How do aneroid and Torricellian barometers work? (2024)

by Chris Woodford. Last updated: July 3, 2022.

Is it going to rain today? Is it going to stay fine? And how canyou tell? One easy way is to measure the air pressure. If it's risingand the pressure is high, chances are it'll be a fine day; if thepressure is falling, it's more likely to be wet, windy, and dull.Instruments that measure air pressure are called barometers andpeople have been using them for weather forecasting and scientificresearch for hundreds of years. Let's take a closer look at howthey work!

Photo: This traditional aneroid barometer dates from the mid-20th century and is calibrated (marked) as a crude weather-forecasting device, but it's simply an instrument that measures the air pressure. The needle turns clockwise when the pressure is rising (indicating fair or dry weather); it turns anticlockwise when the pressure is falling (indicating rain or stormy weather).

What is air pressure?

Photo: Are you feeling under pressure? It's caused by the weight of a column of air (mostly molecules of nitrogen and oxygen) pressing down on you. The higher up you go, the "thinner" the air gets (the fewer the air molecules) and the less the pressure.

If you've ever been scuba diving, you'll know just what pressure feels like. Divedown beneath the surface of the sea and you'll soon feel the weightof water pressing in on you. The deeper yougo, the more water there is above you, the more it weighs, and the more pressure you feel. Butthere's pressure pushing in on your body even if you never go inthe sea.

Look up at the sky and try to imagine the weight of the atmosphere:the huge amount of gas surrounding our planet andpulled to its surface by gravity. Allthat gas might look like a vast, empty cloud of nothing, but it stillhas weight. And it still presses down on your body. That's airpressure. When you're under the sea, the weight of water pressingin on your body makes it hard to breathe from your oxygen tank. Airpressure never has this effect because our bodies are hollow and ourlungs are full of air, so the air presses equally on the inside andoutside of our body at the same time. That's why we don't feelair pressure in the same way we feel water pressure.

Why air pressure changes from place to place

Air pressure varies all across our planet. It's highest at sea level(where there'sthe most amount of air pushing down) and gets lower the higher up yougo. Way up in the atmosphere, there's much less air—so there'sless oxygen to breathe. That's why mountain climbers often have touse oxygen cylinders. It's also why airplanes have to havepressurized cabins (internal passenger compartments, where the air iskept athigher pressure than it would normally be at that altitude) so peoplecan breathe comfortably.

Even in one place, the air pressure is constantly changing. That'sbecause Earth isconstantly spinning and moving round the Sun, so different parts arebeing warmed up by different amounts. When the air cools and falls,it increases the pressure nearer to the ground. Regions of highpressure like this are linked with fine weather. The opposite happenswhen the air warms and rises to create regions of low pressure andwet weather.

How can we measure air pressure?

Photo: A combined digital barometer and altimeter (instrument for measuring height above sea level) used for weather forecasting. Because air pressure varies in a very predictable way with height, some altimeters measure height above sea level simply by measuring air pressure. Photo by Andy Dunaway courtesy of Defense Imagery.

Imagine you're an inventor and your job is to create a machine that canmeasure airpressure. How are you going to do it? Think about air pressing downon you and see if you can imagine building something that willmeasure its pressure. See if you can sketch something now on a pieceof paper. Here's a clue. Imagine the air pressing down is containedinside a giant, invisible tube pressing down on Earth's surfacenext to your feet.

If you imagined something a bit like a pair of scales that canmeasurethe weight of the air in the tube, congratulations! That's prettymuch the solution. A device that can measure air pressure (which wecall a barometer) works by measuring how much the air ispressing down on it.

How barometers work

Modern barometers are completely electronic and showthe pressure reading on an LCD display. The two traditional kinds of barometer are called Torricellian and aneroid (dial) barometers—and here's how they work.

Torricellian barometers

The simplest kind of barometer is a tall closed tube standing upsidedown in a bath of mercury (a dense liquid metal at roomtemperature) so the liquid rises partly up the tube a bit like itdoes in a thermometer. We use mercury in barometers because it'smore convenient than using water. Water is less dense (less heavy, in effect) thanmercury so air pressure will lift a certain volume of water much higher upa tube than the same volume of mercury. In other words, if you use water, you need a reallytall tube and your barometer will be so enormous as to beimpractical. But if you use mercury, you can get by with a muchsmaller piece of equipment.

A piece of apparatus like this is called a Torricellian barometer for Italian mathematician Evangelista Torricelli (1608–1647), a pupil of Galileo's, who invented the firstinstrument of this kind in 1643. He took a long glass tube, sealed at one end, filled it with mercury from a bowl, put his finger over the open end, tipped it upside down, and stood it upright in the mercury bowl. Since he was careful not to let any air into the tube, the space that formed above the mercury column was a vacuum.Indeed, this was the first time anyone had ever produced a vacuum in a laboratory(and a vacuum made this way is called a Torricellian vacuum in honor of its inventor).

