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The 21 cm signal from cosmic hydrogen is one of the most propitious probes ofthe early Universe. The detection of this signal would reveal key informationabout the first stars, the nature of dark matter, and early structureformation. We explore the impact of an emissive and reflective, or `hot',horizon on the recovery of this signal for global 21 cm experiments. It isdemonstrated that using physically motivated foreground models to recover thesky-averaged 21 cm signal one must accurately describe the horizon around theradiometer. We show that not accounting for the horizon will lead to a signalrecovery with residuals an order of magnitude larger than the injected signal,with a log Bayesian evidence of almost 1600 lower than when one does accountfor the horizon. It is shown that signal recovery is sensitive to incorrectvalues of soil temperature and reflection coefficient in describing thehorizon, with even a 10% error in reflectance causing twofold increases in theRMSE of a given fit. We also show these parameters may be fitted using Bayesianinference to mitigate for these issues without overfitting andmischaracterising a non-detection. We further demonstrate that signal recoveryis sensitive to errors in measurements of the horizon projection onto the sky,but fitting for soil temperature and reflection coefficients with priors thatextend beyond physical expectation can resolve these problems. We show thatusing an expanded prior range can reliably recover the signal even when theheight of the horizon is mismeasured by up to 20%, decreasing the RMSE from themodel that does not perform this fitting by a factor of 9.

Modelling a Hot Horizon in Global 21 cm Experimental Foregrounds