Lettuce continue

It amazes me how hardy plants are. Even after experiencing several nights in the teens, some unprotected plants in the garden are green, not growing, but green.
And the lettuce experiment continues.
On Monday I picked a large basketful of lettuce. Even after some very cold nights, lots of lettuce is still good. Some of the leaves are obviously ruined. Some just have ruined bits on them. Others are as lovely and crisp as if it were a balmy spring.
Underneath the plastic, which is draped over 10-foot PVC bent to make a tunnel about 4 feet wide and 2 1/2 to 3 feet tall, large Christmas lights add a few degrees of warmth during cold nights. Buckets painted black and filled with water absorb heat from sunlight during the day and radiate it at night. This past Sunday night, when the low hit 15, I filled the buckets with hot water. The next two nights, when the lows were predicted in the low teens, I put sheets over the plastic to keep the heat from dissipating as rapidly or as much.
The sheets seem critical to keeping the temperature up.
Last week I put a remote temperature sensor inside one of the low tunnels. A receiver indoors tells me what the temperature in the tunnel is, and records the high and low every 24 hours. On the nights without the sheets, the temperature inside seemed not much different than the outdoor temperature. However, the sensor for our regular outdoor reading is about 4 feet above the ground on a tree. The slope of the land puts the lettuce beds possibly 10 feet lower than that sensor, enough of a drop that the temperature there could be a few degrees colder, since cold air flows downward.
Even with the lights on, the temperature was not much higher inside the tunnels. That was on nights when the low was in the 20s, without the extra protection of the sheets. With the sheets over the tunnels, the temperature was quite a bit different. The low inside the tunnel last night was 24 degrees, while the exterior low was 15.
Today, when the high hit 46 and the sun shone through light, wispy clouds, the temperature inside the tunnel went up to 90. But the temperature drops rapidly when the sun falls low, and trees to the west cast shadow over the tunnels.
The lettuce is heavily mulched with hay, so the roots should be relatively warm, enough to keep the plants alive. As warm as it gets inside the tunnels during the day, the lettuce might even do a little growing.
The forecast lows for this weekend are something like 13 for Saturday night and a blistering 6 (that's degrees Fahrenheit) on Sunday night, with the highs those two days in the low 20s. Ouch.
However, Thursday and Friday's highs are about 50 or a little higher. So on Friday I will pick more lettuce, do a little cleanup, water, get more buckets situated inside and put blankets over the lettuce beneath the tunnel. Then before I go to bed on Saturday and Sunday nights, I will fill the buckets with hot water and put sheets over top of the tunnels again.
My friend who had lettuce growing all last winter has a low tunnel that is taller and wider than mine. I wonder if the larger area held just a bit more heat. I don't know. She told me that on really cold nights she placed candles inside the tunnel. But she has some ground space between her lettuce and the side of the tunnel. I have lots of dry, flammable hay. No candles for me.
We'll see whether this weekend puts an end to the lettuce experiment, or whether it keeps on going.