As expected the temperature inside the polytunnel heats up really quickly when the sun is out. I believe this is due to infrared radiation being unable to pass through the polythene (or glass) so heat builds up. (but with thick cloud cover the temperatures inside and out are very similar in the day time). In the evening I've noticed that the temperature inside falls quickly and by early morning it is a degree or two colder than the outside air temperature. I wonder why this is so as I'd expect it to stay at least as warm as the outside temperature if heat is being lost by conduction alone from a hotter body (the PT) to a colder body (air outside). As a consequence, the tatties I was growing for Xmas have mostly died off even though the frosts outside have been very light. Could it be that other radiation wavelengths continue to escape all night but are not being topped up by the sun, especially as there's no IR effect? If someone could explain the physics of it, I'd appreciate it!