Atmospheric pressure: what is it, formula, value and units

What is atmospheric pressure?

Atmospheric pressure or barometric pressure is the force exerted by the air column of the atmosphere on the Earth's surface at a certain point.

This force is inversely proportional to altitude. The higher the altitude, the lower the atmospheric pressure, and the lower the altitude, the higher the atmospheric pressure.

The highest atmospheric pressure is that produced at sea level. Therefore, this measurement is taken as a reference of normal atmospheric pressure.

Units of atmospheric pressure

There are various units of measurement to represent atmospheric pressure. The one used in the SI is the so-called Pascal (Pa) or hectopascal (hPa). However, bars (b), millibars (mb), “atmospheres” (atm), millimeters of mercury (mm Hg) and Torricellis (Torr) are also used.

Atmospheric pressure formula

The formula for calculating atmospheric or barometric pressure is governed by the principles of the hydrostatic fundamental equation. Let's see below.

Pa = ρ.gh

In this formula,

• Pa is equal to the pressure exerted on a point of the fluid. P is equal to the density of the fluid. G is equal to the acceleration of gravity. H is equal to the depth.

Thus, if:

• ρ = 13550 kg / m3 (density of mercury) g = 9.81 m / s2h = 0.76 m (height of the mercury column)

So,

• Pa = 101 023 Pa

Value of atmospheric pressure at sea level

The value of the normal atmospheric pressure (at sea level) is 760 mm, which is equivalent to 760 torr; at 1,013.2 mb (millibars); 101 325 Pa (pascals); at 1013.2 hPa (hectopascals) or also at 1 atm (atmosphere).

Instrument to measure atmospheric pressure

Arenoid barometer

The instrument for measuring atmospheric pressure is known as a barometer. That is why atmospheric pressure is also called barometric pressure. There are different types of barometers. The most important are the following:

Mercury barometer

Basic diagram of the mercury barometer.

It is the first barometer in history, invented by Evangelista Torricelli in 1643. It is a glass tube open at the lower end and closed at the upper end. This tube is filled with mercury, the level of which varies according to the weight of the column of air that rests on the device. Let's see how it is possible.

In Torricelli's so-called experiment, the scientist completely filled a one-meter-long tube with mercury and closed it with a finger. Then he turned it over, placed it at a certain angle in a container, also with mercury, and released the mouth of the tube.

In doing this, the liquid descended, but the descent stopped at a height of 76 cm, creating a vacuum at the upper end. From this it was induced that the pressure in the vacuum equals 0. With these data, Torricelli managed to calculate the atmospheric pressure.

Arenoid barometer

Internal mechanism of an arenoid barometer

Invented in 1843 by Lucien Vidie, this barometer consists of a silver metallic capsule. This capsule is in contact with a lever attached to gears, which, in turn, are attached to an indicator needle. The capsule contracts when there is more pressure or expands when there is less pressure, which drives the movement of the gears and activates the indicator needle.