What Is Atmospheric Pressure

Short answer

• Pressure is force per unit area. It is measured in pascals, millibars, hectopascals, inches of mercury, and pounds per square inch.
• Atmospheric pressure, also called air pressure or barometric pressure, is the pressure exerted by Earth's atmosphere. At a weather station it is approximately the weight of the column of air stretching from that point up to the top of the atmosphere.
• Standard sea level pressure is about 101,325 pascals, or 1,013.25 hectopascals, or 29.92 inches of mercury. Real sea level readings usually fall between about 980 and 1,040 hectopascals.
• Pressure falls with height because there is less air above you as you climb. About half of the air molecules in the atmosphere sit within the first 18,000 feet, about 5.6 kilometers, above the surface.
• A barometer measures atmospheric pressure. Mercury barometers read pressure as the height of a mercury column; aneroid barometers read it as the flexing of a sealed metal capsule; modern instruments mostly use electronic sensors calibrated against mercury standards.
• Low pressure systems tend to bring cloud and precipitation, and high pressure systems tend to bring fair, calm weather. The link works through rising and sinking air, not through the pressure number alone.
• Weather maps show sea level pressure, not the raw reading at each station. Converting to a sea level value lets forecasters compare stations at different elevations on an equal footing.

Introduction

Stand outside on a calm day and the air feels like nothing at all. Yet that same air presses on every square inch of your skin, your roof, and the ground under your feet with a force of about 14.7 pounds per square inch at sea level. You do not notice it because the force is balanced, steady, and acting in every direction at once.

This article explains that force: what pressure means, what atmospheric pressure is, why it falls as you climb, how barometers measure it, why forecasters convert readings to sea level, and how high and low pressure shape the weather. It does not repeat the layer by layer tour of the atmosphere; that is covered in the companion article on the layers of Earth’s atmosphere, linked in the closing notes at the end.

What Pressure Means

Pressure is force divided by area. Press your palm flat against a table and the force spreads over the whole palm. Press the same force through a fingertip and the same push concentrates on a much smaller area, so the pressure is higher. That is why a sharp knife cuts and a blunt one does not, even when you push both with the same effort.

In a gas, pressure comes from molecules in motion. Air molecules move in random directions, colliding with each other and with any surface in their way. Each collision delivers a tiny force, far too small to feel alone. But air contains an enormous number of molecules, and the sum of all those collisions on a surface adds up to a considerable push. The National Oceanic and Atmospheric Administration (NOAA) makes exactly this point in its Jetstream teaching material: molecules strike surfaces and exert a force, and the total observed pressure is the sum of a very large number of individually tiny forces.

Two features of gas pressure matter here. First, pressure acts in all directions, not only downward, because molecules move randomly and strike surfaces from every side. Second, pressure in a container rises if you add more molecules or heat the gas. Adding molecules means more collisions with the walls; heating molecules makes them move faster, so each collision hits harder. NOAA illustrates the first point with a car tire: pumping in more air adds molecules, increases collisions with the inner wall, and raises the pressure.

Keep pressure distinct from density and temperature. Density is mass per unit volume, or how much air is packed into a given space. Temperature measures the average motion of molecules. Pressure is the push those molecules exert on a surface. The three are related, but they are not the same quantity, which is why high pressure does not automatically mean hot air and low pressure does not automatically mean cold air.

What Atmospheric Pressure Is

Atmospheric pressure is pressure with a specific source: the air of Earth’s atmosphere. The atmosphere is held to Earth by gravity, so air piles up around the planet, densest near the surface and thinning with height. The pressure at any point is, to a close approximation, the weight of the air above that point.

Penn State’s introductory meteorology course states the idea in its most useful form: the pressure exerted by air molecules at a weather station is approximately the weight of the air in a column that extends from a fixed area on the ground to the top of the atmosphere. At sea level, that column weighs roughly 14.7 pounds on each square inch of surface. Penn State notes that the total force on an area the size of a single baseball base, 18 inches by 18 inches, is more than 2 tons. That is the weight pressing on a patch of ground no bigger than a dinner tray, and it presses on every such patch, all the time.

