  We model the ascent of warm, moist air in the Earth's atmosphere by turbulent
convection and expansion with the KPZ equation, familiar in the physics
literature on surface growth. Clouds form in domains where the interface
between the rising air and its surrounding air achieves an elevation higher
than that necessary for condensation. The model predictions are consistent with
the perimeter fractal dimension and the cumulative frequency-size distribution
of cumulus cloud fields observed from space.
