What Does a Barometer Measure? | Wonderopolis Wonder of the Day #213 What Does a Barometer Measure? What does a barometer measure? How much pressure does the atmosphere exert on you at all times? How do changes in air pressure signal changes in the weather? Tags: Listen Can you feel the pressure ? It's around you…all the time…everywhere you go. What is it? Atmospheric pressure — often referred to simply as air pressure — is the constant force exerted on you by the weight of little particles of air. These tiny air particles, called air molecules, can't be seen, but they are all around you. They have weight, which means they constantly “push" down on you. If you look straight up in the air, you can imagine a tall column of air above your head reaching all the way to the edge of the Earth's atmosphere . The weight of that column of air is the amount of air pressure exerted on you. If you move to a higher elevation (climb a mountain, for example), the air pressure will be lower. Why? The length of that column of air above you has decreased by the amount of your increase in elevation .

As you move to a higher elevation , you may notice that your ears have to “pop." This balances the pressure between the inside and outside of your ear. Since there are fewer air molecules the higher you go, you will also probably need to breathe faster to breathe in more molecules to make up for the deficit . Air molecules also take up space. Because there tends to be a lot of empty space between air molecules, air can either fill a big area or it can be compressed to fit into a smaller area. When it's compressed, air is said to be under high pressure . Earth's atmosphere presses down on you with a force of almost 15 pounds per square inch. You may be wondering why it doesn't feel that heavy or why you're not crushed under the weight. Remember that thing you do called breathing? The air inside your body balances out the pressure from air in the atmosphere , which prevents you from being squished by the pressure of the atmosphere . You don't sense air pressure as a constant force , because the air inside you balances outside pressure and you're used to that feeling. If you watch the weather report frequently, you're sure to hear the weatherman talk about barometric pressure .

Weather forecasters use a special tool called a barometer to measure air pressure . Barometers measure atmospheric pressure using mercury , water or air. You'll usually hear forecasters give measurements in either inches of mercury or in millibars (mb). Forecasters use changes in air pressure measured with barometers to predict short-term changes in the weather . Changes in air pressure signal the movement of high- or low- pressure areas of air, called fronts. Air molecules in high pressure areas tend to flow toward low pressure areas. We call this flow of air molecules wind . The larger the difference in pressure between areas, the stronger the winds will be. As weather forecasters monitor air pressure , falling barometer measurements can signal that bad weather is on the way. In general, if a low pressure system is on its way, be prepared for warmer weather with storms and rain. If a high pressure system is coming, you can expect clear skies and cooler temperatures. Wonder Words (18) Test your knowledge Wonder What's Next? We’re about to erupt with excitement. Join us in Wonderopolis tomorrow for a Wonder of the Day that will blow your top! Try It Out How's the weather?

Gather together a few friends or family members to check out one or more of the following fun activities: How accurate is the weatherman where you live? Keep track and find out! Over the course of the next week, watch the weather report at the same time each day. Take notes in a journal. Make sure to record predictions for temperature and precipitation. Then make careful observations about the weather on your own. Did the weatherman get it right? How often? Would you like to be a weather forecaster one day? Why or why not? Are you ready to predict the weather? You can be a weather forecaster in the making when you Build Your Own Barometer with just a few simple supplies. Be sure to ask an adult for help. Once you have built your barometer, put it to the test. Can you track changes in atmospheric pressure? Do they predict changes in the weather? Do you think your barometer is accurate? Why or why not? Air pressure can be a hard scientific concept to grasp. After all, air is invisible, which means it's not the easiest thing to observe! Can you see air? If it's really windy outside, you might see the effects of wind, but can you see the wind itself? Not really!

Air pressure is the same way. Science tells us that it's always pushing down on us, but it's hard to observe. To see air pressure in action for yourself, you can show your friends and family the amazing power of air pressure with this super-cool Unspillable Water Experiment ! USATODAY.com How a barometer measures air pressure Source: USA TODAY research by Chad Palmer Air pressure and differences in pressure are among the most important weather makers. The centers of storms are areas of relatively low air pressure, compared to pressures around the storm. High air pressure generally brings good weather. Keeping track of how the pressure is changing is important for forecasting the weather. Differences in air pressure between places cause the winds to blow - air moves from high toward low pressure. The instruments that measure air pressure are called barometers, from Greek words for weight and measure. Evangelista Torricelli invented the mercury barometer in 1643 and today's mercury barometers are much like those of the 17th century. The drawing above shows the basic principles of a barometer. A glass tube from which the air has been removed is inserted into a dish of mercury.

