The authors have previously reported a model experiment on the unsteady natural convection in a triangular domain induced by the absorption of solar radiation. This issue is reconsidered here both analytically and numerically. The present study consists of two parts: a scaling analysis and a numerical simulation. The scaling analysis for small bottom slopes reveals that a number of flow regimes are possible depending on the Rayleigh number and the relative value of certain non-dimensional parameters describing the flow. In a typical situation, the flow can be classified broadly into a conductive, a transitional or a convective regime determined merely by the Rayleigh number. Proper scales have been established to quantify the flow properties in each of these flow regimes. The numerical simulation has verified the scaling results.