Comparison of the chemical equilibrium (blue) and non-equilibrium (orange) models of this work to sample spectra (black) forming a T–Y spectral sequence. Left and right columns: non-equilibrium models with the strong and weak K_zz mixing relationships with surface gravity, respectively. Spitzer IRAC photometry is plotted as squares and WISE photometryas circles. Top panel: observed spectrum of the T9 dwarf UGPS 0722 (Lucas et al. 2010; Leggett et al. 2012), with Spitzer IRAC and WISE photometric points from Kirkpatrick et al. (2012). Middle and bottom panels: 0.9− 1.7 μm HST WFC3spectra of the Y0- and Y1-type dwarfs WISE 1206 and WISE 1541 (Schneider et al. 2015), with Spitzer IRAC photometry also from Schneider et al. (2015) and WISE photometry from Cutri et al. (2013).