Example from our Cloudy models. The cumulative emission of different bands and emission lines are presented (upperpanel), together with some physical parameters (density, electronic density and temperature, lower panel) as a function of the depth in the cloud, measured with the parameter, Ay. The emission of all of the tracers was normalized to a maximum of 1 to allow a direct comparison. The age of the central ionizing source is 4 Myr, and the initial values of physical parameters are log(n₀)= 2.5 and log(U) = −3.