Track surface temperature is the key, especially if you are on Supercorsas or any other 'road legal track tyre'. It might seem like a faff checking this stuff but its literally the difference between staying upright and crashing. Anything below 10 degrees and you are in serious trouble with SC1 or SC2 Pirelli/ K1,K2 Metzeller. SC0 are produced specifically to go down to 5 degrees track temp. You're actually better off on road tyres at this time of year if they can serve the performance that you need since they are much happier at very low temperatures and will handle down to 5 degrees or even less.
This is a very handy weather website that actually has a track surface temp tab so you can get an idea of how the air temperature affects the track temp through the day
http://www.myweather2.com/Motor-Racing/United-Kingdom/Cadwell-Park-Circuit.aspx In a nutshell it depends on how much rain there has been in the previous few days. If it has been wet then the track surface temp will stay just below the air temp and match it around midday. If it's been dry for a few days then the surface temp will be cold in the morning but then rise above the air temperature around noon.