  We use a simple organism lifecycle model to explore the viability of an
atmospheric habitable zone (AHZ), with temperatures that could support
Earth-centric life, which sits above an environment that does not support life.
To illustrate our model we use a cool Y dwarf atmosphere, such as
$\mathrm{WISE~J}085510.83-0714442.5$ whose $4.5-5.2$ micron spectrum shows
absorption features consistent with water vapour and clouds. We allow organisms
to adapt to their atmospheric environment (described by temperature,
convection, and gravity) by adopting different growth strategies that maximize
their chance of survival and proliferation. We assume a constant upward
vertical velocity through the AHZ. We found that the organism growth strategy
is most sensitive to the magnitude of the atmospheric convection. Stronger
convection supports the evolution of more massive organisms. For a purely
radiative environment we find that evolved organisms have a mass that is an
order of magnitude smaller than terrestrial microbes, thereby defining a
dynamical constraint on the dimensions of life that an AHZ can support. Based
on a previously defined statistical approach we infer that there are of order
$10^9$ cool Y brown dwarfs in the Milky Way, and likely a few tens of these
objects are within ten parsecs from Earth. Our work also has implications for
exploring life in the atmospheres of temperate gas giants. Consideration of the
habitable volumes in planetary atmospheres significantly increases the volume
of habitable space in the galaxy.
