Fluid mechanics

What is atmospheric pressure? Units and barometers

What is atmospheric pressure? Units and barometers

Atmospheric pressure (or barometric pressure) is the pressure exerted by the atmosphere on the earth's surface.

In most cases the value of this quantity is equivalent to the hydrostatic pressure exerted by the weight of the air column present on the measurement point.

Pressure is the force exerted per unit area, in this case the force exerted by the air at a given point on the surface. If the force exerted by the air increases at a certain point, the pressure will also increase.

Atmospheric pressure is one of the thermodynamic parameters of the state of the atmosphere, it changes according to place and time.

These pressure differences have a thermal origin and are directly related to solar radiation and the processes of heating air masses. 

Atmospheric pressure varies with altitude: the higher the altitude, the lower the atmospheric pressure. Therefore, at sea level is the place where the atmospheric pressure is highest.

What is the unit of atmospheric pressure?

In the international measurement system, atmospheric pressure is measured in Pascals (Pa). Although it is also very common to use the atmosphere (atm). An atmosphere is the median pressure exerted by the Earth's atmosphere at sea level at different points on Earth.

In addition to the pascal, other units are also used:

  • inch or millimeters of mercury (mmHg)

  • kilopascal (kPa)

  • atmosphere (atm)

  • millibar (mbar)

  • hectopascal (hPa)

However, the most common units for measuring atmospheric pressure are the last three. At an example level, 1 atm is equivalent to 760 mm of mercury or 101.325 Pa.

Atmospheric pressure gauges

Barometers are the instruments that measure atmospheric pressure. They are used in many areas of physics, but especially in meteorology. Barometers can also be used as an altimeter, taking into account the change in atmospheric pressure with altitude.

How is atmospheric pressure measured?

Atmospheric pressure is measured with the barometer. There are two types of barometers:

Aerobic barometer

These meters contain a liquid in a glass tube in such a way that one of the two surfaces of the liquid is in contact with air. In fact, the adjective "aerobic" (aero- + -bio) means that the liquid touches the atmospheric air pressure in direct contact.

Evangelista Torricelli's mercury barometer is an example of an aerobic barometer. To invent his barometer, Torricelli grabbed a glass tube and filled it with mercury. Later, he immersed it in a container filled with mercury. The mercury inside the tube rose and fell as a function of atmospheric pressure.

Aneroid barometer

These barometers do not use liquid but a hermetically sealed box or metal frame into which the air is partially emptied. A surface of this cavity is in contact with atmospheric air and, under the influence of air pressure, elastically deforms.

Connected to the surface, there is a mechanical system to amplify the displacements caused by pressure changes. In this way, the deformation can be recorded on graph paper or displayed on a screen.

Data de publicació: November 18, 2021
Última revisió: November 18, 2021