This morning outside at 6:00 and 0 degrees at 9:00 with lowest temperatures recorded in the greenhouse of between -2.2 to -0.2 over night from the 10 monitoring locations and that was with the terracotta pot candle heater.
looking at the weather forecast for the next 10 days I think the Terracotta pot candle will be used most nights in the next 10 days
What I really don't understand is why is the temperature dips a degree after the sun has gone at 16:48 but from 23:00 or 11pm until 3am the following morning is rising by 4 degrees.
I'm so glad the chitting spud are in the warm part of the greenhouse, the over wintering onions are hardy enough to stand the cold.
UPDATE I asked the question of our weather guru on the Grapevine Grow Your Own Forum and got this answer
What is happening here is that last night we had high pressure with light winds, allowing the cold air to sink towards the ground with nothing much to mix it up. The sky was clear so heat radiated out into space, and you had a frost. During the day these conditions persisted, but the sun warmed the air so it got a bit warmer. Once the sun goes down the heat again starts to radiate into space - a clear or partly clear sky is showing up to about 8pm. After that it starts to cloud over as the pressure falls and this stops the heat radiating into space so fast. Also notice that the wind starts to pick up. This mixes the air and prevents the cold air sinking so the temperature at ground level actually rises.
Now we all know!