Context : Understanding of clouds is instrumental in interpreting current and future spectroscopic observations of exoplanets .
modeling clouds consistently is complex , since it involves many facets of chemistry , nucleation theory , condensation physics , coagulation , and particle transport .

Aims : We aim to develop a simple physical model for cloud formation and transport , efficient and versatile enough that it can be used , in modular fashion for parameter optimization searches of exoplanet atmosphere spectra .
In this work we present the cloud model and investigate the dependence of key parameters as the cloud diffusivity K and the nuclei injection rate \dot { \Sigma } _ { n } on the planet ’ s observational characteristics .

Methods : The transport equations are formulated in 1D , accounting for sedimentation and diffusion .
The grain size is obtained through a moment method .
For simplicity , only one cloud species is considered and the nucleation rate is parametrized .
From the resulting physical profiles we simulate transmission spectra covering the visual to mid-IR wavelength range .

Results : We apply our models toward KCl clouds in the atmosphere of GJ1214 b and toward MgSiO _ { 3 } clouds of a canonical hot-Jupiter .
We find that larger K increases the thickness of the cloud , pushing the \tau = 1 surface to a lower pressure layer higher in the atmosphere .
A larger nucleation rate also increases the cloud thickness while it suppresses the grain size .
Coagulation is most important at high \dot { \Sigma } _ { n } and low K .
We find that the investigated combinations of K and \dot { \Sigma } _ { n } greatly affect the transmission spectra in terms of the slope at near-IR wavelength ( a proxy for grain size ) , the molecular features seen at \sim \mu m ( which disappear for thick clouds , high in the atmosphere ) , and the 10 \mu \mathrm { m } silicate feature , which becomes prominent for small grains high in the atmosphere .

Conclusions : Clouds have a major impact on the atmospheric characteristics of hot-Jupiters , and models as those presented here are necessary to reveal the underlying properties of exoplanet atmospheres .
The result of our hybrid approach – aimed to provide a good balance between physical consistency and computational efficiency – is ideal toward interpreting ( future ) spectroscopic observations of exoplanets .