Atmospheric Pressure – Concept, discovery, how it is measured

We explain what atmospheric pressure is, why it varies and how it was discovered. Also, how it is measured and what units are used.

atmospheric pressure
Atmospheric pressure is equivalent to the weight of a column of air.

What is atmospheric pressure?

Atmospheric pressure or barometric pressure is the force exerted by the set of mixed gases that make up the atmosphere, on the earth’s surface and the elements that are on it. This force is given per unit area, that is, it is equivalent to the weight of the column of air that extends from a point on the Earth’s surface to the upper limits of the atmosphere.

Atmospheric pressure and its variations over a period of time are common data in the study of atmospheric climate. However, air varies in density as it moves away from the ground and is also affected by temperature, so it is usually not easy to calculate the atmospheric pressure of a given point with a high margin of certainty.

In ancient times, the very idea of ​​atmospheric pressure was unknown, and its everyday effects, such as the practical limitation of ascent of certain materials, was understood as evidence of the horror vacuis, that is, the “terror of the void” that nature manifested, since it was thought that the air lacked weight and rose on its own.

This was so until, In 1643, the weight of the air was discovered by the Italian physicist Evangelista Torricelli (1608-1647), through the first experiments that led to the creation of the barometer. His most famous experiment consisted of comparing the behavior of mercury and water when introduced into a curved tube at the end, known today as the Torricelli tube.

His experiences served the French polymath Blaise Pascal (1623-1662), to measure the weight of atmospheric air in different geographical locations and different altitudes, such as the top of the Puy-de-Dôme volcano in southern France. But it was not until 1654, thanks to the experiments with the Magburg hemispheres of the German physicist Otto von Guericke (1602-1686), that the existence of atmospheric pressure was publicly demonstrated.

Units of atmospheric pressure

Atmospheric pressure is first and foremost a form of pressure, so is measured in the International System of Units (SI) in Pascals (Pa), a unit that pays tribute to the French physicist and is understood as the pressure exerted by a force of 1 newton (N) on a surface of 1 square meter (m2) normal to it, that is: Pa = kg / ms2.

However, to measure atmospheric pressure, it is common to use other types of units, such as atmospheres (atm), bars (b), millibars (mb), or millimeters of mercury (mmHg). Each one deserves a separate explanation:

  • 1 atmosphere is the pressure exerted by the air to balance 760 mm of mercury under normal conditions on the earth’s surface, and is equivalent to 101,300 Pa.
  • 1 bar (from the Greek word for “weight”: bars) is equivalent to 0.986923 atm, and therefore to 100,000 Pa.
  • 1 millibar it is equivalent to 1000 bars, and therefore 100 Pa and 0.0010197 atm.
  • 1 millimeter of mercury It is equivalent to the pressure necessary to increase the height of one millimeter of mercury inside a Torricelli tube. It is equivalent to 133.3 Pa and is a unit widely used in medicine.

How is atmospheric pressure measured?

atmospheric pressure barometer how it is measured
The barometer measures in baros the pressure that the air exerts on the liquid it contains.

To measure the atmospheric pressure of a certain place, we need a device called a barometer. Its fundamental principle, which reproduces the experiences of Torricelli in the seventeenth century, consists of a column of liquid (generally mercury) introduced into a tube whose upper portion is closed.

In this way, the weight of the air in the atmosphere exerts more or less force on the liquid, forcing it to remain inside the tube up to a certain point, equivalent to the force itself received.