Emission spectrum of WASP-121b obtained from Trappist (at 0.9 μm; see Delrez et al. 2016), HST/WFC3 (between 1 and 2 μm), and Spitzer (at 3.6 μm; see Evans et al. 2017). The observations are compared to the dayside spectrum from a SPARC/MITgcm with solar metallicity. Also shown are models with the same thermal structure but with either H⁻ opacities or water dissociation neglected when calculating the spectrum. Both H⁻ opacities and water dissociation are responsible for the weak water signature in the WFC3 bandpass.