The word “approximately” deserves attention. The column weight picture is an excellent approximation and the picture meteorologists use to explain pressure behavior. Strictly speaking, air is in motion, with its own dynamics, but for understanding why pressure falls with height and why high and low systems behave as they do, the column of air is the right mental model.

The atmosphere has no sharp top edge where the column ends. It thins gradually, with no lid and no line where air stops and space begins. For pressure purposes, the practical point is simpler: almost all of the air is near the ground, so almost all of the column’s weight sits in the lowest part of the atmosphere. Climb a little, and you have already left a meaningful fraction of the column below you.

Why We Do Not Feel Crushed

If 14.7 pounds presses on every square inch of your body, why are you not flattened by it? Three facts answer the question. First, pressure acts in all directions, so the pushes on you largely balance each other, with no net force trying to squash you in one direction. A sheet of paper survives for the same reason: air presses on both faces with nearly equal force.

Second, your body is not a sealed, empty container. Your lungs, sinuses, and other air filled spaces contain air at about the same pressure as the air outside, so the pressure inside balances the pressure outside, and your tissues transmit pressure evenly rather than presenting a hollow shell to be crushed.

Third, you are adapted to it. You have lived your entire life under roughly this pressure, so it is your baseline, not an extra load. You notice pressure when it changes, not when it is steady. That is why your ears pop on a fast elevator ride or a descending airplane: the outside pressure changes faster than the air in your middle ear can equalize, and you feel the temporary difference until the two sides match again. A sealed snack bag carried up a mountain puffs up for the same reason: the trapped air still pushes outward at close to its original valley pressure, while the falling outside pressure opposes it less and less.

Why Pressure Falls With Height

Pressure falls with height for one straightforward reason: as you climb, there is less air above you. At the surface, the full column of the atmosphere stands overhead. Higher up, a large part of that column is already below you, pressing on the ground far beneath your feet rather than on you.

Gravity makes the fall steepest near the ground. It pulls air molecules toward Earth’s surface, so air is compressed and crowded together at low altitudes and spreads out thinly higher up. NOAA puts a striking number on this crowding: although the atmosphere extends hundreds of miles up, one half of the air molecules in the atmosphere are contained within the first 18,000 feet, about 5.6 kilometers, above the surface. Climb through that first layer and you have already passed half the atmosphere’s molecules. Climb the same distance again and you pass half of what remains. The fall in pressure therefore starts fast and then slows, because each equal step upward removes a smaller absolute amount of air than the step before it.

NASA’s Glenn Research Center states the same relationship in general terms: pressure can be related to the weight of the air over a given location, and as altitude increases there is always less air above than was present at a lower altitude, so air pressure decreases as altitude increases.

This fall creates a practical problem for forecasting. A mountaintop station will always read lower pressure than a sea level station nearby, even under the same weather system, simply because it has less air above it. Comparing raw readings from stations at different elevations would mix up elevation with weather. Forecasters solve the problem by converting every station reading to the value it would have at sea level, as NOAA explains. All surface weather charts use this sea level pressure, so a high over the mountains and a low over the coast on the same chart describe weather, not terrain.

Temperature also changes pressure, through density. NOAA explains that warm air is less dense than cooler air, because its molecules move faster and sit farther apart. Warm a column and it expands and lightens, tending to lower surface pressure; cool it and it contracts and grows denser, tending to raise surface pressure.

Units and Conversions

Pressure has accumulated more units than almost any other everyday quantity, because different professions adopted different instruments. Science uses the pascal. Meteorology in most of the world uses the hectopascal or the millibar. Weather broadcasts in the United States still commonly use inches of mercury. Engineering in the United States uses pounds per square inch.

The standard atmosphere is a defined reference value, not a measurement taken on a particular day. Chemistry LibreTexts defines the atmosphere unit, symbol atm, from the mercury column: 1 atm is exactly 760 millimeters of mercury, or 760 torr, and it gives the equivalent in pascals as 101,325. NOAA’s Jetstream material rounds the same standard to 29.92 inches of mercury and 1,013.2 millibars at sea level, and Penn State rounds it to about 1,013 millibars. The small differences between 1,013.2 and 1,013.25 are rounding, not disagreement.