The air pressing down on the mercury in the dish forces some of the mercury up into the glass tube. At the Earth's surface, air pressure pushes the mercury about 30 inches up into the tube - give or take close to a couple of inches. The U.S. National Weather Service reports air pressure at the surface in inches of mercury while air pressure aloft is reported in millibars, also known as hectopascals (hPa). Scientists, however, generally use pressures in hectopascals. For more on air pressure, go to our Understanding air pressure file. Another type of barometer, called an aneroid barometer , is more commonly used today in meteorology and aviation because it takes up much less space and is more accurate. Barometer Basics: How it Works and Helps Forecast Weather How Does a Barometer Measure Air Pressure? An aneroid barometer shows a high pressure (fair weather) reading. Peter Dazeley/Photographer's Choice RF/Getty Images By Jenny Worboys Updated March 29, 2016. 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. continue reading below our video Test Your General Science Knowledge 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 What does it mean when a barometer is rising or falling? | HowStuffWorks What does it mean when a barometer is rising or falling? Barometers are used to predict the weather. iStockPhoto ­A barometer measures air pressure: A "rising" barometer indicates increas­ing air pressure; a "falling" barometer indicates decreasing air pressure. In space, there is a nearly complete vacuum so the air pressure is zero. On Earth, because there are many miles of air molecules stacked up and exerting pressure due to the force of gravity , the pressure is about 14.7 pounds per square inch at sea level (see this page for a good explanation of atmospheric pressure). Up Next How Weather Works ­The interesting thing about air pressure is that it is different at different points on the planet and it changes over time. Why might that be?

If you've read How Hot Air Balloons Work , you know that hot air is less dense (lighter) than cooler air. Therefore, on any given day you would expect the air over a desert to­ have a lower pressure than the air over an ice cap. And that would be true. These same sort of pressure differences occur all over the planet for various reasons. These pressure differences have a big effect on the weather, so if you know the current air pressure at your house, as well as the pressure trend, you are able to predict certain things about the weather . As a very loose rule, a high-pressure area will be clear, and a low-pressure area will be cloudy and rainy. 1 How a Barometer Works How a Barometer Works How a Barometer Works Enjoying the use of your analog barometer - with the needle! Your barometer is just as accurate a weather forecaster as the TV meteorologist you watch with the news. Under most conditions it forecasts weather for the 12 to 24 hours ahead. Its mission is to measure air or atmospheric pressure and indicate its rising or falling.

The pressure is shown on your barometer's dial, usually expressed in "inches" referring to "inches of mercury" (inch Hg). The early barometers measured the pressure by the rise or fall of a column of mercury. This mercury barometer was invented back in 1643 by a pupil of Galileo named Evangelista Toricelli. Meteorologists use "millibars" in charting atmospheric pressure and your barometer has a second scale or ring which reads in millibars (mb), or "hectopascals" (hPa). The latter is used in some countries as a unit of pressure. Standard air pressure at standard elevation (sea level) at 15°C and 45° latitude is 1013 hPa or 29.92 inches of mercury. Most dial type barometers employ an air pressure sensor with a limited range of sensitivity. Generally the working altitude range covers only about 3000 to 4000 feet. So be sure to choose a barometer that has a sensor range factory set for use at your elevation (e.g. 0-3000 ft., 3000-7000 ft.

or 7000 ft and above). Looking at setting your barometer... When you receive your barometer it must be adjusted before being put to use and a local weather broadcast will give barometric pressure for your approximate location. The adjusting screw is located in a hole in the back of the barometer and can be reached with a small screw driver to adjust your instrument to this reading. Turn the adjusting screw while observing the face of the dial so the indicating hand moves clockwise to match the broadcast reading for your elevation. Tap the display lightly and further adjust the hand if necessary to the desired setting. The center knob on the barometer's front moves the set hand. Use it to mark or place over the location of the black inside hand at the time you set the barometer. This outside set hand will then serve as a reference marker so you can easily see if pressure is rising, falling or steady. Your barometer's reading may not exactly coincide with a broadcast due to difference in location, time of reading and other factors. What about correcting to sea level?

