The ambient pressure on an object is the

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

of the surrounding medium, such as a gas or

liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...

, in contact with the object.

Atmosphere

Within the atmosphere, the ambient pressure decreases as elevation increases. By measuring ambient

atmospheric pressure Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1,013. ...

, a pilot may determine

altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...

(see pitot-static system). Near

sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...

, a change in ambient pressure of 1 millibar is taken to represent a change in height of .

Underwater

The ambient pressure in water with a free surface is a combination of the

hydrostatic pressure Hydrostatics is the branch of fluid mechanics that studies fluids at hydrostatic equilibrium and "the pressure in a fluid or exerted by a fluid on an immersed body". The word "hydrostatics" is sometimes used to refer specifically to water and o ...

due to the weight of the water column and the atmospheric pressure on the free surface. This increases approximately linearly with depth. Since water is much denser than air, much greater changes in ambient pressure can be experienced under water. Each of depth adds another bar to the ambient pressure. Ambient-pressure diving is

underwater diving Underwater diving, as a human activity, is the practice of descending below the water's surface to interact with the environment. It is also often referred to as diving (disambiguation), diving, an ambiguous term with several possible meani ...

exposed to the water pressure at depth, rather than in a pressure-excluding atmospheric diving suit or a

submersible A submersible is an underwater vehicle which needs to be transported and supported by a larger ship, watercraft or dock, platform. This distinguishes submersibles from submarines, which are self-supporting and capable of prolonged independent ope ...

Other environments

The concept is not limited to environments frequented by people. Almost any place in the universe will have an ambient pressure, from the hard vacuum of deep space to the interior of an exploding supernova. At extremely small scales the concept of pressure becomes irrelevant, and it is undefined at a gravitational singularity.

Units of pressure

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 Meteorology is the scientific study of the Earth's atmosphere and short-term atmospheric phenomena (i.e. weather), with a focus on weather forecasting. It has applications in the military, aviation, energy production, transport, agricultur ...

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 other circumstances be measured in

pounds per square inch The pound per square inch (abbreviation: psi) or, more accurately, pound-force per square inch (symbol: lbf/in2), is a unit of measurement of pressure or of stress based on avoirdupois units and used primarily in the United States. It is the pre ...

(psi) or in standard atmospheres (atm). The ambient pressure at sea level is approximately one atmosphere, which is equal to , which is close enough for bar and atm to be used interchangeably in many applications. In underwater diving the industry convention is to measure ambient pressure in terms of water column. The metric unit is the

metre sea water The metre (or meter) sea water (msw) is a metric unit of pressure used in underwater diving Underwater diving, as a human activity, is the practice of descending below the water's surface to interact with the environment. It is also o ...

which is defined as 1/10 bar.

Examples of ambient pressure in various environments

Pressures are given in terms of the normal ambient pressure experienced by humans – standard atmospheric pressure at sea level on earth.

Atmosphere

Underwater

Other environments

Units of pressure

Examples of ambient pressure in various environments

References

Further reading