clear skies and radiation frost is a fascinating subject. Inland, it's frost 99% of the time. In an inland coastal position like here, it depends on the atmospheres boundary layers.. those layers can rise and fall through the night. Sometimes, it falls sharply here, then recovers later in the night as warm air boundary layer falls. It's quite unpredictable whether frost will occur in borderline conditions. Other times the warm air boundary level is so high which means harsh frost, even from the same conditions.
I know I will have recorded my first air frost by the 20th, and many will too, except mild, and true coastal locations.