Note that when using a weather barometer (usually with a scale range between 28 and 31 inches Hg) at a location above sea level, the reading must be corrected back to sea level. This is automatically accomplished when you initially match your barometer's reading to that reported by local TV or radio weather forecast. These reported readings have already been "corrected" to sea level, thus eliminating any pressure differences due to elevation. © barometer - National Geographic Society barometer A barometer measures atmospheric pressure. Photograph by Senior Airman Andy Dunaway, U.S. Air Force Storm Glass A storm glass is a type of barometer used centuries ago. A storm glass is a sealed glass container with an open spout, partly filled with colored water. If the water level in the spout rises above the water level in the container, observers expect low pressure and stormy weather. A barometer is a scientific instrument used to measure atmospheric pressure , also called barometric pressure . The atmosphere is the layers of air wrapped around the Earth. That air has a weight and presses against everything it touches as gravity pulls it to Earth. Barometers measure this pressure. Atmospheric pressure is an indicator of weather .

Changes in the atmosphere, including changes in air pressure, affect the weather. Meteorologist s use barometers to predict short-term changes in the weather. A rapid drop in atmospheric pressure means that a low-pressure system is arriving. Low pressure means that there isn’t enough force, or pressure, to push clouds or storm s away. Low-pressure systems are associated with cloud y, rain y, or wind y weather. A rapid increase in atmospheric pressure pushes that cloudy and rainy weather out, clearing the skies and bringing in cool, dry air. A barometer measures atmospheric pressure in units of measurement called atmospheres or bars. An atmosphere (atm) is a unit of measurement equal to the average air pressure at sea level at a temperature of 15 degrees Celsius (59 degrees Fahrenheit). The number of atmospheres drops as altitude increases because the density of air is lower and exert s less pressure. As altitude decrease s, the density of air increases, as does the number of atmospheres. Barometers have to be adjust ed for changes in altitude in order to make accurate atmospheric pressure readings. Mercury Barometer The mercury barometer is the oldest type of barometer, invent ed by the Italian physicist Evangelista Torricelli in 1643.

Torricelli conduct ed his first barometric experiments using a tube of water. Water is relatively light in weight, so a very tall tube with a large amount of water had to be used in order to compensate for the heavier weight of atmospheric pressure. Torricelli’s water barometer was more than 10 meters (35 feet) in height, which rose above the roof of his home! This odd device caused suspicion among Torricelli’s neighbors, who thought he was involved in witchcraft . In order to keep his experiments more secretive, Torricelli deduce d that he could create a much smaller barometer using mercury , a silvery liquid that weighs 14 times as much as water. A mercury barometer has a glass tube that is closed at the top and open at the bottom. At the bottom of the tube is a pool of mercury. The mercury sits in a circular, shallow dish surrounding the tube. The mercury in the tube will adjust itself to match the atmospheric pressure above the dish. As the pressure increases, it forces the mercury up the tube. The tube is marked with a series of measurements that track the number of atmospheres or bars.

Observer s can tell what the air pressure is by looking at where the mercury stops in the barometer. Aneroid Barometer In 1844, the French scientist Lucien Vidi invented the aneroid barometer . An aneroid barometer has a sealed metal chamber that expand s and contract s, depending on the atmospheric pressure around it. Mechanical tools measure how much the chamber expands or contracts. These measurements are align ed with atmospheres or bars. The aneroid barometer has a circular display that indicate s the present number of atmospheres, much like a clock. One hand moves clockwise or counterclockwise to point to the current number of atmospheres. The terms stormy, rain, change, fair, and dry are often written above the numbers on the dial face to make it easier for people to interpret the weather. Aneroid barometers slowly replaced mercury barometers because they were easier to use, cheaper to buy, and easier to transport since they had no liquid that could spill. Some aneroid barometers use a mechanical tool to track the changes in atmospheric pressure over a period of time. These aneroid barometers are called barographs.

