Traditionally in the determination of heat loads insummer, internal solar heat gains are calculated using nearextremesolar irradiation data. The latter are typically from theASHRAE Handbook—Fundamentals clear sky model in thefenestration chapter. Using these and other gains, the plant isthen sized based on the synoptic weather data (dry-bulb, wetbulb,etc.) near-extreme values. These are based on the valuesin the ASHRAE Handbook—Fundamentals using 0.4%,1.0%, and 2% exceeded values. Combining independent nearextremevalues and modeled data can lead to more severedesign conditions than are likely in reality. A method of determiningthese solar and synoptic near-extreme values frommeasured values is proposed based on examination of thecumulative distribution functions. Examination shows themethod to be robust and sensitive to changes in weather parameters.Such a method is also found to provide more realisticdesign conditions.