High Tunnel Gets Colder Than Outside

[Nan_PA_6b]

So if covers make for lower than outside temps, does this mean things will get colder under a row cover? (Which is supposed to extend the season). And when it's going down to near freezing, does this mean we should not cover our plants to protect them, if they'll only get colder under cover than out in the open?

Nan

[FourOaks]

Originally Posted by Nan_PA_6b

So if covers make for lower than outside temps, does this mean things will get colder under a row cover? (Which is supposed to extend the season). And when it's going down to near freezing, does this mean we should not cover our plants to protect them, if they'll only get colder under cover than out in the open?

Nan

So far any evidence points to row covers helping.

Has anyone done a long term study on this effect?

Back in March of 16 I noticed the temp. issue as well. I said this:

This morning I noticed the temp. in the GH was 40. The outside temp was 45.

When I first heard about this, I wrongly assumed it had to do with wind being sealed out, and not bringing in warm temps. I recall leaving the end door open one night, walking in and the moment I crossed the "threshold" you could feel the temp. change.

After that, I started leaving the sides rolled up. Problem solved. Or, use backup heat.

Either way, a real solution would be nice.

[bower]

Nan, as far as I understand it, this only happens in certain circumstances, when cold air sinks and becomes trapped inside the high tunnel instead of flowing away. Specifically in still clear weather with little wind to circulate the air and disperse the cold air which naturally sinks, afaict.

This would not apply to row cover in any circumstances. In fact plants inside a high tunnel could be protected from this effect with a layer of row cover inside. In case of a low tunnel you could also throw row cover or a blanket over the outside to prevent the cold air from penetrating and becoming trapped in the structure. Row cover is permeable and yet it is better at retaining warmth than plastic.

It would be a mistake to assume that the "colder inside" condition is always the case. It may be more frequent or less in one high tunnel or another because of the lay of the land and the frequency of windless cold clear weather in that location.

[FourOaks]

http://www.ag.auburn.edu/landscape/temperature.html

Radiation Cooling: On clear, cold nights, plants and other objects within a greenhouse will lose heat to the outside by radiation cooling. Because objects within the greenhouse are much warmer than those outside, they lose heat by emitting infrared radiation through the glazing and into the clear sky. Under such conditions, the foliage temperature may be 5°F cooler than the surrounding air. Radiation cooling occurs very little when skies are cloudy.

[Nan_PA_6b]

"Radiation cooling occurs very little when skies are cloudy."

Well, then I'm safe. Pittsburgh is pretty cloudy, especially in winter.

But this phenomenon seems to negate the advantage of having a high tunnel, which is to protect the plants within from cold. Ya can't win.

Nan

[BigVanVader]

I've found mine does NOT get colder when I leave the ends open. I am also on a slope so the cold air may flow down and away from the coldframe. Either way I keep it opened and use row cover and I still have lettuce alive after weeks in the teens.

[BlackBear]

Originally Posted by BigVanVader

I've found mine does NOT get colder when I leave the ends open. I am also on a slope so the cold air may flow down and away from the coldframe. Either way I keep it opened and use row cover and I still have lettuce alive after weeks in the teens.

ah ha ....I leave my tunnel unsealed

and both ends open . I have a north/south ends configuration .. and gentle slope

to the east . I have the ability to close both ends to control air flow ..like a Carburetor. The most common thing I do is leave a strip opening on the north and south wide open then of course in prime grow season it is Max. ventilation and everything is open on both ends and even wall partial rolled etc. I still wait a bit later
for getting seedlings going so the protection will be sufficient when placed in tunnel and chances of cold damage are lessened .

[BlackBear]

Originally Posted by Cole_Robbie

My coldest weather is usually from about Christmas through the end of January. It's a good time to give up gardening and let the cold weather kill the bugs and disease I have cultivated all summer.

I am just about to start my first seeds of the year inside, which should be moved out into the greenhouse about mid-Feb. When I only have a few flats of small plants, I can stack them on shelves, cover them with plastic, and put a heater underneath. They are fine that way, even with night temps into the low 20's. My problems arise when weather that cold arrives later in the spring, when I have a greenhouse full of larger plants.

