The Solar Prospecting Tools are based on 10+ year means (1997/1999 – 2018) from Version 1.0 of the Vaisala solar data set. The uncertainty ranges shown in the tools were determined from the global validation study we completed at the time the tools were released. At the time, the data set was as accurate as anything else available and in many places much better than previous sources. However, we have continued to develop our algorithms and subsequent versions, such as can be found in the Solar Time Series, are more accurate. Nevertheless, the Solar Prospecting Tools are still useful in comparing different locations to each other: although the precise values may change with later versions, the ranking of the sites will stay the same.

When we validated the dataset behind the Solar Prospecting Tools in 2018, the global uncertainty values were 5% for GHI, 11% for DNI, and 15% for DIF. These are the values behind the error bars you see in the annual and monthly averages.

Note that in some areas with highly reflective terrain, such as salt flats, some desert areas, and some areas with permanent snow, the satellite algorithms have difficulty distinguishing clouds from the terrain. As a result, the cloudiness estimates in these areas are higher than they should be, so that the amount of global horizontal and direct normal irradiance is underestimated and the diffuse irradiance in these areas is overestimated. Known areas affected by this problem include the Bonneville Salt Flats on the Nevada-Utah border and the White Sands Missile Range in New Mexico.