How Is Barometric Pressure Measured and Why?

By: Dylan Ris  | 
barometric pressure
Barometric pressure, also known as atmospheric pressure, is a measurement of the air pressure in Earth's atmosphere. trekandshoot/Shutterstock

You've probably experienced the scenario where, on a clear, sunny day, a weather forecaster comes on TV and declares with great certainty that you'll be waking up to rain the next morning. How can that forecaster be so certain about weather changes? There isn't a cloud in the sky, and there aren't any storms reported in neighboring areas. What does this person know that you don't? The answer is simple, actually. The forecaster knows that the barometric pressure is falling.


How Is Barometric Pressure Defined?

Barometric pressure is a measurement of the air pressure in Earth's atmosphere. For this reason, it is also known as atmospheric pressure. The pressure is created by fluid gas molecules in the air, and it extends in all directions. Notably, it exists as downward pressure exerted on the ground on Earth, which makes it relatively easy to measure.

Barometric pressure is greater when air molecules are more densely concentrated. The pressure lets up when the molecules are more spread out. Thus, barometric pressure is consistently higher at sea level than it is at high mountain altitudes where the difference is that the air is "thinner" and less dense with molecular particles. However, sea level air pressure can also get quite low, particularly during major storms like hurricanes.


How Is Barometric Pressure Measured?

Scientists typically measure barometric pressure and barometric pressure fluctuations using a standard unit called an atmosphere (atm). In the English system of measurement, which is popular in the United States, one atmosphere equals 29.9213 inches (1,013 millibars) of mercury (in. Hg). Other conversions include:

  • 1 atm = 760 millimeters of mercury (mmHg)
  • 1 atm = 1,013.25 millibars (mbar)
  • 1 atm = 101,325 pascals (Pa)
  • 1 atm = 1,013.25 hectopascals (hPa)

All of these units describe how much air molecules exert pressure on objects around them. Pressure is directly related to density: When the concentration of particles becomes denser, pressure rises, and when the particles become less dense, pressure decreases. Thus, denser air will produce higher measurements of barometric pressure.


What Is a Barometer?

A barometer is a scientific instrument designed to measure barometric pressure. Barometers as we know them date back to 1643 when the Italian inventor Evangelista Torricelli measured air pressure using a glass tube filled with mercury and open on its downward-pointing end. The tube itself sat in a reservoir filled with more mercury.

Torricelli's contraption (and those that came after it) measures air pressure based on the level of the mercury. When air presses downward onto the pool of mercury, the force pushes some of the mercury higher in the glass tube. When the pressure goes down, there are fewer forces pushing the mercury up the tube, and thus its level falls.


Today, scientists and meteorologists can measure atmospheric pressure without a mercury-based barometer. They instead use a ​​ capacitive pressure sensor, which can detect pressure changes by monitoring fluctuations in electrical capacitance, or ability to collect and store energy in the form of an electrical charge. These devices are used to detect air pressure in all sorts of applications, from industrial manufacturing to aviation to meteorology. Despite this, good old mercury-based barometers remain in regular use.


What Does Barometric Pressure Tell Us About Weather?

For centuries, humans have used barometers and barometric pressure readings to predict the weather. The general rule of barometric pressure is simple: high pressure typically portends hot, dry weather while low pressure portends cool, damp weather. Here's why:

  • High pressure means that the atmosphere is filled with heavy, dense air. That dense air pushes away any air that's closer to the surface, including moisture-laden clouds. Typically, high pressure brings about very clear skies, allowing the sun to cut through and warm Earth's surface.
  • Low pressure means that surface-level air can move upward. In an area with low atmospheric pressure, there's nothing to stop surface-level air from rising and forming clouds. These clouds block the sun and often produce moisture in the form of rain and snow. Low pressure can also form heavy winds, as surface-level air moves freely. When an exceptionally low-pressure area forms over the ocean, it can sometimes spawn a hurricane.


What Is a Normal Barometric Pressure?

People in the United States are used to digesting barometric pressure readings in terms of inches of mercury. This measurement refers to how many inches high a column of mercury will be in a traditional barometer. The standard barometric pressure at sea level is 29.92 inches (1,013 millibars) of mercury (also measured as 1 atmosphere). In general, a normal range for barometric pressure is between 28.5 (965 millibars) and 30.7 inches (1,040 millibars) of mercury.

Meteorologists have observed extreme barometric pressure readings that have gone well beyond this normal range. In December 1968 in Siberia, a barometer recorded a pressure reading of 32.01 inches (1,084 millibars) of mercury. This reading, taken during an extremely cold, dry weather spell, is thought to be the highest barometric pressure ever recorded.


On the other extreme, an October 1979 typhoon over the Pacific produced a stunningly low barometric pressure reading of 25.9 inches (877 millibars) of mercury. Indeed, nearly all the lowest pressure barometer readings in the world have been read measuring hurricanes, cyclones and typhoons. For accuracy's sake, these are the lowest pressure readings at sea level. Extremely low pressure readings have also occurred high in the mountains, where air is far less dense.