Hydrogen density structure (upper panels) and cumulative intensity of some fine-structure emission lines (lower panels) of clouds with Z_gas = 0.2 Z_⊙ (left hand panels) and Z_gas = 3.0 Z_⊙ (right hand panels), as a function of the hydrogen column density. For both cases, n_H = 10³ cm^-3 and U = 10^-2.5 are adopted (where n_H is the hydrogen density at the illuminated face of clouds). Solid, dashed, and dotted lines in the upper panels denote the densities of H⁺, H⁰, and H₂, respectively. In the lower panels, solid lines show cumulative intensities for fine-structure emissions mainly from Hii regions, while dashed lines are for emissions that are emitted also, or mostly from PDRs.