Predicted pressure at which the atmosphere becomes optically thick to continuum radiation along the transit chord due to absorption by H⁻, averaged over the HARPS waveband, as a function of atmospheric temperature and metallicity. The dashed lines indicate the metallicity and temperature chosen in this study, for which the continuum is predicted to become optically thick at a pressure of 3.5 mbar. For solar metallicity, the continuum is formed between 3 and 4 mbar for temperatures between 3500 and 5500 K. For most of this parameter space, the continuum is formed between pressures of 2 and 6 mbar. This indicates that the peak pressures obtained in Table 2 are robust against errors in the assumed temperatureand metallicity to within a factor of order unity.