How is frost possible at 39 degrees F?

[gary350]

TL;DR Summary

How is frost possible at 39 degree F

How is frost possible at 39 degree F

 

[phinds]

and just where was the 39 degrees measured?

 

[Drakkith]

Without more context I'm not sure your question can be answered. Are you talking about finding frost outside on a tree when the temperature was 39 F, or some other scenario? Where was the temperature measurement taken? Locally, several miles away, the next town over? Etc.

 

[anorlunda]

When I go outside with an IR thermometer gun, I can measure different temperatures on different materials and locations.

I can get ice on the windshield of my car although the surroundings near the car are above freezing.

https://en.wikipedia.org/wiki/Radiative_cooling#Nocturnal_surface_cooling

 

[gary350]

I have a vegetable garden every year. I watch TV weather every evening this time of the year. Sometimes weather man or weather woman said, frost tonight. Sometimes it is 34 degrees F, sometimes 36, sometimes 37, sometimes 35. My 2 house thermometers are very accurate, my temperature never matches TV but that does not matter. Sometimes we do have frost at, 33, 34, 35, 36, 37 degrees one of my therometers is accurate to 1/10 deg. TV said, clean sky or clouds determines if we have frost. TV says, it is, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, rare to have frost at 40 degrees but it is possible. TV says, it is rare to have frost at 39 degrees but possible. Sometimes frost on west side of yard but no frost east side of yard. Sometimes frost right side of yard but not left side of yard. Sometimes plants 2 ft tall have frost. Sometimes plants 6" tall have frost. Sometimes front yard has frost but no frost in back yard. Sometimes frost on house roof but no frost in yard. Sometimes frost when there is a full moon and sometimes frost by dark of the moon. Sometimes frost at 6am and sometimes frost at 5am. Is that enough details? Question is, how is it possible to have frost if temperature is warmer than freezing, warmer than 32 degrees F.?

If soil is 48 degrees F. grass still frosts. Still that is not the point of the question. How is it possible to have frost at 37 degrees F when freezing temperature is 32 degrees F.

I know from observation frost can be spotty, hear and there often offected but mass temperature of house, large tree or something else. But with my very accurate thermometer 5 ft high center of yard near nothing how can grass frost below when thermometer reads 37 degrees F how did frost happen 1" from thermometer?.

I have looked at frost very close it looks like ice to me. If I breath on frost or hold my hand very close to frost it melts and becomes water. If it looks like ice and tomato plants die it was frost. Even if thermometer is only 1" from grass and grass has frost thermometer reads 37 degrees F.

Only explination weather man gives, dew point and clear sky determines if we have frost at 37 degrees F or any other temperature.

 

[russ_watters]

But with my very accurate thermometer 5 ft high center of yard near nothing how can grass frost below when thermometer reads 37 degrees F how did frost happen 1" from thermometer?...

Only explination weather man gives, dew point and clear sky determines if we have frost at 37 degrees F or any other temperature.

The clear sky is the big hint. The ground (or any other surface) radiates heat into space, making it colder than the surrounding air. That causes both dew and frost, as the moisture condenses and the freezes out of the air. Indeed, you may have noticed you get more frost when the temperature is warmer (say, 25-35F) than when it is colder. That's because warmer air holds more moisture.

 

[hutchphd]

The leaf of your tomato plant radiates isotropically at 300K. Space radiates back at 3K. The ground radiates from under the tomato at 300K but that is just on the bottom. Space wins ! Tomato loses. I covered my tomatoes 3 hours ago...

 

[gary350]

That does not explain why water freeze at 32 degrees and frost freeze at a warmer temperature.

 

[hutchphd]

The leaves of the tomato are colder than the ambient air because of radiant energy loss.

 

[gary350]

The leaves of the tomato are colder than the ambient air because of radiant energy loss.

That makes sense but I don't totally under stand why. If I was a college physics major I would probably under stand it better.

 

[NTL2009]

The clear sky is the big hint. The ground (or any other surface) radiates heat into space, making it colder than the surrounding air. ..

But isn't the air also radiating heat into space?

 

[russ_watters]

But isn't the air also radiating heat into space?

Yes, but it is much more transparent than a solid surface. In the wavelength range where its peak emissions are (about 10 microns) it is still very transparent:
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/irwindows.html

Hmm...I've never thought to point my infrared thermometer at the sky though. That could give some indication on the transparency at that wavelength.

[edit] So it's 47F outside and partly cloudy. With an IR thermometer my deck measures 37F, a cloudy area of the sky 15F and a near-clear area of the sky -30F. I believe the principle of operation of the infrared thermometer is intensity of IR light at a range of wavelengths(and an assumed emissivity), as a proxy for finding peak wavelength. This tells me the ground radiates at a healthy rate, but actually trying to calculate it would be very difficult.

[edit2] It might be possible to measure the radiant power without too much difficulty. You'd need an insulated, glass-covered pot of water or block of metal pointed at the sky, and heated to ambient temperature. At ambient temperature it isn't gaining or losing any heat via conduction/convection, so the heating power would be the radiant power. If the radiation were perfect, and the atmosphere completely transparent, you'd get about 300W per square meter.

