The size distribution of moist regions in the tropical atmosphere exhibits two modes of organization, one at synoptic scales and the other at planetary scales. We show the existence of these two modes in two data sets: column-integrated precipitable water and outgoing long-wave radiation. A simple two-column model of radiation and convection in the Tropics helps interpret the presence of these two modes. The coupling between radiative effects and low-level atmospheric dynamics provides a plausible mechanism for scale selection.