  Ram pressure stripping can remove significant amounts of gas from galaxies
that orbit in clusters and massive groups, and thus has a large impact on the
evolution of cluster galaxies. In this paper, we reconstruct the present-day
distribution of ram-pressure, and the ram pressure histories of cluster
galaxies. To this aim, we combine the Millennium Simulation and an associated
semi-analytic model of galaxy evolution with analytic models for the gas
distribution in clusters. We find that about one quarter of galaxies in massive
clusters are subject to strong ram-pressures that are likely to cause an
expedient loss of all gas. Strong ram-pressures occur predominantly in the
inner core of the cluster, where both the gas density and the galaxy velocity
are higher. Since their accretion onto a massive system, more than 64 per cent
of galaxies that reside in a cluster today have experienced strong
ram-pressures of $>10^{-11}$ dyn cm$^{-2}$ which most likely led to a
substantial loss of the gas.
