1. A method for managing photovoltaic fleet output with the aid of a digital computer, comprising the steps of: providing a computer with a physical distance between two locations in a geographic region suitable for operation of a photovoltaic fleet and evaluating a temporal distance as a ratio of the physical distance and cloud speed above the photovoltaic fleet within the geographic region; measuring sky clearness over the two locations using ground-based sensors; providing the computer with recorded irradiance measures for the one of the locations comprising a time series of irradiance measures recorded at successive times, comprising the steps of: collecting raw irradiance measures from sensors at ground-based weather stations located in the geographic region as point statistics; and setting each recorded irradiance measure as an average of the values of the raw irradiance measures; providing the computer with forecast irradiance measures for the one location, each forecast irradiance measure corresponding to one of the recorded irradiance measures; bounding in the computer statistical error between the recorded irradiance measures and the forecast irradiance measures by evaluating in the computer a mean and standard deviation; relating in the computer the sky clearness that was measured over the two locations with a correlation coefficient by solving in the computer an empirically-derived exponential function of the temporal distance; generating in the computer an input set of forecast clearness indexes as a ratio of each of the forecast irradiance measures for the one location, and clear sky irradiance; and weighting in the computer each forecast clearness index by the correlation coefficient to form an output set of forecast clearness indexes for the other location and proportioning in the computer the mean and standard deviation to the output set of forecast clearness indexes; and comparing the output set of forecast clearness indexes to the sky clearness that was measured over the two locations.