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A new class of exoplanets has emerged: the ultra hot Jupiters, the hottestclose-in gas giants. Most of them have weaker than expected spectral featuresin the $1.1-1.7\mu m$ bandpass probed by HST/WFC3 but stronger spectralfeatures at longer wavelengths probed by Spitzer. This led previous authors topuzzling conclusions about the thermal structures and chemical abundances ofthese planets. Using the SPARC/MITgcm, we investigate how thermal dissociation,ionization, H$^-$ opacity and clouds shape the thermal structures and spectralproperties of ultra hot Jupiters with a special focus on WASP-121b. We expandour findings to the whole population of ultra hot Jupiters through analyticalquantification of the thermal dissociation and its influence on the strength ofspectral features.  We predict that most molecules are thermally dissociated and alkalies areionized in the dayside photospheres of ultra hot Jupiters. This includesH$_{\rm 2}$O, TiO, VO, and H$_{\rm 2}$ but not CO, which has a strongermolecular bond. The vertical molecular gradient created by the dissociationsignificantly weakens the spectral features from water while the $4.5\mu m$ COfeature remain unchanged. The water band in the HST/WFC3 bandpass is furtherweakened by H$^-$ continuum opacity. Molecules are expected to recombine beforereaching the limb, leading to order of magnitude variations of the chemicalcomposition and cloud coverage between the limb and the dayside. Overall,molecular dissociation provides a qualitative understanding of the lack ofstrong spectral feature of water in the $1-2\mu m$ bandpass observed in mostultra hot Jupiters. Quantitatively, however, our model does not provide asatisfactory match to the WASP-121b emission spectrum. Together with WASP-33band Kepler-33Ab, they seem the outliers among the population of ultra hotJupiters in need of a more thorough understanding.

From thermal dissociation to condensation in the atmospheres of ultra hot Jupiters: WASP-121b in context