The variation in the pressure as a function of wavelength is given according to Eq. (3) (solid line). Twice the solar NaI abundance is assumed. A temperature of 900 K (corresponding to the sodium condensation temperature) is also assumed and the two spectral reference domains are indicated as vertical long dashed lines. In those regions the evaluated pressure (thick solid lines) is indeed on the order of 3 mbar as indicated by the horizontal dashed line.