How Does a Barometer Measure Air Pressure?

aneroid barometer high pressure
An aneroid barometer shows a high pressure (fair weather) reading. Peter Dazeley/Photographer's Choice RF/Getty Images

A barometer is a widely used weather instrument that measures atmospheric pressure (also known as air pressure or barometric pressure) -- the weight of the air in the atmosphere. It is one of the basic sensors included in weather stations.

While an array of barometer types exist, two main types are used in meteorology: the mercury barometer and the aneroid barometer.

How the Classic Mercury Barometer Works

The classic mercury barometer is designed as a glass tube about 3 feet high with one end open and the other end sealed.

The tube is filled with mercury. This glass tube sits upside down in a container, called the reservoir, which also contains mercury. The mercury level in the glass tube falls, creating a vacuum at the top. (The first barometer of this type was devised by Italian physicist and mathematician Evangelista Torricelli in 1643.)

The barometer works by balancing the weight of mercury in the glass tube against the atmospheric pressure, much like a set of scales. Atmospheric pressure is basically the weight of air in the atmosphere above the reservoir, so the level of mercury continues to change until the weight of mercury in the glass tube is exactly equal to the weight of air above the reservoir. Once the two have stopped moving and are balanced, the pressure is recorded by "reading" the value at the mercury's height in the vertical column.

If the weight of mercury is less than the atmospheric pressure, the mercury level in the glass tube rises (high pressure).

In areas of high pressure, air is sinking toward the surface of the earth more quickly than it can flow out to surrounding areas. Since the number of air molecules above the surface increases, there are more molecules to exert a force on that surface. With an increased weight of air above the reservoir, the mercury level rises to a higher level.

If the weight of mercury is more than the atmospheric pressure, the mercury level falls (low pressure). In areas of low pressure, air is rising away from the surface of the earth more quickly than it can be replaced by air flowing in from surrounding areas. Since the number of air molecules above the area decreases, there are less molecules to exert a force on that surface. With a reduced weight of air above the reservoir, the mercury level drops to a lower level.

Mercury vs. Aneroid

We've already explored how mercury barometers work. One "con" of using them, however, is that they're not the safest things (after all, mercury is a highly poisonous liquid metal).

Aneroid barometers are more widely used as an alternative to "liquid" barometers. Invented in 1884 by French scientist Lucien Vidi, the aneroid barometer resembles a compass or clock. Here's how it works: Inside of an aneroid barometer is a small flexible metal box. Since this box has had the air pumped out of it, small changes in external air pressure cause its metal to expand and contract. The expansion and contraction movements drive mechanical levers inside which move a needle. As these movements drive the needle up or down around the barometer face dial, the pressure change is easily displayed.

Aneroid barometers are the kinds most commonly used in homes and small aircraft.

Cell Phone Barometers

Whether or not you have a barometer in your home, office, boat, or plane, chances are your iPhone, Android, or other smartphone has a built-in digital barometer! Digital barometers work like an aneroid, except the mechanical parts are replaced with a simple pressure-sensing transducer. So, why is this weather-related sensor in your phone? Many manufacturers include it to improve elevation measurements provided by your phone's GPS services (since atmospheric pressure is directly related to elevation).

If you happen to be a weather geek, you get the added benefit of being able to share and crowdsource air pressure data with a bunch of other smartphone users via your phone's always-on internet connection and weather apps.

Millibars, Inches of Mercury, and Pascals

Barometric pressure can be reported in any one of the below units of measure:

  • Inches of Mercury (inHg) - Used mainly in the United States.
  • Millibars (mb) - Used by meteorologists.
  • Pascals (Pa) - The SI unit of pressure, used worldwide.
  • Atmospheres (Atm) - Air pressure at sea level at a temperature of 59 °F (15 °C)

When converting between them, use this formula: 29.92 inHg = 1.0 Atm = 101325 Pa = 1013.25 mb

Using Pressure to Forecast Weather

Changes in atmospheric pressure are one of the most commonly used ways to forecast short-term changes in weather. To learn more about this, and why slowly rising atmospheric pressure typically indicates settled, dry weather whereas decreasing pressure often indicates the arrival of storms, rain, and windy weather, read How High and Low Air Pressure Drives Your Daily Weather.

Pressure Lessons, Labs, & Activities

Teachers, looking for air and pressure-related experiments?

Edited by Tiffany Means