In my cloudy/overcast climate I use solar garden lights as "Indicators ".

When there is sufficient energy /light in the days they light up . It is nice to see the tunnel light up again at night . We actually have mild winter temps. but still the low light levels and fog/ clouds etc. Make December /January not a good bet for passive green house . This year the tunnel "Lit up " January 30th. Hence my reference to Ground hog day to the real start date for me . But I also wait another 10 days for seedling start inside heated shop and seedling bench.

[Cole_Robbie]

Last night was interesting weather, perhaps the kind where a high tunnel would get colder than outside. My forecast low was 34. At 1 AM, it was 36, but the grass was crunching under my feet and there was a thick layer of frost already. Inside the high tunnel, my plants are on a bench over horse troughs of water. The thermometer read 42 by the plants. But there was a thick layer of ice on the inside of my plastic, which I guess was frozen condensation.

It does seem counter-intuitive - if heat rises, why do I have ice only on the ceiling? I think it is an example of atmospheric radiative cooling.

As a reference, a cold front had just moved through - clear skies and dry air. I think the conditions for frost at least correlate with the radiative cooling, if not entirely the same.

Looking at the NWS site, my nearest station recorded a low last night of 29. They were off by 5 degrees in forecasting the low.

[bower]

Cole, are your horse troughs open or closed?

I remember I ran across some information about the effect of moisture on heat loss, not good. Just looking for that in my reading notes file..... I'm still trying to figure out what to do with my airleaky yet not perfectly vented greenhouse this spring so trying to read up...

first note/quote:
"..Convection heat transfer is the physical movement of a warm gas or liquid to a colder location. As air is heated in a greenhouse, it rises to the roof and loses some of its heat to the roof glazing materials. Then the cooler, heavier air sinks towards the floor until it is warmed by the heater or by warmer floors and benches. Heat losses by convection inside the greenhouse occur through ventilation and infiltration (fans and air leaks). Heat transfer by convection includes not only the movement of air but also the movement of water vapor. When water in the greenhouse evaporates, it absorbs energy. When water vapor condenses back to a liquid, it releases energy. So when water vapor condenses on the surface of an object, it releases energy to the outside environment."

second note/quote:
"It takes about 1000 X as much energy to evaporate a given amount of water as it does to warm it up one degree F. If water is evaporated from plant or soil surfaces and the vapor is carried to a cooler region, say the glazing, it may condense and give off much of that heat to the glazing, which in turn will conduct and radiate the heat away. Thus condensation on the glazing, besides decreasing the ability of the glazing to transmit light, also contributes to heat loss from the greenhouse. "

Keeping the heat in the greenhouse,pdf.
Chapter 3 Conrad Heeschen
and
http://www.greenhouse-management.com..._loss_gain.htm

And the graphic is from one of those refs as well... somewhere out there.

[PotGarden]

This is an intriguing problem, and it is counterintuitive. This article might be helpful for the underlying physics of the problem simplified (at least I thought so after looking at some physics sites explanation of radiative cooling with long mathematical equations! ).

http://www.hko.gov.hk/education/edu0...adcoolinge.htm

[PotGarden]

Here's a link from Michigan State University that's meant for us non-physicists and had some strategies for how to deal with temperature control. Interesting stuff.

https://www.google.com/url?sa=t&sour...ogZFh3coa4-Et4

[FourOaks]

Originally Posted by PotGarden

Here's a link from Michigan State University that's meant for us non-physicists and had some strategies for how to deal with temperature control. Interesting stuff.

https://www.google.com/url?sa=t&sour...ogZFh3coa4-Et4

Thanks for the link. Certainly some interesting stuff there. Some common sense, but stuff that one may not actively think about. For example, waiting for the temp to hit the 40-45 degree range before removing frost cover.

I had that situation this morning. It was 38 in the seedling house. I waited till it was in the 40s for sure before removing the covers.