Unit	Symbol	Standard sea level value	Where you meet it
Pascal	Pa	101,325	Science, the SI unit: one newton per square meter
Hectopascal	hPa	1,013.25	Weather maps outside the United States
Millibar	mb	1,013.25	Weather maps; one millibar equals one hectopascal
Atmosphere	atm	1	A reference unit equal to standard sea level pressure
Millimeter of mercury	mmHg	760	Mercury barometers
Torr	torr	760	Vacuum work; equal to one millimeter of mercury
Inch of mercury	inHg	29.92	United States weather broadcasts and home barometers
Pound per square inch	psi	14.7	Tires and engineering in the United States

Two conversions do most of the practical work. One millibar equals one hectopascal exactly, so 1,010 hectopascals and 1,010 millibars are the same reading in different letters. One inch of mercury is about 33.86 hectopascals, so a fall of one tenth of an inch on a home barometer is a fall of roughly 3.4 hectopascals, a noticeable change on a weather map. For rough mental arithmetic, sea level pressure is about 1,000 hectopascals, about 30 inches of mercury, and about 15 pounds per square inch.

How Pressure Is Measured

The instrument that measures atmospheric pressure is the barometer, from Greek roots meaning weight measure. Three designs have dominated its history, and each turns pressure into something a person can read: a height, a flex, or an electrical signal.

The mercury barometer was invented in the seventeenth century by Evangelista Torricelli, an Italian scientist. Chemistry LibreTexts describes the design plainly: a glass tube is evacuated, meaning the air is removed from inside it, and its open end is immersed in a reservoir of mercury. Air molecules push down on the mercury in the reservoir, and because the inside of the tube is a vacuum, nothing pushes back from inside, so outside air pressure forces mercury up into the tube. Higher pressure pushes the column higher; lower pressure lets it fall back. At standard sea level pressure the column stands at 760 millimeters, or 29.92 inches, which is the origin of inches of mercury as a pressure unit. The mercury barometer is accurate and conceptually transparent, which is why it remained the reference standard long after more convenient instruments appeared. Its drawbacks are plain: mercury is heavy, toxic, and fragile in glass, so the instrument belongs on a laboratory wall, not in a backpack.

The aneroid barometer, whose name means without liquid, replaces the mercury column with a sealed, flexible metal capsule from which most of the air has been removed. Chemistry LibreTexts describes it as a small evacuated metal capsule with a spring: rising outside pressure squeezes the capsule, and falling pressure lets the spring push it back out. Levers and gears magnify that tiny flexing into the movement of a needle across a dial, the familiar round barometer face found in homes and on ships. NOAA’s Jetstream teaching material traces the idea to Gottfried Wilhelm Leibniz, who conceived a sealed bellows design around 1700, and the first working version to French scientist Lucien Vidie, who built it in 1843. The aneroid design made the barometer portable, but NOAA notes that it still needed calibration against a mercury barometer, and that even in the 1990s National Weather Service offices still checked their aneroid instruments against mercury standards.

Today, electronic sensors have replaced the metal aneroid capsule in most barometers, NOAA notes, while adding that those sensors still need calibration to stay accurate. A modern weather station typically converts the flexing of a tiny diaphragm into an electrical signal, logs the reading every few minutes, and transmits it automatically. A barograph is the recording version of the same idea: it traces pressure continuously, so the rise and fall over hours and days can be seen as a curve rather than as isolated readings. For weather purposes the curve matters more than any single reading, because it is the trend, rising, falling, or steady, that hints at what comes next.

Every barometer reads the pressure where it stands, called station pressure. Forecasters adjust that reading to sea level pressure using the station’s elevation and the temperature of the air column, so stations at different heights can be compared. NOAA states the purpose directly: converting station readings to a sea level value gives a common denominator for stations around the world. Home barometers add a wrinkle: many carry traditional dial labels such as fair, change, and rain, and many can be adjusted for the owner’s elevation. Those labels are folklore built on a real tendency, falling pressure often precedes unsettled weather, but they are not a forecast. A barometer in Denver, if never adjusted for elevation, will sit permanently in the rain zone of its dial, because its station pressure is genuinely lower than sea level pressure. Treat its movement, rather than its absolute label, as the useful signal.

