@macroarray I think it may be based on this sample, which is up near 50,000K: https://fluxometer.com/rainbow/#!id=Outdoors/Blue Sky LA 2PM
This is especially blue but it is common to find the blue sky at 20,000-30,000K. CCT does go up dramatically as things get bluer, it's approximately 1/linear.
Most meters will only say CCT in a particular range (e.g., our measurement software goes to 100,000) - maybe the number is too large for this meter to report...