= Solution
The inferred dayside <water> mixing ratio is about $500$ times below the solar-equilibrium reference. Several explanations are possible. A high <atmospheric carbon-to-oxygen ratio>, especially near or above unity in hot <carbon monoxide>-dominated gas, locks much of the oxygen in CO and leaves little <water>. A genuinely oxygen-poor or low-metallicity envelope also reduces <water>, although bulk elemental abundances must apply to both hemispheres. <Atmospheric photochemistry> can destroy <water> at low pressures. In portions of the dayside that are substantially hotter than the stated minimum, <thermal dissociation> can also reduce <water>, especially at low pressure; \b[$1800\,\mathrm K$ alone does not establish strong dissociation throughout the emitting region]. Temperature/<opacity> degeneracies or incomplete treatment of <exoplanet clouds> and horizontal structure in an <atmospheric retrieval> can bias the inferred abundance.
At the cooler terminator, $10^{-4}$ is only a factor of five below the reference. \b[Moderately reduced oxygen abundance, an enhanced C/O ratio, and <exoplanet cloud>/<atmospheric haze> dilution of spectral features] are plausible explanations. <Water> is comparatively stable at $1000\,\mathrm K$, so strong <thermal dissociation> is not the natural explanation there. A <exoplanet transmission spectrum> samples a slant path through the limb, and the degeneracy among <exoplanet cloud> height, reference pressure and gas abundance can mimic a lower mixing ratio.
The two measurements need not describe the same pressure range or longitude. \b[Local dissociation/photochemistry on the hotter dayside and reformation on the cooler limb can produce a real spatial difference]. Even at one elemental C/O ratio, hot <carbon monoxide>-rich chemistry can leave less oxygen for <water> than cooler <methane>-rich chemistry, an example of <carbon partition and atmospheric water abundance>. Transport can modify or homogenize those tendencies, depending on the reaction and advection timescales. Alternatively, inconsistent assumptions in the emission and transmission retrievals can create an apparent discrepancy. A reconciliation should use one bulk elemental inventory, separate dayside/limb temperature profiles and contribution pressures, and consistent <exoplanet cloud> and transport physics; it should not assign independent planetary metallicities to the two hemispheres.
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