Everything on the earth’s surface is under pressure due to weight of the atmosphere. The amount of pressure applied equals the weight of a column of air one square inch in cross sectional area that extends to the top of the atmosphere. There are three system commonly used in aviation for measuring pressure. It’s have pounds per square inch, inches of mercury and millibars. The general gas laws used on air density and could derive that for a particular gas, pressure and temperature determine density. This gives rise to the expression density altitude.