In order to understand this, it is important to define some basic concepts:
| Pressure | The force applied to a unit area of surface. or rephrased, The force (per unit area) exerted on a surface by the gas (e.g.air) or liquid (e.g.water) in contact with it. |
Therefore, we can paraphrase the above explanation of ‘pressure’ to define both air and water pressure:
| Air Pressure | The force (per unit area) exerted by the weight of air above an object. e.g. the higher a person goes above sea level = less air pushes down on them = decreasing air pressure. |
| Water Pressure | The force (per unit area) exerted by the weight of water above an object. e.g. the deeper a person dives below sea level = more water from above pushing down on them = increasing water pressure. In addition, the diver will also experience the weight of air from above the water surface. |
This leads on to explaining what barometric pressure is:
| Barometric pressure is also known as air pressure or atmospheric pressure and derives its name from the instrument used to measure it called a Barometer. |
Units of Measurement for Barometric Pressure
The International System (SI) unit of pressure is the Pascal (Symbol: Pa) which is defined as one Newton per square meter.
However, the unit for measuring atmospheric pressure for international meteorological purposes remains the millibar (mbar or mb).
1 mb = 100 Pa = 1 hPa.
Metric units of measurement for atmospheric pressure include:
Pascal (Pa)
Kilopascal (kPa) Note :1kPa=1000Pa
Millibar (mbar or mb) Note: 1mbar=1 hectopascal (hPa)
Bar (bar) Note: 1bar=1000mbar=100KPa which is roughly equal to the atmospheric pressure on Earth at sea level ( 0.987 atm)
Equivalent units of measurement include:
Mercury:
Millimeter of mercury (mmHg)
Centimeter of mercury (cmHg) Note: 1cmHg = 10 mmHg
Inches of mercury (inHg)
Others:
Pounds per square inch (psi or alternatively lbf/in2, lbf/in2, lbf/sq in or lbf/sq in)
Pounds per square foot (psf)
Atmospheres (atm) Note: A standard atmosphere = 101325Pa /101.325kPa /1013.25mbar /760 mmHg /29.92 inHg /14.70 psi