase in the evolution of massive stars.
Recent radiative hydrodynamic simulations suggest that their atmospheres may be
the location of large-scale convective motions. As supergiant convection is
expected to generate supersonic motions and shocks, we seek constraints on
these atmospheric motions and their possible relation with mass-loss rates. We
present high-resolution, visible spectroscopy of a sample of red supergiants
(spectral type M I) and analyse them with a tomographic technique. We observe
steep velocity gradients, characterising both upward and downward supersonic
motions, which are time variable on time scales of a few hundred days. These
convective motions will 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.
