Atmospheric pressure level p [bar] at which the vertically integrated optical depth of cloud particles reaches unity (τ = 1) as a function of wavelength. Four cloud particle cases are shown: monodisperse distribution of compact particles (black), Gaussian distribution of compact particles (red), monodisperse distribution of highly micro-porous particles f_por = 0.9 (blue), and Gaussian distribution of highly micro-porous particles (green). The y-axis is inverted because we integrate from the top of the atmosphere. The atmosphere is T_eff = 1800 K, log (g) = 3.0.