High Pressure, Low Pressure, and Weather

On a surface weather map, pressure is drawn as isobars: lines joining places of equal sea level pressure, as NOAA explains in its upper air teaching material. The centers of the patterns are marked H for high and L for low. Those two letters carry most of what a forecast map has to say about the next day or two.

A low pressure system is a region where the column of air weighs less than the columns around it. Penn State calls a low a lightweight. A high is the opposite: its column weighs more than its neighbors, making it a heavyweight. The weight difference is surprisingly small. Penn State notes that the difference between an ordinary high and a fairly strong low is only about five percent of total pressure, illustrated by a high of 1,018 millibars beside a low of 986 millibars, a gap of just 32 millibars.

The reason a small difference has large weather consequences is vertical motion. Near the ground, friction turns surface winds slightly toward low pressure. Surface winds therefore converge toward a low, forcing air to rise, and diverge away from a high, forcing air to sink. Rising air cools as it ascends, and cooler air holds less water vapor, so vapor condenses into cloud droplets and, if the lift continues, into precipitation. Sinking air does the reverse: it compresses and warms as it descends, suppressing cloud formation and favoring clear skies. That is the physical chain behind the familiar rules: lows bring cloud and precipitation, highs bring fair and calm conditions.

Wind itself is air moving from high pressure toward low pressure, trying to equalize the difference. The stronger the pressure difference over a given distance, the stronger the wind. Chemistry LibreTexts makes the storm connection explicit: a large difference in pressure between two areas is an important criterion for generating storm reports, and the greater the difference, the stronger the winds that develop. That is why tightly packed isobars on a map signal strong winds, while widely spaced isobars signal light winds.

Typical values help anchor the map. Penn State gives a practical range: nearly all sea level pressures lie between 950 and 1,050 millibars, with most readings between 980 and 1,040 millibars. A very strong winter high may reach around 1,050 millibars, while a formidable low bringing heavy snow might read 960 to 980 millibars. The extremes belong to tropical cyclones. Penn State notes that very intense hurricanes can have sea level pressures down near 900 millibars, citing Hurricane Maria at its peak in 2017 with a minimum sea level pressure of 908 millibars, and that a handful of hurricanes and typhoons globally have dropped slightly below 900 millibars.

Reading a single pressure value tells only part of the story. Forecasters watch the trend, the direction and speed of change, at least as closely as the value. NOAA’s Jetstream material preserves a set of nineteenth century barometer rules attributed to Rev. Dr. Brewer, writing in 1848, and immediately qualifies them: the rules hold true for many locations but not all of them. In the tropics, except for tropical cyclones, there is very little day to day pressure change, and none of the rules apply. In the United States, the largest storm related pressure changes generally occur in Alaska and the northern half of the country. The general tendency still holds, a falling barometer often precedes wind and rain, and a rising barometer often precedes fair weather, but the tendency is a clue, not a forecast.

Pressure also has a small daily rhythm. NOAA notes a twice daily rise and fall driven by the Sun’s heat, lowest around 4 a.m. and 4 p.m. and highest around 10 a.m. and 10 p.m., largest near the equator. Three cautions keep pressure in proportion. High pressure does not guarantee sunshine and low pressure does not guarantee rain. Pressure is one variable among several, and a forecast built on pressure alone will often be wrong. And the pressure and weather link is strongest in the midlatitudes and weakest in the tropics, where pressure barely moves outside tropical cyclones.

Common Confusions

High pressure does not mean the air feels heavy. All surface air is under roughly the same order of pressure; a high differs from a low by only a few percent, so the words high and low describe a comparison between neighboring columns, not an absolute state. Low pressure does not suck air upward: air moves from high to low because the higher pressure side pushes harder, not because the low side pulls. The unit called an atmosphere is not the same thing as the atmosphere itself: one atmosphere, symbol atm, is a pressure unit equal to standard sea level pressure, while the atmosphere is the whole envelope of gas around Earth. Finally, pressure alone does not describe the air overhead, since two columns of the same height can weigh different amounts if one is warmer and less dense than the other, which is why forecasters track temperature and wind as well as pressure.

