Non-ripening tomatoes and nighttime temperatures
In 2020, we had an extended period when tomatoes would not ripen. That was new to me, but apparently that’s pretty common in the South. The lack of ripening is due to excess heat. But it’s not a daytime excess. It’s due to warm nights, as many varieties of tomatoes will not begin the ripening process (enter the “breakers” stage) if nighttime temperatures consistently exceed 70 or maybe 72 F. See Post #G22-43 for full details.
Source: Calculated from historical weather data from NOAA, for Dulles International Airport.
As explained in that prior post, the non-ripening is a subtle thing. Tomatoes that have already begun the ripening process will continue to ripen. But those that have not yet started that will remain green. So, at some ill-defined lag after the nights warm up, the supply of ripe tomatoes gradually dries up.
That “warm nights” thing is a pity, because climate models have long predicted that global warming will raise nighttime temperatures more than daytime temperatures. So it would seem that warmer nights are in the pipeline.
If you look at the graph above, that 2020 stretch of warm nights began in the middle of July.
Here’s the extended forecast for Vienna VA today:
It’s worth pointing out two things.
First, we’re surely in for at least a few nights above 70F. And, depending on whom you believe (and your misplaced trust in 10-day forecasts), we might be in for an extended period with nighttime temperatures over 70F.
Second, it won’t take one whole lot of warming to push all those forecasts above the 70F threshold. That’s going to make it tough to grow a whole lot of varieties of tomatoes around here, I think. But we’re likely talking the better part of a century from now. I hope.
Everybody talks about the weather, but nobody does anything about it: Radiative cooling.
Not me. I’m going to try a radiative cooling experiment. I’m going to see if I can use radiant barrier to reduce the nighttime temperature in parts of my garden.
It’s an unusual idea, but it’s not rocket science.
As I noted in earlier posts on this topic (G21-014 , G21-015 , G22-005 , etc.) a garden bed is like a big window, looking straight up into outer space. As such, it continuously radiates heat energy (long-wave infrared) upward, toward the cold of outer space. By my calculation, on a cold spring night, you lose more heat from radiation than from conduction.
That’s why a radiant barrier is what you want, for frost protection, for your garden beds. That can be a space blanket or similar material. But that’s also why a glass cloche works to prevent freezing overnight. And why a simple, thin-walled glass mason jar provides excellent frost protection for tender plants (Post G22-006). And, by contrast, why polyethylene sheeting does diddly-squat to prevent overnight freezing (G22-005).
But, weirdly enough, you can also use a radiant barrier for cooling, by preventing ambient radiation from reaching your garden bed at night. In effect, you make it so that your garden bed “sees” only the cold of outer space, directly over head. If the air is sufficiently transparent to long-wave infrared, your garden bed then cheerfully radiates energy off into outer space, and cools as a consequence.
This technique works OK in the dry desert, with a clear sky, which may explain in part why various Middle Eastern cultures have used it for millennia, to make small amounts of ice, in the desert (reference). That said, even under those optimal conditions, temperatures had to be near-freezing to start with. This reference suggest an upper limit of 5 C, or about 41 F. Ideally, a combination of insulation, evaporative heat transfer, and radiative losses would generate small amounts of ice, under those conditions.
By contrast, the main problem with using that here is water. Water vapor is the most important greenhouse gas. It’s plentiful in the atmosphere, and it absorbs and re-emits infrared across many parts of the infrared spectrum. Between the humidity and the clouds, a lot of what gets radiated into space will be, in effect, reflected (re-emitted) right back down to earth.
Which is, in a nutshell, the greenhouse effect.
OTOH, I only need a few degrees. If this can pull a 9F differential in the dry desert, maybe it can drop the temperature 3F on a cloudless Virginia summer night. After all, I’m just trying to trick those tomatoes into starting the ripening process. My understanding is, once that gets going, they will continue to ripen.
So it’s worth a shot, just out of intellectual curiosity. I’m going to set up a small enclosure made of radiant barrier — basically, a big tube with the open ends facing ground and sky. Cap that with a piece of clear polyethylene sheet to provide an IR-transparent barrier to the outside air. Then use temperature loggers to track nighttime temperatures inside and outside the enclosure. I might get lucky.
Otherwise …
The nice thing about this method is that there’s zero energy consumption.
Probably ought to consider a shade cloth, as well, but I can’t quite figure out how that would be much help in terms of nighttime temperatures. Plausibly, the cooler the soil stays during the day, the cooler the area may be at night.
But if I’m willing to expend a bit of energy, I think a mist-cooling device would plausible achieve a sufficient drop in temperature. Mist coolers work by converting sensible heat (temperature) into latent heat (water vapor, instead of liquid water). I went over that in my post on the true energy cost of humidifiers, Post #1669.
That said, bathing my plants in mist all night just seems like a recipe for every tomato leaf disease known to mankind. So that’ll only be used as a last resort.
Otherwise, short of sticking a window AC under a tarp, and using that, I guess I’m at the mercy of Mother Nature here. If it’s too hot to ripen tomatoes, then it’s too hot to ripen tomatoes. Grow something else for the time being.
Any sufficiently advanced technology is indistinguishable from magic. (Arthur C. Clarke)
My garden beds emit “black body” radiation. That is, they toss out radiation at every frequency, with a peak in the long infrared. As a consequence, some of that is bound to be absorbed and re-emitted by atmospheric gasses.
But suppose, through some miracle of modern science, you could create a material that radiates infrared only on those bands of frequencies where the atmosphere is transparent to infrared. That is, frequencies that aren’t absorbed and re-emitted by common atmospheric greenhouse gasses.
Then — and frankly, this is where I lose it — you could, in theory, create a material that would literally cool itself below ambient temperature. If the air outside is 80F, your miracle-o’-modern-science could be 78F, with no power input. Just from enhanced “emissivity” in the right part of the spectrum.
Or, as these folks put it, emphasis mine:
... the PDRC coating demands a significant solar reflectance (Rsolar) in the spectral region (0.3–2.5 m) and a significant thermal emissivity (LWIR) in the environmental long-wave infrared (LWIR) propagation region (8– 13 m). As a result, during the day, the energy loss to frigid space ... is far more than the warming from daylight, resulting in electricity-free spontaneous refrigeration.
In other words, you could sit a piece of this stuff out in the sunshine, and it would remain cooler than the ambient air. With no energy input.
That’s close enough to magic for me.