 

[anorlunda]

So it's 47F outside and partly cloudy. With an IR thermometer my deck measures 37F, a cloudy area of the sky 15F and a near-clear area of the sky -30F. I believe the principle of operation of the infrared thermometer is intensity of IR light at a range of wavelengths(and an assumed emissivity), as a proxy for finding peak wavelength. This tells me the ground radiates at a healthy rate, but actually trying to calculate it would be very difficult.

The next time we are asked for a simple science experiment to do at home this would be great. IR thermometers are cheap. The measurements are easy to do. But at the back end, it takes a lot of thinking to explain the differences.

But isn't the air also radiating heat into space?

Almost everything radiates some. You're missing the point that some things radiate better than other things. You could probably take an object, paint half of it, then measure a nighttime temperature difference between the painted and unpainted halves.

Different materials also have different specific heat. That relates to how much heat flow it takes to change the temperature a given amount. Remember that the night doesn't last forever, so the speed at which things cool affects the temperature at dawn.

Different objects also have different shapes and orientations, and some are in contact with the ground or other objects, so they exchange heat with each other. Other objects are more isolated.

There is a long list of physics effects that lie hidden under the simple question in this thread. Put that together with @russ_watters ' simple experiment and you have a rich science lesson.

 

[Drakkith]

That does not explain why water freeze at 32 degrees and frost freeze at a warmer temperature.

When leaves and other objects radiate heat away they cool down. In some cases, such as when there is a clear night and the air is close to freezing, their temperature can drop below freezing and frost can then form on them. The air isn't a very good emitter of heat, not compared to most solid objects, so it can remain warmer than the trees and ground.

 

[jbriggs444]

The fact that air is transparent is at least a partial clue that it cannot be emitting very strongly. Objects that are transparent can neither absorb nor emit in the frequencies where they are transparent. If it emitted, you would see the air. If it absorbed, you would not see objects on the far side of the air.

Emittance and absorbance at any particular frequency have to match. Otherwise you could construct a clever device to violate the second law of thermodynamics.

The fact that @russ_watters can point his IR thermometer at the night sky and get a reading is an indication that the night air is transparent to IR and, hence, does not radiate [much] IR.

 

[russ_watters]

Additional info and avenue for investigation:

• Pointing the thermometer at the zenith at noon on a very clear day yields -30F.
• Pointing the thermometer lower toward the horizon yields -10F.

Conclusion: where the atmosphere is thicker, it is more opaque. The variation is probably a predictable function of the angle of elevation.

I also hypothesize that the reading for the underside of clouds is highly correlated to their altitude and the temperature of the atmosphere at that altitude. I'll try that next time it's cloudy (probably tomorrow).

 

[Klystron]

In the context of cooler air temperatures surrounding a large ash tree, a professional landscaper used the term 'micro-climate' to describe the cooler temperatures and the fact that this tree retains healthy green leaves well into Autumn when similar trees nearby have lost their leaves.

My front porch and home work area lie within the umbra of this tree. While frost conditions are rare in this desert climate, we enjoy the cooler environment in the shade. As rough documentation, my air conditioning power bills are consistently lower than identical units outside this 'micro-climate'.

 

[NTL2009]

Lots of great replies in this thread - they helped me obtain a better understanding of my consumer grade IR detector. Thanks.

I had thought that these things just couldn't read a shiny metal surface. I now think I understand that it reads them just fine, but since they are "hardwired" to assume an emissivity of ~ 0.95, the displayed conversion to surface temperature is off. So if you can estimate the emissivity of the object, and apply a correction factor, you could use it for surfaces that are not near 0.95. Though I'm also reading that estimating the emissivity isn't easy, minor changes in the polish and/or oxidation of the surface cause a big change in the number. The sources I read said to do it emperically with an actual surface temperature measurement (thermo-couple) where possible.

I found this correction formula here:

https://cr4.globalspec.com/thread/80955/Correction-Factor-for-Emissivity

Secondly there is a power to four between the temperature and the instrument sensor output.

j* = εσT⁴

https://en.wikipedia.org/wiki/Stefan–Boltzmann_law

j* is what the instrument is measuring under ideal conditions. (watts per square meter)

ε is emissivity, a number between 0 and 1 (approx. 0.98 for human skin)

σ is a constant

T is temperature in Kelvin

So with the instrument set to 0.95 a simplified equation for conversion could be expressed as:

0.95σ(Tinstrument)⁴= εnewσ(Tcorrected)⁴

since σ appears on both sides we can substitute it with 1

0.95(Tinstrument)⁴= εnew(Tcorrected)⁴

Tcorrected = ⁴√((0.95(Tinstrument)⁴)/(εnew))

(Sorry, I'm not Latex experienced, I just used superscript for "raised to the 4th power", and "4th root of". I think it worked OK)

 

[jbriggs444]

I tend to over use braces.$$T_{\text{corrected}}=\sqrt[4]{\frac{0.95 T_{\text{instrument}}^4}{\epsilon_{\text{new}}}}$$
$$T_{\text{corrected}}=\sqrt[4]{\frac{0.95 T_{\text{instrument}}^4}{\epsilon_{\text{new}}}}$$

 

[NTL2009]

That looks better - thanks!