Frequently Asked Questions

What is atmospheric pressure in simple terms?

Atmospheric pressure is the push of the air around Earth on any surface, caused by the weight of the air above that surface and by the collisions of air molecules with it. At sea level it is about 14.7 pounds per square inch, about 1,013 hectopascals, or 29.92 inches of mercury. You do not feel it as a crushing force because it pushes in all directions at once and the air inside your body pushes back with nearly equal force.

What is the difference between pressure and atmospheric pressure?

Pressure is the general concept: force divided by area, wherever it occurs, in a tire, in a water pipe, or in a gas container. Atmospheric pressure is the specific case where the fluid is Earth’s atmosphere and the force comes from the air above. A tire gauge reading of 32 pounds per square inch, for example, is not an atmospheric reading at all.

Why does atmospheric pressure decrease with altitude?

Because there is less air above you as you climb. Gravity concentrates air near the surface, so about half of all air molecules sit within the first 18,000 feet above the ground. Climbing leaves part of the column below you, so the weight pressing down at your altitude falls. The fall is fastest near the ground, where the air is densest, and slows with height as the remaining air thins out.

What is standard atmospheric pressure?

Standard atmospheric pressure is a defined reference value: 101,325 pascals, or 1,013.25 hectopascals, or 760 millimeters of mercury, or 29.92 inches of mercury, or about 14.7 pounds per square inch. It is close to the average pressure at sea level, but it is a fixed definition, not a daily measurement. Real sea level pressure on a given day usually falls between about 980 and 1,040 hectopascals.

How is atmospheric pressure measured?

With a barometer. A mercury barometer measures the height of a mercury column pushed up by air pressure. An aneroid barometer measures the flexing of a sealed metal capsule as pressure changes. Modern weather stations mostly use electronic pressure sensors, calibrated against mercury standards. Whatever the design, forecasters convert the station reading to sea level pressure before plotting it on a weather map.

What does high pressure mean for weather?

High pressure generally means sinking air, which warms as it descends and suppresses cloud formation, so it tends to bring fair, calm, and often clear weather. It does not guarantee sunshine: a high can also trap fog, haze, or cold air near the ground, especially in winter. On a map, a high is marked H, and its winds circulate outward from the center.

What does low pressure mean for weather?

Low pressure generally means rising air, which cools as it ascends, so water vapor condenses into clouds and often into precipitation. Lows therefore tend to bring cloud, wind, and rain or snow. On a map, a low is marked L, and its winds converge toward the center. The strongest lows, tropical cyclones, can have sea level pressures near or below 900 hectopascals, far below the usual 980 to 1,040 range.

Why do weather maps use sea level pressure instead of the pressure where I stand?

Because elevation would otherwise dominate the map. A mountain station always reads lower pressure than a nearby valley station, even under identical weather, simply because less air stands above the mountain. Converting every reading to its sea level equivalent removes elevation from the comparison, so the remaining differences reflect weather systems rather than terrain.

Can atmospheric pressure affect my body?

Yes, mainly through changes rather than through the steady value. Rapid pressure changes can make ears pop as the air in the middle ear equalizes. At high altitude, lower pressure means fewer oxygen molecules per breath, which is why climbers breathe harder and may need time to acclimatize.

Glossary

• Aneroid barometer: a barometer that measures pressure through the flexing of a sealed, evacuated metal capsule, without liquid.
• Atmosphere (unit): symbol atm, a pressure unit equal to standard sea level pressure, 101,325 pascals.
• Barograph: an instrument that records pressure continuously over time, showing its rise and fall as a curve.
• Barometer: any instrument that measures atmospheric pressure.
• Hectopascal: symbol hPa, a pressure unit equal to 100 pascals and to one millibar, used on weather maps.
• Isobar: a line on a weather map joining places of equal sea level pressure.
• Millibar: symbol mb, a pressure unit equal to one thousandth of a bar and to one hectopascal.
• Pascal: symbol Pa, the SI unit of pressure, equal to one newton per square meter.
• Sea level pressure: a station pressure reading adjusted to the value it would have at sea level, used for weather maps.
• Station pressure: the pressure actually measured at an instrument’s location, before sea level adjustment.
• Torr: a pressure unit equal to one millimeter of mercury, named for Torricelli.

