It's been low 30's here (43rd Ave and Peoria) in the mornings. Here's what I find weird. I covered my two raised beds with old flannel sheets and then placed a large trellis on top to hold the sheets in place.

I the mornings, the last two, I checked the temp just outside the raised bed, then put the temp probe under the flannel sheet and it registered a few degrees colder. For example, it was 33ð outside and 30ð under the sheet.

Does that make sense? The probe is accurate, it's one my hubby uses for air conditioning work. All the seedlings that have sprouted in the raised beds are fine. How can it be 30ð under the covering with no damage to the sprouts???

I have a similar thing going on with my temporary greenhouse. This morning, it was 46 on my patio but in the "greenhouse" the thermometer said that it was 30.My light in there has a photo cell on it to shut it off at daybreak so I thought that maybe that was the problem. A few hours ago, it was 52 on the patio and 39 in the greenhouse. The funny thing is that I have 2 120 watt floodlights in there. I think I'm going to try a different thermometer.
If my current thermometer is right, I will probably begin to see some damage on my Adeniums. If they make it after tonight, I am thinking that I will bring them in on cold nights and not chance it.

52ð on the patio and 39ð in the greenhouse, that's a big difference. (I tried two different thermometers too). Wonder how that works. If I could figure a Google phrase, I'd try it. Wikipedia knows all.

Location has a lot to do with it too. I'm in a cold "trough" a low lying area that cold air sinks into. I will average 25 nights of a brief frost, say 2 hours before sunrise, per year. In the same year, the local area forecast will call for frost maybe 5 nights.

If it's a clear night, plants and thin puddles of water can lose enough heat to what is called "night sky radiation" to freeze themselves even though air temps did not reach freezing.

To prevent this, put something between the plant and the night sky ... like a frost cover.

I'm in Michigan, and I once asked a weatherman why this happens. He said the air temperature is measured at eye level. Of course, cold air sinks, so it is often colder at ground level. Windless, cloudless nights will often show frost here even at 40 degrees. Covering works because it retains the ground heat.
Frost damages/kills plants because the water molecules expand when they freeze, breaking cell walls.

you have now figured out how they made ice in the desert in ancient times ... welcome to the desert ...

http://yarchive.net/space/ice_in_desert.html

Here is a link that might be useful: Making Ice in the Desert