also play a role in controlling the spectra of at least some brown dwarfs and most extrasolar giant planets. Indeed one suggested classification scheme (Sudarsky, Burrows & Pinto 2000) for extrasolar planets hinges on the specific atmospheric condensates present. Yet despite the importance of condensates there exists no simple model for predicting the parameters most relevant to radiative transfer: the vertical profile of condensate mass and its distribution over particle size.

Chemical equilibrium models (e.g., Lewis 1969; Fegley & Lodders 1994) predict which species are expected to condense in an atmosphere, yet they provide no guidance as to the expected particle sizes. Other models (e.g., Rossow 1978; Lunine et al. 1989; Carlson, Rossow, & Orton 1988) predict some parameters, but lack a simple, self-consistent recipe for exploring the possible phase space in which clouds might exist.

A single example motivates the need for cloud models in substellar atmospheres. With increasingly later spectral type the warm L-dwarfs become progressively redder in their $J-K$ color (e.g., Kirkpatrick et al. 1999; Mart[í]{}n et al. 1999; Fan et al. 2000). Spectral fitting and models (e.g., Leggett, Allard, & Hauschildt 1998; Chabrier et al. 2000; Marley 2000) demonstrate that this is due to the progressive 