Ammonia-to-methane ratios derived for late-T dwarfs and Y dwarfs with a 95% confidence level from NIRSPEC data (blue) and HST data (red) compared with those of Jupiter and Saturn from IRTF data (red) as functions of spectral type (upper panel) and T_eff (lower panel). The T_eff of T dwarfs, Y dwarfs, Jupiter, and Saturn adopted in this work are listed in Table 1. The dashed pink curves are the third-degree polynomial fittings for all objects (except Saturn) against spectral type and T_eff, and the dashed black line is a linear fitting just for objects from T8 to Y2. In the lower panel, the results are compared with the scaled theoretical ratios of molecular column densities estimated from Zahnle & Marley (2014), which are shown in light blue.