What Is Frost ?

[jmsieglaff]

Where frost forms the temperature is freezing or less. Where your thermometer is it could be 35 or 40 or whatever. Have a frost prone spot put a thermometer there to see how it differs from your other(s).

Temperature inversions (coldest at ground level) and warming with height are most common on frost conducive nights (e.g.-calm and clear). It is common for
a 3-5F temperature inversion to exist from ground to standard thermometer height on nights like that (6 ft).

On calm nights small variations in terrain height matter, with the coldest air draining and pooling in the lowest spots due to gravity.

So frost can form "when the temperature is above freezing" because your thermometer isn't where the frost is.

Originally Posted by Gardeneer

Right. Radiation heat loss to the space is what happens in the deserts .
So that is what frost at 35F happened last night. It might even happen with tonights's low of 40 with clear starry skies.