Knudsen number, Kn = l_MFP∕(2a) (l_MFP – mean free path, a – particle radius) for different exoplanet and brown dwarf DRIFT-Phoenix model atmosphere results, except for Jupiter and Venus. The number Kn indicates whether the cluster interactions occur through diffusion (Kn < 1) or in a free molecular flow (Kn > 1). Exoplanet and brown dwarf atmospheres are predominately in the regimes of a free molecular flow (Kn > 1). Knudsen numbers for Earth’s atmosphere and typical cloud particle sizes are ≈ 10⁻⁵ and therefore fall in the turbulent, viscous case.