Freezing bulbs: Duration vs. low temperature

Lee Poulsen
Tue, 15 Jan 2013 12:52:43 PST
I've noticed the same thing about the difference in native hemisphere. But I attribute this to the mediating influence of what could be (and sometimes is) called the Southern Ocean. Any frigid cold air masses moving north from the south polar regions has to cross a great deal of liquid (and presumably above-freezing) water before it can reach land areas (South Island, New Zealand; Tasmania and the southern regions of Australia; South Africa; and southern Chile and Argentina). So this will tend to warm up any frigid air mass much more by the time it reaches land again than the parallel case of frigid north polar air masses moving south towards all the populated northern hemisphere land areas (USA, Europe, China, etc.) The north polar frigid air masses mostly ever travel over very cold and dry land on their way south, so they remain far colder far longer than their equivalents in the southern hemisphere. So I think that at the same latitude north vs. south, on the same side of the continent, you're probably going to see more examples of colder extreme low temperatures in the northern latitude than at the equivalent southern latitude. 

[Furthermore, due to the same physical cause--i.e., the huge mass of water pole-ward of southern latitudes vs. the mostly land mass pole-ward of northern latitude regions--you find that the humid sides of the continents (i.e., the eastern sides) have much warmer nighttime temperatures during the summer in the northern hemisphere than do the equivalent latitudes on the east coasts of the southern hemispheres, even right along the ocean coasts. Compare Nanjing or Shanghai, China vs. Sydney Australia. Or Charleston, South Carolina, USA vs Buenos Aires, Argentina. And yet, southern hemisphere cities can grow much more tender plants than the equivalent latitude northern hemisphere cities. Because they never see low temperatures as cold.]

As for the clear plastic vs. nonwoven (or even woven), *opaque* fabric, I think the explanation is due to the blackbody radiation effect from physics. This is typically a problem during cold clear dark windless nights. (If it's really windy, you either don't quite reach temperatures as cold as you do under similar conditions with no winds. Also, when the air is frigid but it's very windy, coverings of any kind aren't as effective.) We call it "radiation frost", but it's due to the principle that the ground or the surface of uncovered plants "sees" the dark black sky of space (which has a temperature of around -270°C/-454°F), so since the surface is much warmer than space, it will spontaneously start radiating that heat energy as infrared radiation towards the sky. This loss of energy causes its temperature to drop, even below the air temperature around it, which is why you can sometimes get a killing frost even when the air temperature is above freezing. (This effect can gradually cool the air temperature near the ground as well.) The fabric (or cloth or overhead tree leaves or eaves of a house) block this visibility, so there isn't this energy radiation effect occurring. Also, I believe all of these "opaque" objects are opaque to infrared, so the infrared radiation can't travel through the cloth or other object. On the other hand, clear plastic allows more infrared to radiate than cloth does, and the cold black sky is visible to the ground and plant leaves through the clear plastic so the blackbody radiation effect is more effective than through cloth. I forgetfully saw this principle at work earlier this winter in my vegetable garden when I used some new plastic-tunnel-with-hoops thing I saw and purchased earlier last year on several pepper plants one day when they were predicting very cold (but not freezing) low temperatures. There was a still surviving tomato plant next to them that was far too big for the plastic hoops, but I threw some Reemay over it just because it was handy. I think the low temperature was about 36°F/2°C at the garden that night. There was a lot of radiation frost that killed all the exposed tomato and pepper plants in the other garden plots of the community gardens. My covered pepper plants turned to mush as well but the one covered tomato plant was still green and alive the next day. (It's dead now; yesterday morning's low temperature was just too cold. Even downtown Los Angeles dropped to 34°F/1°C.) BTW, this is why you will sometimes see a species get killed growing in the ground out in the open when the same species growing under a nearby tree does not after a cold clear windless night.

--Lee Poulsen
Pasadena, California, USA - USDA Zone 10a
Latitude 34°N, Altitude 1150 ft/350 m

On Jan 15, 2013, at 11:23 AM, Jane McGary wrote:

> I've noticed a pattern over the years: "marginal" bulbous plants that 
> survive cold snaps are most likely to come from the Northern 
> Hemisphere, especially the Mediterranean, west and central Asia, and 
> the west coast of North America. Plants from South America and 
> southern Africa are less likely to recover

>  I agree that plastic film alone should not be used, and 
> especially clear plastic.
> The nonwoven fabric 

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