Cold air is denser, molecules slow down blah blah. Essentially warm air higher pressure, cold air lower pressure. Add more air if it's going to be cold for a while, try to keep it around specs. Your tire pressure rises a little when driving after the rubber heats up too.
If you added warm air, the molecules were spaced out more, when it got cold, they crammed in closer. The colder it gets the lower the pressure goes and the TPMS I believe sense a 10% reduction, which isn't much.