The Madison Area Permaculture Guild has erected a 20′ X 40′ hoop house on the grounds of the Token Creek Eco-Inn. As some of you may know, I put a couple of temperature loggers into the hoop house on Nov 27. They filled up January 8, and I just downloaded their contents January 14th to look at them.
If anyone is a data geek, I can give them the original data, but it was gathered at 30-minute intervals, Temperature in Fahrenheit. I put one in a plastic ziplock hanging on the south side of the hoop house, about 5 feet off the ground. The other was in another ziplock laying on a stack of empty pots on the north side near the entrance. I’ve added a third now, in the shadow of a crosspiece outside on the North.
To put it in context, I played around with Weatherspark (http://weatherspark.com) for Madison temperatures, and have put together a graphic with both at approximately the same scale for comparison.
I also had a logger (iButton) in my own little hoop, which is fairly open, it is mostly a shelter for my chickens, is about 6′ tall, 20′ long, and has about 8 square feet of hardware cloth venting, so it does not hold heat in very well.
Attached is a low resolution version of the graph. The top is from the iButtons, the bottom from Weatherspark. I tried to match the dates and times up, should be fairly close. In case you can’t see it, in the Weatherspark graph, a thick horizontal line is 32 degrees, freezing. I tried to put one in the iButton graph, but it disappeared. The horizontal lines are, from the bottom, 20, 40, 60, 80, 100, 120, and 140 at the top.
As you can see, it got over 100 degrees on many days on one of the sensors, the other peaked at 80, with highs for one often 25 degrees higher than the other. I forgot to mark which was which, so I would guess that the one on the south got hotter being higher off the ground and in more direct sunlight.
For the lows, note the coldest night, January 3, 6th from the right (9 degrees according to Weatherspark). The data from the ibuttons recorded 5 and 6.8 those nights, while my hoop recorded 44 as a low that night.
We put some 55-gal water barrels on their sides, 1/2 full of water, along the south side of the hoop house inside. I checked these, too. There was thin ice floating on each, so the temp was probably just above freezing in the barrels. There is snow piled up on the south side, the north is clear.
What this seems to indicate is that the hoop house has some extremes of temperature, could use some way of storing it to moderate the cold, and even out the fluctuations. Perhaps if the barrels were further from the wall, where they are shaded by the snow, they would be warmer.
A closer look at January 2-3 shows interesting, if predictable patterns.
Here the hoizontal and vertical scale are the same, the top graph shows the sun position, next below it cloud cover, then air temperature (probably at the Airport, about 5 miles South) Click for full image.
(I added a horizontal line at 32 degrees, and the green and blue lines are from sensors inside and outside my chicken coop at HedgeCroft) Both the ambient temperature and the hoop house responded to the sky clearing late on the 2nd, and bumped back up with the brief cloudiness at around 7pm. The hoop house started a quick rise in temperature just after sunrise (there are no obstacles to the Southeast). I have no clue why the 20 degree drop 9:30-11 AM, though I see a similar dip on many days at a similar time, maybe there is a shadow from an obstruction to the south? I’ll look it over next time I’m out there. If so, it would probably affect the temperature most on cloudy days. Anyway, enough fun for now.
Great info Math. Couple of thoughts:
Is the ground frozen inside the Token Creek hoophouse?
Eliot Coleman published a very good book in 2009 called “The Winter Harvest Handbook.” He’s been growing produce in cold houses in ME, at nearly our same latitude and growing zone, for decades. One thing he points out is that the day length is more of a limiting factor than temperature, though both issues are manageable and about 20 crops can be grown year around here in unheated hoop houses. Its a very interesting book by a master produce gardener.
I’ll check the ground next time I’m there, I didn’t think of it. Except for the barrels, which might do better in a row in the center and standing up, there is no heat storage other than the ground, and that is fairly light in color, with the dead grasses. The barrels are an experiment to see how much they freeze, which is why they are 1/2 full and on their sides, to avoid splitting as the water freezes.
Regarding the hoop house, is it your intention to keep it warmer, or are you just measuring? At Growing Power they use big piles of compost to heat their hoop houses.
We are considering compost, though we’d have to transport it some distance, and unlike Growing Power, we don’t have lots of helparound. But the idea is to moderate the temperature swings, by storing the heat produced on sunny days for release at night. Since active systems require some energy to circulate the heat, and there is no electricity at present, I’m thinking a passsive thermal mass might help.