As long as the air isn't confined in any way, the molecules get agitated when it gets hot and fly apart a bit more therefore there are less molecules per given volume, therefore the pressure is less.

This is the principal behind an intercooler in an engine. If the air going towards the inlet is hot but is passed through a heat exchanger to cool it down before actually going into the cylinder, more air gets into the cylinder (it's denser) and more power is produced.

-David C.