  The James Webb Space Telescope (JWST) is now observing Y dwarfs, the coldest
known brown dwarfs, with effective temperatures T_eff <= 475 K. The first
published observations provide important information: not only is the
atmospheric chemistry out of equilibrium, as previously known, but the
pressure-temperature profile is not in the standard adiabatic form. The rapid
rotation of these Jupiter-size, isolated, brown dwarfs dominates the
atmospheric dynamics, and thermal and compositional changes disrupt convection.
These processes produce a colder lower atmosphere, and a warmer upper
atmosphere, compared to a standard adiabatic profile. Leggett et al. (2021)
presented empirical models where the pressure-temperature profile was adjusted
so that synthetic spectra reproduced the 1 <= lambda um <= 20 spectral energy
distributions of brown dwarfs with 260 <= T_eff K <= 540. We show that spectra
generated by these models fit the first JWST Y dwarf spectrum better than
standard-adiabat models. Unexpectedly, there is no 4.3 um PH_3 feature in the
JWST spectrum and atmospheres without phosphorus better reproduce the 4 um flux
peak. Our analysis of new JWST photometry indicates that the recently
discovered faint secondary of the WISE J033605.05-014350AB system
(Calissendorff et al. 2023) has T_eff = 295 K, making it the first dwarf in the
significant luminosity gap between the 260 K WISE J085510.83-071442.5, and all
other known Y dwarfs. The adiabat-adjusted disequilibrium-chemistry models are
recommended for analyses of all brown dwarfs cooler than 600 K, and a grid is
publicly available. Photometric color transformations are provided in an
Appendix.
