Temperature Variance in Small Greenhouse.
 

I have a small, 8' X 8' poly fabric (opaque) greenhouse (similar to material used for high tunnels and quickie car garages). It will get warmer than the ambient air temperature during a sunny day. I understand that. Conversely, it often gets colder than the ambient air temperature during the night (sometimes by as much as three or four degrees). I don't understand that.

I also understand that this is something that is not uncommon with small greenhouse, at least of a certain type.

What is it? Why is it? And is there a simple fix (like a small fan circulating air or opening it up at night?)

Thanks

Paul, the problem with plastic greenhouse is that when there is a temperature inversion (calm clear cold weather at night causes cold air to sink) the cold air from the inversion actually gets trapped in the greenhouse and it is colder than outdoors. Same as when you have a "frost pocket" that is a place in the landscape where cold air gets trapped on this kind of night.



The easy solution for a small greenhouse is to cover it with a tarp when you're expecting a clear and cool night. That would allow it to retain its warmth and keep out the cold air that's sinking.


You can also leave it open at night to allow air circulation, but that means your low will be at best the same as outdoors.

I wouldn't worry about a few degrees at night unless you are near frost. In that case the plants should be small and can go in a low tunnel inside the greenhouse. This traps the ground heat and can give extra frost protection.
I looked into this some time back, and my recollection is radiant cooling causes the internal temp to drop below ambient at night.

Thanks. Both explanations make sense (and are complementary with cool air on a still night settling on the plastic surface with radiant cooling ensuing).

Regardless, the forcast low tonight is in the mid-forties which could mean 40 for a couple hours in my greenhouse. I'm not in this to make money, I'm in for early tomatoes. I threw a little electric heater in there. Right now, it's 51 outside, 62 in my greenhouse.

Small greenhouses are like small fish tanks. They are more unstable than the big ones.

But, fun!