Having winter covered my own greenhouse with 6-mil polyethylene for the first time, I was curious to see the difference between outside air and soil temperature at different times of the day. I happened to pick the coldest day of the year for this little study. We used a thermometer we have outside and one in the greenhouse soil. The reading on the left, below, reflects indoor temperature in the room with the sun beaming in. The reading on the right reflects the soil temperature in the raised beds of our greenhouse. No auxiliary heat is used in the greenhouse.