Cumulus Support

I recorded 9.6 today, and thought that was a bit high.

The sky was the bluest I have ever seen it!

However, I checked DEFRA's website showing official UV levels in Camborne, and they recorded 9.5. Even Leeds recorded 8.8 - 8.9

What's the reasoning behind the high levels? Lack of Ozone?

A very warm and relatively thick/deep troposphere with a compensatory thinner/shallower stratosphere above. The stratosphere is where the ozone is. This is part of the reason. The other part is the relatively high water vapour content in the lower part of the current airmass with relatively dry air above. Also, if you have any low-level cloud around - especially thin stratus - it can contribute to the level sensed by your sensor via reflection and refraction.