Barograph s are barometers connected to needles that make marks on a roll of adjacent graph paper . The barograph records the number of atmospheres on the vertical axis and units of time on the horizontal . A barograph’s tracking tool will rotate , usually once every day, week, or month. The spikes in the graph show when air pressure was high or low, and how long those pressure systems lasted. A severe storm, for instance, would appear as a deep, wide dip on a barograph. Digital Barometers Today’s digital barometers measure and display complex atmospheric data more accurately and quickly than ever before. Many digital barometers display both current barometric readings and previous 1-, 3-, 6-, and 12-hour readings in a bar chart format, much like a barograph. They also account for other atmospheric readings such as wind and humidity to make accurate weather forecasts. This data is archive d and stored on the barometer and can also be downloaded onto a computer for further analysis . Digital barometers are used by meteorologists and other scientists who want up-to-date atmospheric readings when conducting experiments in the lab or out in the field. The digital barometer is now an important tool in many of today’s smartphone s.

This type of digital barometer uses atmospheric pressure data to make accurate elevation readings. These readings help the smartphone’s GPS receiver pinpoint a location more accurately, greatly improving navigation . Developers and researchers are also using the smartphone’s crowdsourcing capabilities to make more accurate weather forecast s. App s like PressureNet automatically collect barometric measurements from each of its users, creating a vast network of atmospheric data. This data network makes it easier and faster to map out storms as they develop, especially in areas with few weather station s. Term to change or modify something to fit with something else. air force pressed on an object by air or atmosphere. align to put in a straight line. altitude process of studying a problem or situation, identifying its characteristics and how they are related. aneroid barometer tool that determines atmospheric pressure by measuring how much a metal chamber expands or contracts. app (application) specialized program downloaded onto a mobile device. archive to keep records or documents. associate atmosphere (atm) Noun (atm) unit of measurement equal to air pressure at sea level, about 14.7 pounds per square inch. Also called standard atmospheric pressure.

atmospheric pressure Noun force per unit area exerted by the mass of the atmosphere as gravity pulls it to Earth. bar Noun (b) unit of measurement for pressure; 1 bar is about equal to the atmospheric pressure at sea level. barograph barometer that tracks changes in atmospheric pressure over time. barometer atmospheric pressure as read by a barometer. chamber to make up for a loss or injury, usually in money, goods, or services. complex to transmit, transport, or carry. contract to shrink or get smaller. crowdsourcing technique that enlists the public to assist with a specialized task. data (singular: datum) information collected during a scientific study. decrease to reach a conclusion based on clues or evidence. density having to do with numbers (or digits), often in a format used by computers. display to grow or get larger. forecast to predict, especially the weather. GPS receiver Noun device that gets radio signals from satellites in orbit above Earth in order to calculate a precise location. graph paper paper marked with small boxes, or intersecting horizontal and vertical lines. gravity physical force by which objects attract, or pull toward, each other. horizontal left-right direction or parallel to the Earth and the horizon.

humidity to explain or understand the meaning of something. invent low-pressure system Noun weather pattern characterized by low air pressure, usually as a result of warming. Low-pressure systems are often associated with storms. measurement process of determining length, width, mass (weight), volume, distance or some other quality or size. mercury chemical element with the symbol Hg. mercury barometer tool that determines atmospheric pressure by measuring how much mercury moves in a glass tube. metal category of elements that are usually solid and shiny at room temperature. meteorologist person who studies patterns and changes in Earth's atmosphere. navigation series of links along which movement or communication can take place. observer someone who watches, or observes. physicist person who studies the relationship between matter, energy, motion, and force. predict to know the outcome of a situation in advance. pressure force pressed on an object by another object or condition, such as gravity. previous earlier, or the one before. rain to turn around a center point or axis. sea level Noun mobile telephone with additional features, such as a web browser or music playing device. storm severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.

storm glass glass container filled with water or another liquid that responds to changes in atmospheric pressure. suspicion to move material from one place to another. vast up-down direction, or at a right angle to Earth and the horizon. weather area with tools and equipment for measuring changes in the atmosphere. wind Noun movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun. witchcraft What is a Barometer? (with pictures) What is a Barometer? Edited By: L. S. Wynn Last Modified Date: 12 December 2016 Copyright Protected: Adorable animal families that will make you "aww" A barometer measures atmospheric pressure . The air in the atmosphere exerts a force called pressure that constantly changes due to moving weather systems. Therefore, in conjunction with other meteorological instruments , this device can be used to help predict clear or rainy weather. In 1643, Evangelista Torricelli invented the first barometer. He figured that if he had a vacuum, an airless space, he could compare the ever-present atmospheric pressure with zero.

