I Why is there no frost below a certain temperature?

[gary350]

It is 21°F here this morning and no frost on anything not even car windows. I have noticed this before when it gets cold enough there is no frost. Maybe all the moisture freezes out of the air and there is no moisture to make frost? I don't know? At what temperature is there no frost?

 

[anorlunda]

That's a good question. I think I found the answer in the very first sentence of the Wikipedia article.

https://en.wikipedia.org/wiki/Frost
Frost is a thin layer of ice on a solid surface, which forms from water vapor in an above-freezing atmosphere coming in contact with a solid surface whose temperature is below freezing,[1][2] and resulting in a phase change from water vapor (a gas) to ice (a solid) as the water vapor reaches the freezing point.

So if the atmosphere is below freezing, you no longer have the conditions for frost.

Often you can answer your own questions better by checking sources like Wikipedia first.

 

[kuruman]

I should add that the solid surface temperature drops below freezing when there are clear skies overnight and there is heat loss on the solid surfaces due to radiational cooling. When I lived in a cold climate, it was frequently the case that when I parked my car in the driveway outside the garage, I only had to scrape frost from the windshield (rear or front) that faced away from the house. I attributed that to radiation from the house keeping the near windshield warm enough to prevent frost formation.

 

[Lnewqban]

Maybe all the moisture freezes out of the air and there is no moisture to make frost? I don't know?

Please, see:
https://hvacrschool.com/relative-humidity-of-air-below-freezing/

Absolute humidity (content of water vapor in the mass of air) of cold air is less than that of warmer air, but is not zero.

 

[256bits]

That's a good question. I think I found the answer in the very first sentence of the Wikipedia article.
So if the atmosphere is below freezing, you no longer have the conditions for frost.

Often you can answer your own questions better by checking sources like Wikipedia first.

I not sure the wiki explanation is correct, especially the part about the above atmospheric temperature.
Frost can form at any temperature of the air.
The particular explanation given is why frost can form on plants, or car windows, when the air is above 32 F.

It is a similar argument of why droplets of water form on a cold drink.
The surface of the drink has to be at/below the dew point of the air, meaning in the vicinity of the glass surface the relative humidity is 100%, or supersaturated , and dew will form.

For frost to form on a surface, the relative humidity at the surface has to be below the frost point.
While winter air is dry, simply speaking, it does contain water vapour.
That water vapour can condense out of the air onto a cold surface which is below the frost point forming that annoying cover on car windshields, or the beautiful rendition of nature's art on trees.

So during the daytime, when the air is warmer, but below freezing, the air will contain a certain amount of moisture. For one particular situation such as on a clear night where surfaces radiate their heat out into space, the relative humidity near the surface can become 100^, promoting the formation of frost on the surface.

One may note that the different types of frost, such as the hard to scrape of stuff versus the light crunchy stuff depends upon the moisture content of the cold air, the temperature, and the rate and amount of deposition as solid water onto the surface.

Look up hoar frost .
Hoar frost requires slightly different conditions. It forms when the water vapour in the air comes into contact with solid surfaces that are already below freezing point. Ice crystals form immediately, and the ice continues to grow as more water vapour is frozen. On a still night, it can grow well on tree branches, where the surface temperature is unlikely to rise above zero for several hours
https://www.woodlandtrust.org.uk/blog/2019/02/what-is-hoar-frost/