Galactic fountain of high-velocity clouds

We examine whether high-velocity clouds of neutral hydrogen (not the Magellanic Stream), observed in every longitude quadrant of the sky, can condense from a hot, dynamic corona above the plane of the Galaxy. Supernova-heated gas that arises above the disk must either flow outward as a wind or remain bound to the Galaxy in a dynamic corona in which gas is constantly in motion: a static corona is not stable. Since clouds cannot condense in a wind, we favor a corona and show that it is consistent with current values for the supernova heating rate above the disk. Gas continuously entering the corona from the disk rises up (in Z) and outward (in ω) in an attempt to reach static equilibrium with the corona. The gas behaves adiabatically until radiative losses become important. Then, coronal gas is susceptible to thermal instabilities and neutral clouds condense from the corona. Clouds form toward the top of the corona at about one density scale height because thermal instabilities grow most rapidly there. After formation, clouds fall ballistically toward their point of origin