Torricelli placed a vacuum contained in a glass tube on top of a larger container of mercury. The air pressed down on the mercury's exposed surface and pushed it up into the tube. The higher the mercury level, the greater the air pressure , and originally, the units of air pressure were just millimeters of mercury. People could finally measure the force of air. Since the advent of "Torricelli's tube," others developed the aneroid barometer that works without liquid. In this instrument, a flexible metal accordion box that resembles a bellows is partially squeezed to a medium pressure. Then, if air pressure rises, the bellows contract because the air inside takes up less volume. This tool is often connected to a recording device, together forming a barograph . A pen moves against a rotating cylinder whenever the bellows moves and creates a visual aid to the pattern of falling and rising air pressure. The barograph remains a basic instrument of modern meteorology. Ad Used in conjunction with a thermometer, this device can help make general weather predictions. While weather is very complex, storms more or less follow certain patterns of high and low pressure systems.

In simplified terms, a rising barometer means wind, frost, or clear skies, while a falling one indicates coming storms. A steady reading might mean precipitation or sun. Weather forecasters look at the relative change at different places, taking into account how air pressure changes with elevation. Some home devices still convert units to inches or millimeters of mercury, whether or not mercury is present. The International Meteorological Society has declared the universal unit of pressure to be the hectopascal. For example, 1016 hectopascals equals 30 inches of mercury (762.1 mm). Ad How do aneroid and Torricellian barometers work? Tweet by Chris Woodford . Last updated: January 1, 2017. Is it going to rain today? Is it going to stay fine? And how can you tell? One easy way is to measure the air pressure. If it's rising and the pressure is high, chances are it'll be a fine day; if the pressure is falling, it's more likely to be wet, windy, and dull. Instruments that measure air pressure are called barometers and people have been using them for weather forecasting and scientific research for hundreds of years.

Let's take a closer look at how they 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? If you've ever been scuba diving, you'll know just what pressure feels like. Dive down beneath the surface of the sea and you'll soon feel the weight of water pressing in on you. The deeper you go, the more water there is above you, the more it weighs, and the more pressure you feel. But there's pressure pushing in on your body even if you never go in the sea. Look up at the sky and try to imagine the weight of the atmosphere: the huge amount of gas surrounding our planet and pulled to its surface by gravity . All that gas might look like a vast, empty cloud of nothing, but it still has weight. And it still presses down on your body. That's air pressure.

When you're under the sea, the weight of water pressing in on your body makes it hard to breathe from your oxygen tank. Air pressure never has this effect because our bodies are hollow and our lungs are full of air, so the air presses equally on the inside and outside of our body at the same time. That's why we don't feel air pressure in the same way we feel water pressure. Photo: Are you feeling under pressure? How much pressure? If you have a smartphone with a built-in barometer, you can nip across to your favorite app store and download a variety of barometer and weather-station apps that will tell you precisely! Why air pressure changes from place to place Air pressure varies all across our planet. It's highest at sea level (where there's the most amount of air pushing down) and gets lower the higher up you go. Way up in the atmosphere, there's much less air—so there's less oxygen to breathe. That's why mountain climbers often have to use oxygen cylinders. It's also why airplanes have to have pressurized cabins (internal passenger compartments, where the air is kept at higher pressure than it would normally be at that altitude) so people can breathe comfortably.

