  We have studied head-on collisions between equal-mass, mildly supersonic
(Mach number 1.5) HI clouds, in a standard Two-phase ISM (T_cl = 74 K, n_cl =
22 cm^-3, \chi = 100). We explore the role of various factors, including the
radiative cooling parameter \eta = t_rad/t_coll (t_coll=R_c/v_c), evolutionary
modifications on the cloud structure (by colliding clouds ``evolved'' through
independent motion within the intercloud medium (WIM)), and the symmetry of the
problem (by colliding initially identical clouds, evolved to different ages
before impact). The presence of bow shocks and ram pressure from material in
the cloud wake, developed during such evolution through the WIM, significantly
alters these interactions with respect to the standard case of non-evolved
clouds.
  In general, in our adiabatic collisions the clouds are disrupted and convert
their gas into a few low density contrast clumps. By contrast, for symmetric
radiative cases we find that the two clouds coalesce, with almost all the
initial kinetic energy radiated away. On the other hand, for both adiabatic and
radiative collisions, asymmetric collisions have a much greater tendency to
disrupt the two clouds. Fragmentation of the clouds may occur, and
instabilities are in general enhanced. In addition, radiative cooling is less
efficient in our asymmetric interactions, so that those parts of the clouds
that initially seem to merge are more likely to re-expand and fade into the
WIM. Since the majority of real cloud collisions should be asymmetric for one
reason or another, we conclude that most gasdynamical diffuse cloud collisions
will be disruptive, at least in the absence of significant self-gravity or of a
significant magnetic field.
