An inverted mason jar gives six or seven degrees Fahrenheit of frost protection. I never would have guessed that.
I had to do this experiment twice, because I didn’t believe the results the first time. For two nights running, I left a pair of temperature loggers (digital recording thermometers) on a raised garden bed, one beneath an inverted wide-mouth quart Ball (mason) jar, one in the open.
In both cases, the goofy little mason jar provided 6 to 7 degrees of warmth. That wasn’t quite enough to prevent frost inside the jar this morning (the temperature dipped below 20F around dawn). But that is, nevertheless, an impressive amount of frost protection from a device that has, for all intents and purposes, almost zero value as either insulation or thermal mass.
Now I know why the glass cloche has stood the test of time. And I think I know how it works. It doesn’t insulate well. It doesn’t provide thermal mass. Instead, a glass cloche (or mason jar) works because, in addition to forming a pocket of still air over the plant, it serves as an excellent radiant barrier. It captures the long-wave radiation emitted by the warm soil underneath.
That’s the only plausible way in which that thin layer of glass could have maintained that large a difference between the inside and outside air. It wasn’t just providing some passive insulation. It was actively powered, all night long, by upwelling long-wave infrared. (Edit: About 1.6 watts of it — see next post.)
The results, two nights running
The first night I did this, I didn’t bother to make sure that the readings on the data loggers matched before I started the experiment. (That had never been an issue in the past, so I figured it was unnecessary.) I just set them to record the temperature every half-hour, chucked one under a jar, and left the other out in the open, side-by-side on my raised garden bed.
Thus, on the top graph above, the first recorded temperature occurred with the experiment already in progress. I looked at the results the next day and said, that has to be an instrument failure of some sort.
So last night, I fired up those data loggers hours ahead of time. The initial reading is the inside of my house. Then I put them outside and covered one with a mason jar around 6 PM. And I made sure to swap the treatment and control units.
And yet, I got the same results. I therefore conclude that the results are real. A wide-mouth quart Mason jar provides 6 or 7 degrees F of frost protection.
Also, notice that the gap between the lines does not close over time. If the glass were merely providing insulation, I would expect that gap to fall as everything in the garden cooled off, soil surface included. Just as your house cools off if you turn off the heat. And, to be sure, everything did cool off. This morning, the mulch under the mason jar was, in fact, frozen.
It certainly looks like there was some small, steady flow of power into that mason jar, keeping it warm. And I think there was. This isn’t a totally passive system, in the sense of using no energy. It’s powered by the upwelling long-wave infrared from the warm(ish) garden soil below.
And so, after two suggestions from the internet that were total failures (floating row cover, plastic sheet), I am pleased to find one that is real and now makes sense from every perspective.
Implications and additional research
I’m pretty sure that this works because the glass is forming an excellent radiant barrier. That thin glass jar would provide R-1 (U.S. units) insulation at best. And it certainly isn’t providing thermal mass the way a Wall O’ Water (r) does. But ordinary glass is more-or-less completely opaque to the long-wave infrared that would be given off by the warm soil of the raised bed. So I think this works exactly the same way my tin-foil-hat (radiant barrier bed cover) works. It prevents the emitted infrared from escaping into the night sky.
I need a few more simple experiments to nail that down.
First, I need to redo this using a plastic jar roughly the same size and shape as a quart mason jar. My understanding is that most ordinary plastics (e.g., HDPE, PETE, and so on) are almost completely transparent to infrared. (I will, of course, check that for whatever plastic my jar is made from). If I’m correct about why a glass jar works, then a plastic jar shouldn’t provide much frost protection at all.
Practical takeaway number 1: Don’t buy a plastic cloche and expect it to provide any significant frost protection.
Edit: See G22-008. It depends on the type of plastic, I think. A cloche made out of a thick piece of PETE ought to work fairly well. By contrast, one made out of polyethylene sheet shouldn’t do much at all.
Second, I should redo this after attempting to block the infrared radiation from entering the jar. I think it should be adequate to rest the jar on a piece of radiant barrier. The low infrared emissivity of the radiant barrier should prevent the ground from radiating into the inside of the jar. It might be sufficient just to sit a sealed mason jar, bottom-side-down, on that bed and measure the overnight temperature. You’d have the same degree of contact with the ground, but the glass bottom would stop the infrared radiation from entering the jar. The right-side-up lidded mason jar shouldn’t provide much frost protection at all.