Photo: A Torricellian barometer (sometimes called a mercury barometer) is an inverted (upside-down) glass tube standing in a bath of mercury. Air pressure pushes down on the surface of the mercury, making some rise up the tube. The greater the air pressure, the higher the mercury rises. You can read the pressure off a scale marked onto the glass.

At sea level, the atmosphere will push down on a pool of mercury andmake it rise up in a tube to a height of approximately 760mm (roughly 30in). We call thisair pressure one atmosphere (1 atm). Go up a mountain, and take yourTorricellian barometer with you, and you'll find thepressure falls the higher you up go. The atmosphere no longer pushes down on the mercury quite so much so it doesn't rise so far in the tube. Maybe it'llrise to more like 65cm (25 in). The pressure on top of Mount Everest is slightlyless than a third of normal atmospheric pressure at sea level (roughly 0.3 atm).

Aneroid barometers

Torricellian barometers are useful and accurate, but mercury ispoisonous—and no-one really wants a great lake of mercury slopping around in theirhome. That's why most people who own barometers have ones witheasy-to-read dials, which are called aneroid barometers.

Photo: An aneroid barometer in close-up. You can clearly see the spring that makes the pointer rise or fall as the pressure changes. The sealed metal box is immediately behind the spring.

Instead of having a pool of mercury that the atmosphere pushes downon, they have a sealed, air-tight metal box inside. As the airpressure rises or falls, the box either squashes inward a tiny bit orflexes outward. A spring is cunningly attached to the box and, as thebox moves in and out in response to the changes in air pressure, thespring expands or contracts and moves the pointer on the dial. Thedial is calibrated (marked with numbers) so you can read the airpressure instantly.

Artwork: An aneroid barometer is built around a sealed box (blue, sometimes called an aneroid cell) that expands or contracts with increasing pressure. As it moves, it pulls or pushes a spring (red) and a system of levers (orange), moving a pointer (black) up or down the dial (yellow).

Aneroid barometers measure the air pressure when you knock their glassfaces. When you first inspect them, the needle shows the pressure as it was when youlast looked at them—however long ago that might have been. Give theglass a sharp tap and the needle will jump to a new position showingthe pressure as it is now. The way the needle moves is important. Ifit moves clockwise, up the dial, the pressure is increasing so theweather is likely to be getting hotter, drier, and finer; if theneedle turns counterclockwise, the pressure is decreasing and theweather is likely to get cooler, wetter, and poorer.

Barographs

Air pressure changes all the time. If you're in the business of keeping weather records, you don't want tohave to keep peering at a barometer and noting down the reading every two minutes. Wouldn't it be great ifa machine could do that job for you automatically? That's what a barograph is: it's a barometer that keeps a constant record of air pressure measurements. Old-fashioned barographs (like the one pictured below) were entirelymechanical. They used aneroid barometers to measure the pressure and a simple lever recorded themeasurement on a piece of paper. A clockwork mechanism made the recording paper turn slowly on a drum so thebarograph could keep a record for hours or days at a time. Today, pressure is more likely to be measureddigitally and recorded by computer-based equipment.

Artwork: A simple mechanical barograph invented by William G Boettinger of Bendix Aviation in 1937. At its heart, there's an aneroid barometer (red), which expands and contracts according to changesin air pressure. These movements are magnified by the levers (yellow) and recorded by a pen pressingagainst the paper drum (blue). Artwork from US Patent 2,165,744: Temperature compensating means for a measuring instrument courtesy of US Patent and Trademark Office.

Electronic barometers

We live in a digital age now and mechanical barometers, charming though they are as wall decorations, are rather inconvenient and old-fashioned. So how do we measure air pressure in the modern world? Typically using chip-based barometers that detect pressure differences with tiny synthetic rubber sensors. Essentially, as the air pressure changes, a small rubber membrane flexes in or out and its electrical resistance changes accordingly; measuring the resistance (with a circuit called a Wheatstone bridge) gives an indirect measurement of the pressure. Sensors that work like this way are known as piezoresistive (a similar concept to piezoelectricity).

Animation: How an electronic barometer works (simplified): as the pressure changes, a rubber membrane (top, red) flexes back and forth. As it stretches, its resistance increases. A Wheatstone-bridge type of electric circuit connected to the membrane (gray/blue, bottom) measures the resistance and a chip converts it into a pressure measurement.

Some smartphones have chip-based barometers like this built into them, which are broadly analogous to the chip-based accelerometers you'll also find in your phone. Both are examples of what are called MEMS (micro electro mechanical systems) technology, which essentially just means chips that have a combination of tiny, moving mechanical parts and electronic sensors and controls. You can buy digital MEMS pressure and temperature sensors for use with hobbyist microcontrollers like the Arduino from manufacturers such as Bosch (see the find out more section for references).

