Well, the DENSITY of air does not decrease or increase because density is a fixed rate. As you go higher in altitude, though, air pressure tends to increase.
True, that's what we are told by meteorologists. Density in turn is a function of temperature. Hot air is lighter / less dense than cold air. Anticyclones / high pressure areas are made of cold air,...
"Air pressure can change from day to day. A denser substance has more mass per unit volume than a less dense one. So denser air exerts more pressure than less dense air." -Focus on: California Earth...