  Observations of active regions and limb prominences often show cold, dense
blobs descending with an acceleration smaller than that of free fall. The
dynamics of these condensations falling in the solar corona is investigated in
this paper using a simple fully ionised plasma model. We find that the presence
of a heavy condensation gives rise to a dynamical rearrangement of the coronal
pressure that results in the formation of a large pressure gradient that
opposes gravity. Eventually this pressure gradient becomes so large that the
blob acceleration vanishes or even points upwards. Then, the blob descent is
characterised by an initial acceleration phase followed by an essentially
constant velocity phase. These two stages can be identified in published
time-distance diagrams of coronal rain events. Both the duration of the first
stage and the velocity attained by the blob increase for larger values of the
ratio of blob to coronal density, for larger blob mass, and for smaller coronal
temperature. Dense blobs are characterised by a detectable density growth (up
to 60% in our calculations) and by a steepening of the density in their lower
part, that could lead to the formation of a shock. They also emit sound waves
that could be detected as small intensity changes with periods of the order of
100 s and lasting between a few and about ten periods. Finally, the curvature
of the falling path is only relevant when a very dense blob falls along
inclined magnetic field lines.
