Left panel: physical structure of a generic hot-Jupiter atmosphere. The temperature T(P) and gas density profile ρ_gas(P) are obtained from the Guillot (2010) atmosphere model. The equilibrium density ρ_eq of MgSiO₃ is obtained from Eq. (18). Below the cloud deck (horizontal dashed line), ρ_v = ρ(MgSiO₃) < ρ_eq; the species are present at constant abundance, such that x_v = ρ_v∕ρ_gas ≡ x_v,bot is constant. Above the cloud base (ρ_v > ρ_eq; light blue) cloud formation (not included in this figure) will reduce the vapor density at the expense of condensates. Right panel: same for KCl in the GJ1214 b atmosphere.