Even in one place, the air pressure is constantly changing. That's because Earth is constantly spinning and moving round the Sun, so different parts are being warmed up by different amounts. When the air cools and falls, it increases the pressure nearer to the ground. Regions of high pressure like this are linked with fine weather. The opposite happens when the air warms and rises to create regions of low pressure and wet weather. How can we measure air pressure? Imagine you're an inventor and your job is to create a machine that can measure air pressure. How are you going to do it? Think about air pressing down on you and see if you can imagine building something that will measure its pressure. See if you can sketch something now on a piece of paper. Here's a clue. Imagine the air pressing down is contained inside a giant, invisible tube pressing down on Earth's surface next to your feet. If you imagined something a bit like a pair of scales that can measure the weight of the air in the tube, congratulations! That's pretty much the solution. A device that can measure air pressure (which we call a barometer) works by measuring how much the air is pressing down on it.

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. How barometers work Modern barometers are completely electronic and show the 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 upside down in a bath of mercury (a dense liquid metal at room temperature) so the liquid rises partly up the tube a bit like it does in a thermometer . We use mercury in barometers because it's more convenient than using water. Water is less dense (less heavy, in effect) than mercury so air pressure will lift a certain volume of water much higher up a tube than the same volume of mercury. In other words, if you use water, you need a really tall tube and your barometer will be so enormous as to be impractical.

But if you use mercury, you can get by with a much smaller 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 first instrument of this kind in 1643. 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 and make it rise up in a tube to a height of approximately 760mm (roughly 30in). We call this air pressure one atmosphere (1 atm). Go up a mountain, and take your Torricellian barometer with you, and you'll find the pressure 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'll rise to more like 65cm (25 in). The pressure on top of Mount Everest is slightly less than a third of normal atmospheric pressure at sea level (roughly 0.3 atm). Aneroid barometers Torricellian barometers are useful and accurate, but mercury is poisonous—and no-one really wants a great lake of mercury slopping around in their home. That's why most people who own barometers have ones with easy-to-read dials, which are called aneroid barometers. Instead of having a pool of mercury that the atmosphere pushes down on, they have a sealed, air-tight metal box inside. As the air pressure rises or falls, the box either squashes inward a tiny bit or flexes outward. A spring is cunningly attached to the box and, as the box moves in and out in response to the changes in air pressure, the spring expands or contracts and moves the pointer on the dial. The dial is calibrated (marked with numbers) so you can read the air pressure instantly. Artwork: Left: 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). Photo: Right: 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. Aneroid barometers measure the air pressure when you knock their glass faces. When you first inspect them, the needle shows the pressure as it was when you last looked at them—however long ago that might have been. Give the glass a sharp tap and the needle will jump to a new position showing the pressure as it is now. The way the needle moves is important. If it moves clockwise, up the dial, the pressure is increasing so the weather is likely to be getting hotter, drier, and finer; if the needle turns counterclockwise, the pressure is decreasing and the weather is likely to get cooler, wetter, and poorer. Measuring pressure There are lots of different units you can use for measuring pressure.

Historically, scientists described ordinary atmospheric pressure as "one atmosphere" and said it was equivalent to "76cm (760mm) of mercury," sometimes written 76cmHg or 760mmHg (because Hg is the chemical symbol for mercury). You might also come across an old unit called the Torr: 1 Torr (named for Torricelli) is very roughly equal to 1mmHg (a mercury height of 1mm) or 1.33 millibars (another increasingly archaic unit)—roughly one thousandth (actually 1/760) of atmospheric pressure (0.0013 atmospheres). In modern SI units, one atmosphere is equal to 101,325 Pa (pascals) or 101.325 kilopascals (thousands of pascals or kPa). Pascals and kilopascals are the preferred scientific units for measuring pressure now. 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.) Articles 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. 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. Books For older readers Physical Geography by James F. Petersen, Dorothy Sack, and Robert E. Gabler. Cengage Learning, 2011. 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: How Does a Barometer Work? - YouTube How Does a Barometer Work? Want to watch this again later? Sign in to add this video to a playlist. Need to report the video? Sign in to report inappropriate content. The interactive transcript could not be loaded. Loading... Rating is available when the video has been rented. This feature is not available right now. Please try again later. Published on Sep 28, 2013 A barometer is a very useful invention that is used to measure atmospheric pressure and predict the weather. Category

Question: What does a barometer measure?
Answer: Atmospheric pressure