I ask the same question on GTA. The way i understood the replys temperature had very little affect on velocity. I might shoot some latter today to see if there is a difference between 60 degrees and 45 degrees. Gun is in the outbuilding cooling off now. How cold are you talking about?
Going from indoor to outdoor recently, so 68F to maybe 50F or a bit below, I was losing around 20fps and have noticed this before and more.
I heard it said on a youtube vid that it's to do with grease and lubes from factory assembly in the gun getting thicker in the lower temperatures... but I'm not sure in a PCP exactly where the grease is that's causing this - maybe in the hammer spring or valve stem? I am pretty sure that the inside of regulators are supposed to have some lubricant in them but I'm sure someone knowledgeable here could tell us.
After 4 hrs at 45 degrees i shot 5 more shots. The highest velocity was 7 fps slower than any of the 5 shots at 60 degrees. Not very scientific but it appears temperature does have some affect on velocity. The video showed that real cold has a lot to do with velocity loss.
I have noticed a difference when shooting your pcp in extreme temperature changes. I have noticed gun both my matador and my cricket left outside for two hours in 20 degree temps vastly slows down the pellet. On the other extreme, I left my matador in a 110 degree room and noticed my POI was higher.
Admittedly changes of frictions and dilatations depend not only of temperature but also of used materials ( of pallets, lube, barrel, valve, spring ,…… ) . Therefore to find out exact change of FPS/temperature is not easy.
Metal shrinks in the cold and everything gets tighter plus any moisture at all is now ice I would bet if you got super cold the gun would not fire at all.