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/in^{2}, lbf/in^{2}, 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