Numerical and analytical models of sand and dust particle accumulation on photovoltaic modules in dry regions are presented and supported by a laboratory investigation of sand particle accumulation on a glass surface. Both models and the experimental data indicate that the reduction in the free fractional area can be described by an exponential decay resulting from the formation of clusters of particles. Such clusters can support particles in upper layers which reduce the available area for photon capture by a much smaller amount than particles resting directly on the glass surface. The results qualitatively describe existing field data beyond the linear regime and are developed to account for field conditions, including analysis of photovoltaic module tilt, humidity and wind speed. This investigation is intended as a basis of an engineering design tool to assess the case for including photovoltaics in dry regions.