Temperature monitoring of heated propagator tray

I was curious about what the temperature in my heated propagator trays where my chilli and pepper seedlings are.

I am using a Garland 7 windowsill propagator, which is heated but not thermostatically controlled.

This was on a cold winter's day with the temperature only a few degrees above freezing outside, but next to a south facing window, with the window closed. The room heating is only on for an hour in the morning before the recording started, and a few hours in the evening, and radiator is on the other side of the room from this window.

I used a Temper1 USB temperature probe.

The probe itself was on the side of the tray facing the window, buried in the compost. During the day, the temperature reached 32°C inside the tray, though this was at the side directly having sunlight shining on it.

The output of the temper1 sensor can be redirected to a .csv file, which I here plot using Python+matplotlib.

It may be better to place the probe further from the side of the tray where direct sunlight shines on it, to get a more representative signal.

Update 

The temperature on 6th February, when I placed the probe in the side of the tray, rather than the end facing the window. There was snow this morning, and not really any direct sunlight unlike the previous day.

The first stage of potting the chilli seedlings on from the propagator trays. Most of them have been placed 2 to each cell, with some in an individual cell.