Ambient pressure concept of pressure becomes irrelevant, and it is undefined at a gravitational singularity. The SI unit of pressure is the pascal (Pa), which is a very small unit relative to atmospheric pressure on Earth, so kilopascals (kPa) are more commonly used in this context. The ambient atmospheric pressure at sea level is not constant: it varies with the weather, but averages around 100 kPa. In fields such as meteorology and underwater diving, it is common to see ambient pressure expressed in bar or millibar. One bar is 100 kPa or approximately ambient pressure at sea level. Ambient pressure may in
Henry's law a species in the aqueous phase, and formula_8 is the partial pressure of that species in the gas phase under equilibrium conditions. The SI unit for formula_12 is mol/(m Pa); however, often the unit M/atm is used, since formula_17 is usually expressed in M (1 M = 1 mol/dm) and formula_8 in atm (1 atm = 101325 Pa). The Henry solubility can also be expressed as the dimensionless ratio between the aqueous-phase concentration formula_17 of a species and its gas-phase concentration formula_24: For an ideal gas, the conversion is: where formula_27 is the gas constant and formula_28 is the temperature.