Users of whole-building simulation programs spend time and resources in selecting the most appropriate weather file for a simulation, whether it be a historical or actual weather file. However, they rarely check the algorithms used in the programs to process the weather data into the components used in the simulation program. This paper investigates some of the different algorithms used in popular simulation programs to determine effective sky temperatures and sub-hourly solar radiation from the hourly values available in typical weather data sources. The impact of these algorithmic differences on the simulation results are highlighted.