What physically matters is the pressure difference. 2atm:1atm and 1atm:0atm are the same -- they both exert 1.03 kg-force per square centimeter of wall. If you

What physically matters is the pressure difference. 2atm:1atm and 1atm:0atm are the same -- they both exert 1.03 kg-force per square centimeter of wall. If you're asking how does pressure change with depth, it's linear for water. The pressure is just the sum weight of the column of fluid above you. Since water is (basically) incompressible, the density is the same at any depth, so, the fluid weight is proportional to the column height. At ~10 meters, it's 1.03 kgf per cm^2 or 1 atmosphere (on top of one atmosphere of air above, for 2 atmospheres total). It's the opposite for air. That's a compressible gas, so density increases with pressure, which makes pressure an exponential function of altitude. A packet of air is compressed by the air above it, which makes it denser (more mass/volume), so that the pressure below increases at a steeper rate.