  We report on the first phase of our study of cloud irradiation. We study
irradiation by means of numerical, two-dimensional time-dependent
radiation-hydrodynamic simulations of a cloud irradiated by a strong radiation.
We adopt a very simple treatment of the opacity, neglect photoionization and
gravity, and instead focus on assessing the role of the type and magnitude of
the opacity on the cloud evolution. Our main result is that even relatively
dense clouds that are radiatively heated (i.e., with significant absorption
opacity) do not move as a whole instead they undergo a very rapid and major
evolution in its shape, size and physical properties. In particular, the cloud
and its remnants become optical thin within less than one sound crossing time
and before they can travel over a significant distance (a distance of a few
radii of the initial cloud). We also found that a cloud can be accelerated as a
whole under quite extreme conditions, e.g., the opacity must be dominated by
scattering. However, the acceleration due to the radiation force is relatively
small and unless the cloud is optically thin the cloud quickly changes its size
and shape. We discuss implications for the modelling and interpetation broad
line regions of active galactic nuclei.
