  We present results from moist convection in a mixture of pressurized sulfur
hexa-flouride (liquid and vapor) and helium (gas) to model the wet and dry
components of the earth's atmosphere. To allow for homogeneous nucleation, we
operate the experiment close to critical conditions. We report on the
nucleation of microdroplets in the wake of large cold liquid drops falling
through the supersaturated atmosphere and show that the homogeneous nucleation
is caused by isobaric cooling of the saturated sulfur hexaflouride vapor. Our
results carry over to atmospheric clouds: falling hail and cold rain drops may
enhance the heterogeneous nucleation of microdroplets in their wake under
supersaturated atmospheric conditions. We also observed that under appropriate
conditions settling microdroplets form a rather stable horizontal cloud layer,
which separates regions of super and sub critical saturation.
