I can't answer these question in any depth, but when air slows down, the kinetic energy is converted into heat. So the temperature increases. I suspect most of the heating occurs due to turbulence in the compressor itself rather than slowing of flow downstream, but don't know for sure.
I think folks are assuming you're asking about what happens at altitude when, of course, the cabin pressure is significantly higher than ambient. Your question isn't really clear, but -20C is a pretty cold day in most places.