  Both observations and recent numerical simulations of the circumgalactic
medium (CGM) support the hypothesis that a self-regulating feedback loop
suspends the gas density of the ambient CGM close to the galaxy in a state with
a ratio of cooling time to freefall time >10. This limiting ratio is thought to
arise because circumgalactic gas becomes increasingly susceptible to multiphase
condensation as the ratio declines. If the timescale ratio gets too small, then
cold clouds precipitate out of the CGM, rain into the galaxy, and fuel
energetic feedback that raises the ambient cooling time. The astrophysical
origin of this so-called precipitation limit is not simple but is critical to
understanding the CGM and its role in galaxy evolution. This paper therefore
attempts to interpret its origin as simply as possible, relying mainly on
conceptual reasoning and schematic diagrams. It illustrates how the
precipitation limit can depend on both the global configuration of a galactic
atmosphere and the degree to which dynamical disturbances drive CGM
perturbations. It also frames some tests of the precipitation hypothesis that
can be applied to both CGM observations and numerical simulations of galaxy
evolution.
