Below what freezing temperature is it not possible to have frost?

[gary350]

I once had a job servicing microwave repeater station. About 1000 ft up on the tower are reflectors/antenna they have heaters that came on at 40 degrees F. Heaters stayed on until about minus 18 degrees F then heaters turned off. When it gets cold enough all humidity in the air freezes then it is not possible to have frost with no humidity. I can't remember what negative temperature frost can not form I am thinking -18 degree F but not sure that is correct?

I did online search & not finding the answer probably because I am probably not asking the search question with the correct words. This condition probably has a special name that I don't know.

 

[jrmichler]

Water ice has a vapor pressure, even at low temperatures. If the temperature of a surface is lower than the dew point of the air, then there will be condensation. That condensation is liquid water above 32 deg F, and frost below 32 deg F. While there is very little water vapor in air at low temperatures, there is always some water vapor. There is no minimum temperature below which frost cannot form, but frost is unlikely at temperatures well below zero deg F.

ASHRAE Psychrometric Chart No. 2 goes down to -40 deg F. And here is a paper that gives the vapor pressure of ice for temperatures from 0 down to -100 deg C: https://nvlpubs.nist.gov/nistpubs/jres/81A/jresv81An1p5_A1b.pdf. For more information, try search criteria vapor pressure ice.

 

[russ_watters]

To put some specific numbers to that, at a 30F dewpoint, the absolute humidity is 34 grains/lb (you don't need to know what a gain is, but it is 1/7000th of a pound) and at 0F is 5.5 grains/lb. So there's only 1/6th as much water in the air even available to create frost, and since frost requires cooling the ground below the temperature of the surrounding air, that's a significant barrier to frost formation.

 

[gary350]

Location is southern Illinois town of Mt Carmel which is 40 miles north of Evansville Indiana. Winter wind does not blow very much it was seldom 10 mph. No rain in winter months. 50 years ago when I lived there once it got cold it seldom got above 35 degree F during the day all winter. Winters now are warmer than winters 50 years ago. It was very rare to have clouds in winter, clear sky with lots of stars unless it was snowing. I am guessing maybe geographical location and weather determined the temperature setting on each and every microwave tower in the USA. Weather 500 ft to 1000 ft up could be different than ground level. We went there once a week to take readings and make adjustments. All I remember is antenna heaters come on at 40 deg F and turned off about -18 deg F. I was told heaters are off when colder than -18 because frost and ice can not form on the antenna it is too cold.

https://www.electro-tech-online.com/attachments/microwave-jpg.122706/