Derived physical properties from modelling of hydrogen RRL departure coefficients. For input physical conditions, electron density, and temperature, shown on the axes, we computed a departure coefficient (b_nβ_n) via RRL modelling. Plugging in the b_nβ_n, the integrated optical depth, quantum number, n_e and T_e into Eq. (2), we derive the emission measure (top left panel) and corresponding column density (top right panel) and path length (bottom left panel) of the RRL emission. Additionally we show the electron pressure (bottom right panel). Values representing the warm, ionised phase of a face-on (black) and an edge-on (white) Milky Way are shown with dashes in the colour bars. Hydrogen RRLs are expected to be most prominent from warm, T ∼ 1000 − 10 000 K, gas.