Friday, January 22, 2010
Heating Up the Cold Frame
The cold frame has been outside for about a week now and we have had two sunny days recently. I use a maximum-minimum thermometer and a soil thermometer to record the effect of the cold frame. Most interesting is that the soil inside the frame, except near the front edge where the sunshine doesn't reach, is no longer frozen.
There is no doubt that the sun warms up the air and the soil inside the cold frame. On January 21st, when the high daytime temperature (according to the newspaper) was 32 degrees, the temperature inside reached 64 degrees. It was a bright sunny day. The next day was another mostly clear sunny day and the temperature got up to 61 degrees and the soil temperature (taken in the area that the sun reached) was 46 degrees, although the outside temperature high was 36 degrees.
After I put the cold frame outside I added insulation that I bungee cord over the glass in the afternoon when the sun no longer reaches the cold frame. Two nights ago the outside temperature dropped to 14 degrees but the lowest temperature inside the cold frame was 30 degrees. Last night the outside temperature was 12 degrees but the lowest temperature inside the cold frame was 29 degrees and the soil temperature in the morning before the sun struck the cold frame glass was 36 degrees.
What these mean is that, on those two clear sunny days, the temperature inside the cold frame got up to 25 to 30 degrees higher than the outside air temperature. At night the temperature dropped below freezing but was about 16 degrees above the outside air temperature. The soil temperature remained above freezing although it dropped about 10 degrees.
We have three more warm (but not necessarily sunny) days coming up (projected highs in the mid 30s to low 40s) followed by a return to much colder weather (highs in the low twenties and lows around 10 degrees). I may move a few of the mache and claytonia seedlings that I grew as an experiment into the cold frame tomorrow to see if they can survive the colder weather. Do I leave the insulation on all day if it is going to be cloudy? Do they need sunlight or warmth more?