# Facts about Pressure

Static pressure and stagnation pressure are related by the Mach number of the fluid.

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The pressure exerted by a column of liquid of height h and density ? is given by the hydrostatic pressure equation noted above: p = hg?.

Pressure (symbol "p") is the force applied to a surface (in a direction perpendicular to that surface) per unit area of the surface.

The SI unit for force is the Newton, that for area is square meter, and that for pressure is called a pascal (Pa).

Water is nontoxic and readily available, but mercury's density allows for a shorter column (and so a smaller manometer) to measure a given pressure.

Consequently, although a fluid moving at higher speed will have a lower static pressure, it may have a higher stagnation pressure when forced to a standstill.

A liquid or gas can become the source of a localized pressure if either of them is forced through a narrow opening.

The standard atmosphere (atm) of pressure is an established constant.

The pressure within a fluid (gas or liquid) is a scalar quantity—that is, it has magnitude but no particular direction associated with it in space.

Pressure is still sometimes expressed in kgf/cmІ or grams-force/cmІ (sometimes as kg/cmІ and g/cmІ without properly identifying the force units).

Blood pressure is measured in millimeters of mercury in most parts of the world, and lung pressure in centimeters of water is also common.

Flight by birds and airplanes occurs according to simple principles of pressure whereby the faster a fluid moves in one direction the lesser is the pressure it exerts in the direction perpendicular to its flow.

Such a measurement is sometimes called gauge pressure.

Scuba divers often use a manometric rule of thumb: the pressure exerted by ten meters depth of water is approximately equal to one atmosphere.

Pressure arises from two fundamentally different kinds of sources: ambient and localized.

Similar pressures are given in kilopascals (kPa) in practically all other fields, where the hecto prefix is hardly ever used.

Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely.

At a given spot within a fluid, the hydrostatic pressure (or "head pressure") is the pressure resulting from the weight of the fluid column above that spot.

Localized pressure sources are usually discrete objects, such as the finger pressing on the wall, or the tires of a car pressed against the pavement.

Our understanding of the concept of pressure and our ability to manipulate the pressure in various systems have provided us with many practical benefits.

Pressure is the driving force, and volume is the associated displacement.

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Natural gas pipeline pressures are measured in inches of water, expressed as WC ('Water Column').

Where space is limited, such as on pressure gauges, name plates, graph labels, and table headings, the use of a modifier in parentheses, such as "kPa (gauge)" or "kPa (absolute)," is permitted.

The pressure at any point within the gas is independent of direction.

When the fluid meets a solid surface, this pressure is transmitted to the solid in a direction perpendicular to that surface.

Stagnation pressure is the pressure a fluid exerts when it is forced to stop moving.

Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, which is equivalent to the older unit millibar (mbar).

The pressure of a moving fluid can be measured using a Pitot probe (or one of its variations) connected to a manometer.

The most common choices of fluid are mercury (Hg) and water, and pressure was once expressed in terms of inches of mercury (in Hg).

Ambient sources of pressure are usually a gas or a liquid in which an entity is immersed, such as a human being on the surface of the earth or an octopus in the sea.

The units for pressure are further discussed below.

The pressure acts as a generalized force—pressure differences force a change in volume, and their product is the energy lost by the system due to mechanical work.

Scuba divers often use a manometric rule of thumb: the pressure exerted by ten meters depth of water is approximately equal to one atmosphere.

Given that atmospheric pressure at sea level is about 100 kPa, the absolute pressure in the tire is therefore about 320 kPa.

Humans also become sensitive to ambient pressure during the take off and landing of passenger airlines when they need to swallow to equalize the air pressure on both sides of the ear drum.

Flight by birds and airplanes occurs according to simple principles of pressure whereby the faster a fluid moves in one direction the lesser is the pressure it exerts in the direction perpendicular to its flow.

The name for the unit of pressure, the (Pascal), was added in 1971.

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It extends from the earth's surface to an average of 12 km (7 miles). The pressure ranges from 1000 to 200 millibars (29.92 in. to 5.92 in.). The temperature generally decreases with increasing height up to the tropopause (top of the troposphere); this is near 200 millibars or 36,000 ft.