  We show that the turbulent gas in the star-forming regions of galaxies is
unstable to wind formation via momentum deposition by radiation pressure or
other momentum sources like supernova explosions, even if the system is below
the average Eddington limit. This conclusion follows from the fact that the
critical momentum injection rate per unit mass for unbinding gas from a
self-gravitating system is proportional to the gas surface density and that a
turbulent medium presents a broad distribution of column densities to the
sources. For an average Eddington ratio of <Gamma>~0.1 and for turbulent Mach
numbers >30, we find that ~1% of the gas is ejected per dynamical timescale at
velocities larger than the local escape velocity. Because of the lognormal
shape of the surface density distribution, the mass loss rate is highly
sensitive to the average Eddington ratio, reaching 20-40% of the gas mass per
dynamical time for <Gamma>=1. Using this model we find a large scatter in the
mass-loading factor for star-forming galaxies, ranging from 0.001-10, but with
significant uncertainties. Implications for the efficiency of star formation in
giant molecular clouds are highlighted. For radiation pressure feedback alone,
we find an increasing star formation efficiency as a function of initial gas
surface density. Uncertainties are discussed.
