Atmospheric Pressure
Atmospheric pressure refers to the force exerted by the air above a surface due to its weight as it is pulled toward the Earth by gravity. The weight of the air exerts a force on every surface it contacts, and this phenomenon is known as atmospheric pressure or air pressure.
Devices for Measuring Atmospheric Pressure
There are several types of devices used to measure atmospheric pressure, including:
- Mercury Barometer: The mercury barometer is one of the oldest devices developed for atmospheric pressure measurement. It was invented by the Italian physicist Evangelista Torricelli in 1643. Initially, Torricelli experimented with a water-filled tube; however, due to the low weight of water, an exceedingly long tube was required to measure atmospheric pressure effectively. His water barometer exceeded ten meters in length, prompting skepticism among his neighbors who believed he was practicing witchcraft. To resolve this issue, Torricelli opted to use mercury, which is fourteen times heavier than water, allowing him to create a more compact barometer. This device consists of a sealed glass tube open at one end, immersed in a shallow dish filled with mercury. As atmospheric pressure changes, the height of the mercury in the tube adjusts accordingly. This height can be read directly to obtain pressure measurements.
- Aneroid Barometer: In 1844, the French scientist Lucien Vidie invented the aneroid barometer, which gradually replaced the mercury barometer due to its cost-effectiveness and ease of use and transport. This instrument comprises a sealed metallic chamber that expands or contracts in response to surrounding atmospheric pressure, and mechanical tools are employed to measure the degree of this expansion or contraction.
- Digital Barometers: Modern digital barometers provide complex meteorological data with greater accuracy and speed than their predecessors. Many of these devices display current readings as well as historical data from one, three, six, or twelve hours prior. They often include additional information such as wind speed and humidity to enhance weather predictions.
Units of Measurement for Atmospheric Pressure
Atmospheric pressure can be expressed in various units that exhibit slight differences in their values. Some of these include millimeters of mercury (mmHg), with 760 mmHg being equivalent to standard atmospheric pressure; pounds per square inch (psi), where 14.70 psi corresponds to standard pressure; dynes per square centimeter, which equate to 1,013.25 × 103; millibars (mbar), where 1,013.25 mbar denotes standard pressure; and kilopascals (kPa), equivalent to 101.325 kPa.