  Three-dimensional simulations of the disk-halo interaction show the formation
of a thick HI and HII gas disk with different scale heights. The thick HI disk
prevents the disk gas from expanding freely upwards, unless some highly
energetic event such as chimneys occurs, whereas the thick HII disk acts as a
disk-halo interaction region from where the hot ionized gas flows freely into
the halo.
  The upflowing gas reaches the maximum height at $z\sim 9.3\pm 1$ kpc becoming
thermally unstable due to radiative losses, and condenses into HI clouds.
Because the major fraction of the gas is gravitationally bound to the Galaxy,
the cold gas returns to the disk. The descending clouds will have at some
height high velocities. In a period of 200 Myr of fountain evolution, some 10
percent of the total number of clouds are HVCs.