 

[gmax137]

Maybe simpler?

$$T_{\text{corrected}}=\sqrt[4]{\frac{0.95 }{\epsilon_{\text{new}}}} T_{\text{instrument}}$$

EDIT: Just don't forget to use absolute temperature values

 

[russ_watters]

I had thought that these things just couldn't read a shiny metal surface. I now think I understand that it reads them just fine, but since they are "hardwired" to assume an emissivity of ~ 0.95, the displayed conversion to surface temperature is off. So if you can estimate the emissivity of the object, and apply a correction factor, you could use it for surfaces that are not near 0.95.

No, you were right the first time: they do not read polished metal surfaces. Like at all. A stainless steel pot on the stove will read neat room temperature unless it sees your reflection. Somewhere on the forum there is an ir photo I posted of two boiling pots of water and you can clearly see the coated carbon steel pot's reflection in the polished stainless pot.

 

[russ_watters]

Here is a pot of sanitizing solution I've been using for homebrewing for a few weeks. I turned on the burner and waited 30 seconds for the warming diffuser to heat up before taking the picture. The pot is most definitely still at room temp.

 

[NTL2009]

Ahhh, I see. I just tried that for myself, with just the gas flame reflecting in the SS pot, and it immediately jumps to +300F, so just reflecting any IR in the room.

So reflectivity is a big component of this as well.

 

[gary350]

WE live at the bottom of an ocean of air. It seems to me an ocean of air is no different than an ocean of water other than water it not a gas and transfers heat faster than air.

If I have a plant in the house and the plant is 70 degrees F then I take the plant outside where it is 40 degrees F the plants cools down to 40 degrees F. With an ocean of 40 degree air surrounding the plant does the 40 degree air not keep the plant heated to 40 degrees F. I am having trouble understanding how the plant can become colder than the ocean of air around it.

How does covering up a plant prevent it from getting frost? I always thought Earth is a larger mass of heat than air so covered plants hold Earth's heat inside the tent.

I was reading online, during the day Earth is heated by the sun. Objects on Earth store up heat energy, soil, rocks, lakes, buildings, black asphalt highways, house roots, etc. After dark all those object release heat to keep Earth warm. If Earth naturally wants to be -280 degrees F all the objects giving off heat keeps Earth warm. This helps me to understand better how there can be hot and cold spots in the yard.

 

[anorlunda]

Did you read this reference linked in post #4?
https://en.wikipedia.org/wiki/Radiative_cooling#Nocturnal_surface_cooling

Suppose you have a wooden board, and a plate of steel, both at the same temperature. If you touch them with your palm, the metal feels cooler than the wood. How can that be if they are at the same temperature?

This other article is also relevant:
https://en.wikipedia.org/wiki/Heat_transfer#Mechanisms

The article talks about conduction, convection and radiation. Radiation is the key part of the answer to your question.

You must also consider that things on the surface warm each day and cool each night. The answers for surface objects are different than the answers for the same objects deep inside an underground cavern. In the cavern case, all the objects are likely to have the same temperature as the air, but not outside in your back yard.

 

[Lnewqban]

... With an ocean of 40 degree air surrounding the plant does the 40 degree air not keep the plant heated to 40 degrees F. I am having trouble understanding how the plant can become colder than the ocean of air around it.
...
I was reading online, during the day Earth is heated by the sun. Objects on Earth store up heat energy, soil, rocks, lakes, buildings, black asphalt highways, house roots, etc. After dark all those object release heat to keep Earth warm. If Earth naturally wants to be -280 degrees F all the objects giving off heat keeps Earth warm. This helps me to understand better how there can be hot and cold spots in the yard.

The weather man measures and gives you the mean above-freezing temperature of air where his thermometer is located at.
If you could keep sufficient movement of that same air around each spot of your yard, you could have a more homogeneous above-freezing temperature everywhere.
A leaf that is rapidly loosing heat through radiation can rapidly cool down the stagnant layers of air that are very close to its surface, stealing the energy of the closest molecules of water that exist within the air in form of vapor.

Copied from
https://en.wikipedia.org/wiki/Frost

"Hoar frost, also hoarfrost, radiation frost, or pruina, refers to white ice crystals deposited on the ground or loosely attached to exposed objects, such as wires or leaves.
They form on cold, clear nights when conditions are such that heat radiates out to the open air faster than it can be replaced from nearby sources, such as wind or warm objects.
Under suitable circumstances, objects cool to below the frost point of the surrounding air, well below the freezing point of water.
Such freezing may be promoted by effects such as flood frost or frost pocket. These occur when ground-level radiation losses cool air until it flows downhill and accumulates in pockets of very cold air in valleys and hollows.
Hoar frost may freeze in such low-lying cold air even when the air temperature a few feet above ground is well above freezing."