1. A computer-implemented system for correlating overhead sky clearness for use in photovoltaic fleet output estimation, comprising: a computer comprising a processor and memory within which code for execution by the processor is stored, comprising: a temporal distance module configured to determine a temporal distance comprising a physical distance between two locations, which are each within a geographic region suitable for operation of a photovoltaic fleet, in proportion to cloud speed within the geographic region; an input sky clearness module configured to generate a set of input sky clearness indexes as a ratio of each irradiance observation in a set of irradiance observations that has been regularly measured for one of the locations within the geographic region, and clear sky irradiance; a clearness index correlation coefficient module configured to determine a clearness index correlation coefficient between the two locations as an empirically-derived function of the temporal distance; and an output sky clearness module configured to weigh the set of input sky clearness indexes by the clearness index correlation coefficient to form a set of output sky clearness indexes, which indicates the sky clearness for the other of the locations.