Relationships for determining the maximum permissible distance for extrapolating dairy totals of solar irradiation from measurement sites are established for mesoscale monitoring networks in Egypt. The proposed method involves calculation of the standard deviation of the daily differences in solar radiation receipt for each couple of measurement stations to determine the variability coefficient. This is related to distance between station pairs to establish relationship due to extrapolation distance. Results indicate that the solar climate has much greater spatial coherence which permit data extrapolation of data over long distances. A comparison to temperature extrapolated data confirm the same finding.