Spatial correlations between ramp rates are important determinants for output variability of solar power plants, since correlations determine the amount of geographic smoothing of solar irradiance across the plant footprint. Previous works have modeled correlations empirically as a decreasing function of the distance between sites, resulting in isotropic models. Field measurements show that correlations are anisotropic correlations are different for along-wind site pairs than for cross-wind site pairs. Here, cloud fields are modeled using a spatial Poisson process. By advecting the cloud field using a constant cloud velocity, spatial correlations for ramp rates are obtained. Spatial correlations were shown to be a function of along-wind and cross-wind distance, ramp timescale, cloud speed, cloud cover fraction, and cloud radius. The resulting anisotropic correlation model explains the anisotropic effects well at timescales less than 60 s but performs worse than existing empirical isotropic models at longer time scales. (C) 2013 Elsevier Ltd. All rights reserved.