In this article, we report on the correlation between the irradiance variability observed at two neighboring sites as a function of their distance, and of the considered variability time scale. Correlation is the factor that determines whether the combined relative fluctuations of two solar systems add up when correlation is high, or attenuate when correlation is low. Using one-dimensional virtual networks in 24 US locations and cloud motion derived from satellites as experimental evidence, we observe station pair correlations for distances ranging from 100 m to 100 km and from variability time scales ranging from 20 s to 15 min. Within the limits of the assumptions from one-dimensional virtual networks, results show that the relationship between correlation, distance and time scale is predictable and largely independent of location and prevailing insolation conditions. Further, results indicate that the distance at which station pairs become uncorrelated is a quasi linear function of the considered time scale. (c) 2012 Elsevier Ltd. All rights reserved.