Ambient pressure
The ambient pressure on an object is the pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 of the surrounding medium, such as a gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

 or liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

, which comes into contact with the object.

The SI unit of pressure is the pascal (Pa), which is a very small unit relative to atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

 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
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

 and scuba diving
Scuba diving
Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....

, it is common to see ambient pressure expressed in bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

 or millibar. One bar is 100kPa or approximately ambient pressure at sea level. Ambient pressure may in other circumstances be measured in pounds per square inch
Pounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...

 (psi) or in atmosphere
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...

s (atm). One atmosphere is also approximately the ambient pressure at sea level and is equal to 14.7 psi or 1.01325 bar, which is close enough for bar and atm to be used interchangeably in many applications.

In scuba diving, the ambient pressure on the diver increases linearly with depth. Since water is much denser than air, a diver will experience much greater changes in ambient pressure than can be experienced above water. Each 10 metres (32.8 ft) of depth adds another bar to the ambient pressure.

Within the atmosphere, the ambient pressure decreases with height above ground and by measuring ambient pressure, a pilot may determine height. Near the ground, a change of ambient pressure of 1 millibar is taken to represent a change of height of 9 metres (29.5 ft).
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