For instance, Joe Schmoe sees frost on his car or grass. He says hey it must be 32 out!! He's probably close to being correct. If he looks at his thermo and sees it is 35 with nice signs of frost...I don't think he'll get excited about saying he had his first frost. I think there is a difference in a field that is subject to radiational cooling for sure as you said. Objects on a clear and calm night will act to be cooler than the ambient air temp.