Mass, Density and Pressure Density expresses the amount of mass in a given volume. The density of air changes with pressure. At the surface, air density is near 1 kg m3. At the top of the troposphere, the density is only about 0.1 kg m3. Mass is the amount of matter within a given object or quantity of fluid.
Weight is the force that the mass experiences when in a gravitational field.
The hydrostatic relationship expresses the change in pressure with height. Basically, the pressure at a given point is equal to the weight of all matter above that point. Similarly the change of pressure with height equals the change of matter above a given point with height, or simply the density of the fluid. The hydrostatic balance is valid for most large-scale motions. Lines of constant pressure are called isobars.