Units for measuring air pressure

Photo: Pressure is sometimes measured in bars, but although that'sa metric unit, it's not used for scientific purposes. This is the analog pressure gauge on my home gas boiler.

There are lots of different units you can use for measuring pressure.

Historically, scientists described ordinary atmospheric pressureas "one atmosphere" and said it was equivalent to "76cm (760mm) ofmercury," sometimes written 76cmHg or 760mmHg (because Hg is the chemical symbol for mercury). Youmight also come across an old unit called the Torr: 1 Torr (named forTorricelli) is very roughly equal to 1mmHg (a mercury height of 1mm)or 1.33 millibars (another increasingly archaic unit)—roughly onethousandth (actually 1/760) of atmospheric pressure (0.0013 atmospheres).

In modern SI units, one atmosphere is equal to 101,325 Pa (pascals) or101.325 kilopascals (thousands of pascals or kPa). Pascals and kilopascals are the preferred scientific units for measuring pressure now.You'll sometimes see measurements written in hPa (hectopascals), where 1 hectopascal = 100 pascalsor 0.1 kilopascals. A standard atmospheric pressure of 101,325 Pa is equivalent to 1013.25 hPa.Bars are metric units of pressure (though not SI units) defined such that 1 bar is equivalent to 100,000 Pa.That means measurements in bars and atmospheres are very roughly the same (1 atmosphere equals 1.01 bars).

Photo: An analog car tire pressure gauge gives a quick and accurate measurement of how pumped-up your tires are. Clip the valve (black, left) over the tire valve and press briefly and the gauge on the right shoots out. The higher the pressure, the further it moves. You can read the tire pressure off the calibrated scale. Since this model dates from 1960s or 1970s Britain, the measurements are 6–50 pounds per square inch or 0.5–3.4 bar (roughly the same in atmospheres).

Old barometers tend to be marked in older, Imperial units: inches of mercury, sometimes abbreviated to inHg.Atmospheric pressure at sea level is roughly 30inHg (and you can probably see that all we're doing hereis converting ~76cm or 760mm to ~30in), and the scale on a typical aneroid barometer will run fromabout 26–31inHg.

Find out more

On this website

• Altimeters
• Hygrometers
• Pyranometers
• Thermometers

On other sites

• Air Pressure: A simple introduction from the National Weather Service. Includes an activity showing how to make your own aneroid barometer.
• It's All About Air Pressure: An introduction to air pressure and its practical uses. (Unfortunately, the original version of this page has been deleted, so this link is served via the Wayback Machine.)
• Bosch BMP085 Breakout Boardby lady ada, Adafruit. An introduction to using a piezoresistive MEMS pressure sensor with Arduino.

Articles

• How to Measure the Height of a Building With a ... Barometer? by Rhett Allain. Wired, June 21, 2017. An imaginative activity that teaches students the connection between pressure and altitude, and encourages a creative approach to solving scientific problems.
• Weatherwatch: Under pressure by David Hambling, The Guardian, 21 September 2014. How barometer technology has entered the 21st century with the development of piezoelectric barometers for smartphones.
• The Role of Air Pressure in Starting a Landslide by Henry Fountain. The New York Times, November 2, 2009. Slight changes in air pressure are enough to trigger land movements.
• EU to ban traditional barometers: BBC News, 6 June 2007. Mercury is highly toxic, but moves to ban its use in barometers have proved highly controversial.

Books

For older readers

• Physical Geography by James F. Petersen, Dorothy Sack, and Robert E. Gabler. Cengage Learning, 2016. An introductory textbook with a chapter on atmospheric pressure, winds, and circulation that explains what causes air pressure and how it relates to the weather we experience.
• A Manual of the Barometer by John Henry Belville. Taylor and Francis, 1858. An interesting introduction to barometers, written in the mid-19th century by a meteorologist from the Royal Observatory, Greenwich. Includes the early history of barometers, an explanation of how they're made, and a guide to making measurements. Largely for historical interest, though.

For younger readers

If you're interested in how pressure helps to determine our weather and how you can use pressure measurements in weather forecasting, these books are worth a look:

• Everything Weather by Kathy Furgang. National Geographic, 2012. A vibrant, colorful, 64-page guide in the Nat Geo Kids series. Good for children in the 7–10 range (large photos will interest the younger readers, while the text will engage older ones).
• How the Weather Works by Michael Allaby. Dorling Kindersley, 2006. A more serious, 200-page introduction for children aged 9–12.
• DK Guide to Weather by Michael Allaby. Dorling Kindersley, 2004. A basic, 64-page introduction for ages 8–10.

Patents

• US Patent 20070028683A1: Apparatus and method for sensing pressure utilizing a deformable cavity by Cristian Ionescu-Zanetti, University of California. An example of how a MEMS electronic pressure sensor works.