An odd implication of this is that a wide-mouth jar should work better than a standard-mouth jar, for any given size of jar. The jar is being “powered” by the infrared entering through the mouth. It’s losing heat out the sides and bottom. A wide-mouth jar should have a better power-to-loss ratio. And, I’m pretty sure that a pint should work better than a quart, and a quart should work better than a half-gallon, for exactly the same reason. The ratio of the mouth opening to the total jar surface area is larger.
Practical takeaway 2: Use the smallest wide-mouth jar that you can.
I’ll note how counter-intuitive that is, if you hold some fuzzy-but-incorrect notion of how this works. If you have some notion that this works by “keeping the cold air off your plant”, you’d probably shoot for the largest trapped air volume you could get, and you would be indifferent to the size of the mouth of the jar. Assuming I have it right, and the jar is powered by trapped infrared, both of those would be the wrong choices.
Third, the warmer the underlying soil, the better this works. My raised beds have been warming up nicely, so I have some fairly warm soil underneath that mason jar — somewhere around 50F half-a-foot down, a few days ago. But I bet this would work far worse in soil that’s already quite cold, owing to the lower infrared emissions of the soil.
Practical takeaway 3: The degree of frost protection probably depends on the temperature of the underlying soil, so YMMV.
I guess that’s pretty obvious. Even if you mistakenly thought this worked by providing insulation, you’d think that. My real point is that my measured 6 to 7 degrees may depend on the state of the soil in my bed. YMMV.
Fourth, I need to try this with the mason-jar-equivalent of a beer cozy, just to see how warm the inside of the jar would get if the outside were reasonably well insulated. The second law of thermodynamics assures me that it can’t get hotter than the underlying soil that is generating the infrared. So if the soil deep in the bed is at 50F, the inside of the jar can’t get any higher than that. Still, if this is able to maintain a 7 degree F difference across all of that R-1 (U.S. units) glass, I can’t even guess how well this would work with a little insulation around it. You might be able to pull 15 degrees of frost protection, reliably, from a foam-covered mason jar. Clearly I have to try that.
The second law of thermodynamics is ALWAYS misstated as saying that heat cannot flow from a colder body to a hotter body. This is wrong, and this misstatement has become a staple among global-warming denialists. Heat always flows in both directions. (Which is obvious, if you think about it — how would the heat emitted by the colder body know that it had to dodge around the hotter body?) The proper common-language phrasing of the second law of thermodynamics is that the NET flow of heat is from the hotter body to the colder body. Heat always flows in both directions. It’s just that the flow is greater in one direction than in the other. Hence, “net”.
I won’t correct your grammar. But misstate the second law and I’ll get in your face about it.
Note that this underscores that the amount of warming you get is dependent on the specific conditions of your experiment. The warmer the inside of the jar, the more it re-radiates infrared back into the soil. I think this plausibly explains why I saw the biggest interior-to-exterior temperature difference at the coldest time of the night. The net flow of infrared into the jar is higher, the colder the jar is. The colder it gets, the better this works.
So while this provided an average of 7 F protection over the entire experiment, in the dead of night, it was actually creating a steady 10 F difference between the inside and outside of the jar.
Finally, now I wonder if glass and (uninsulated) plastic greenhouses behave differently in terms of the frost protection offered by an unheated greenhouse. I’d have to say that the probably do. A glass greenhouse is not all that different from an inverted mason jar, scaled up. And a standard single-wall hoop house would be covered with polyethylene sheeting, which I am pretty sure allows all wavelengths of infrared to pass through it. If infrared radiation is what powers the mason jar, it should power a glass greenhouse as well. (But not a plastic-covered hoop house).
Edit: For whom the cloche tolls.
Why a cloche, specifically? Why did that shape stand the test of time?
The penny drops. The light bulb lights. Enlightenment is achieved.
A bell goes off.
Suppose you want to maximize the amount of infrared energy trapped by your glass. And minimize heat losses through that glass to the cold exterior air. What shape would your glass have?
You’d have a great big wide opening at the bottom, to admit infrared energy. And then quickly taper that down, to minimize the glass area above that opening. Given that glass doesn’t like to make sharp turns, you’d end up with something kind of, well, bell-shaped. Broad opening at the bottom, and just enough space inside for your plant.
A cloche. Which, if you didn’t know it, is French for bell. True antique cloches are anywhere from classically bell-shaped, like the Liberty Bell, to vaguely conical, like an old-fashioned bee skep. All combine a broad opening with a rapidly narrowing body. And that turns out to be exactly the right way to get high-performance frost protection out of a piece of glass.
