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Tunnel is colder than outside

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    Tunnel is colder than outside

    I am hoping one of you folks with a science background can help me out here.

    Situation:
    I have a "greenhouse" - specifically this:
    https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1

    it is 10x26. I have black tarp(silage tarp) underneath (specifically, this: https://farmplasticsupply.com/silage-cover-your-size), black side facing up.

    At night, zip-up door and velcro-windows are all closed.

    For whatever reason, the temperature in this structure can be as much as 10F colder than ambient outside temperatures.

    I have a weather station probe (Ambient Weather WS-2000) approximately 25' away. Weather station probe does sit a bit higher, on a pole - probe in the greenhouse is about waist-high.

    I have another "greenhouse" about 25' on the other side of the WS pole (pole sits in between the two, in terms of distance), and it sits about 1F above ambient outdoor at night w/o supplemental heat. This one has a thin gravel floor and is only 10x7.

    What might be causing the temperature in the larger greenhouse to fall and get so much colder than ambient? Is it the tarp? If so, why? Lack of "geo-thermal" heat? I assume lack of airflow could account for a some of it - but ~10F?

    It's just seems wild when it's 40F outside, but 31F in the greenhouse!

    Here is a general shot of what it looks like in the day time
    Click image for larger version

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    Here is a plot of outdoor ambient, and greenhouse 2 (larger, colder) over a week's timeframe. I did not include the smaller greenhouse because I have been heating it with a space heater at night (but on nights I don't it's +/- 1F or so from ambient)

    Click image for larger version

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    Click image for larger version

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