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Nissan develops paint that keeps cars cool in summer heat
(www.asahi.com)
This is a most excellent place for technology news and articles.
Why would it be anything else?
It's clearly too low a number to be °K. And since the only two valid units of measurement for temperature are Kelvin and Celsius, it must be °C.
A differential temperature of 12C is equal to a differential temperature of 12K….. You don’t take the offset into account for differential temperatures.