Conclusion

Atmospheric pressure is not an exotic quantity reserved for meteorologists. It is the ordinary push of the air, made measurable by a single definition, force per unit area, and a single picture, the weight of the column of air overhead. That picture explains why pressure falls with height, why it must be corrected to sea level before it can be compared, and why a difference of only a few percent between neighboring columns can drive winds, build clouds, and organize the weather map into its familiar H and L patterns.

The practical lessons follow from the same picture. Read the trend of a barometer, not just its number. Compare sea level values, not raw station readings. Expect rising air and cloud where pressure is low, and sinking air and clearer skies where pressure is high, while remembering that pressure is one clue among several. And when a snack bag puffs on a mountain road or your ears pop on descent, recognize the same force at work: the air above you has changed, and everything balanced against it must adjust.

Sources & references

• National Oceanic and Atmospheric Administration, Jetstream, "Air Pressure" (noaa.gov/jetstream/atmosphere/air-pressure): molecules striking surfaces create pressure; pressure rises with more molecules or with heat; half of air molecules sit within the first 18,000 feet (5.6 km); station readings converted to sea level values; standard sea level pressure 29.92 inches of mercury and 1,013.2 millibars; common United States units inches of mercury and millibars; twice daily pressure cycle, lowest around 4 a.m. and 4 p.m., highest around 10 a.m. and 10 p.m.; Brewer 1848 barometer rules and their limits, including little day to day change in the tropics outside tropical cyclones.
• National Oceanic and Atmospheric Administration, Jetstream, "Measure the Pressure II: The Dry Barometer" (noaa.gov/jetstream/ll-pressure2): Leibniz conceived a sealed bellows design around 1700; Lucien Vidie built the first working aneroid barometer in 1843; aneroid instruments needed calibration against mercury barometers, including in National Weather Service offices into the 1990s; electronic sensors have replaced the metal aneroid cell in most barometers but still need calibration.
• National Oceanic and Atmospheric Administration, Jetstream, "Constant Pressure vs. Constant Elevation" (noaa.gov/jetstream/upper-air-charts/verses): surface charts use sea level as the common baseline; isobars are lines connecting areas of equal pressure.
• Pennsylvania State University, METEO 3: Weather Revealed, "Under Pressure" (courses.ems.psu.edu/meteo3/l6_p2.html): pressure as force per unit area; station pressure as approximately the weight of the air column to the top of the atmosphere; 14.7 pounds per square inch at sea level, more than 2 tons over a baseball base area; standard mercury column 29.92 inches; about 1,013 millibars at sea level; high and low systems as heavyweight and lightweight columns; ordinary high to strong low difference of about five percent, illustrated by 1,018 and 986 millibars; nearly all sea level pressures between 950 and 1,050 millibars, most between 980 and 1,040; strong winter high around 1,050, formidable low 960 to 980; intense hurricanes near 900 millibars, Hurricane Maria 908 millibars at peak in 2017.
• National Aeronautics and Space Administration, Glenn Research Center, "Earth Atmosphere Equation" (www1.grc.nasa.gov/beginners-guide-to-aeronautics/earth-atmosphere-equation-english/): pressure related to the weight of air over a location; less air above at higher altitude, so pressure decreases with altitude; density also decreases with altitude.
• Chemistry LibreTexts, "Pressure" (chem.libretexts.org, Fullerton College introductory chemistry): pressure as force of gas particle and wall collisions divided by area; atmosphere held by gravity rather than a container; 14.7 pounds per square inch at sea level; pascal as one newton per square meter; 1 atm defined as exactly 760 millimeters of mercury, or 760 torr; torr named for Torricelli, inventor of the mercury barometer in the seventeenth century.
• Chemistry LibreTexts, "Atmospheric Pressure" (chem.libretexts.org, introductory chemistry): barometer as the instrument measuring atmospheric pressure; mercury barometer as an evacuated tube in mercury, with air pressure pushing mercury up the tube; aneroid barometer as an evacuated metal capsule whose expansion and contraction is measured; pressure differences between areas driving winds, with greater differences producing stronger winds.