To the best of my knowledge the following is true.......
ASHRAE has selected 40 deg C as the temp most closely corresponding to the maximum average ambient temperature for most of the US.
ANSI/IEEE and NEMA have used this as a baseline and standardization for equipment design because the impact of solar radiation varies by geographic location, degree of exposure to sunlight, time of day, and season. It would not be economical to design electrical equipment with custom ratings to correspond with a particular set of solar radiation conditions. For similar reasons, it is usually not appropriate to design for all the other influences to simultaneously fall into their most adverse ranges, e.g., maximum ambient, highest load current, maximum solar radiation, or zero wind.
Therefore, it becomes the responsibility of the designer to understand and determine appropriate solar radiation correction factors for the particular installation.