  We show how to construct families of stationary hydrodynamical configurations
that reproduce the observed vertical gradient of the rotation velocity of the
extra-planar gas in spiral galaxies. We then present a simple model for the
lagging halo of the spiral galaxy NGC 891, which is in agreement with the HI
observations. Our method is based on well known properties of baroclinic
solutions, and it is an elementary application of a much more general and
flexible method.
