High/Low/Variable Pressure

A pressure lower than atmospheric pressure is measured as the difference between the external pressure and the ambient pressure; since atmospheric pressure is variable, even if it remains close to 1 kg/cm2, it is possible to have depressions even with absolute pressures higher than 1 atm. 

In general, depression is given in millimeters of mercury or water (100 mm H2O = 7.6 mm Hg). When the depression is very high, we speak of vacuum; in this case the absolute pressure is close to zero, while the absolute depression reaches values close to 760 mm Hg.