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Earth & Space Science · Foundations

Air Pressure

9 min read 5 practice questions
10 key termsSourcesReport an error
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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Quick check
  8. Study tools
  9. Sources & references

In 30 seconds

comes from the weight of the air piled above you, pressing down on everything. A measures it, and at sea level the standard reading is about 1013.25 hectopascals, or 29.92 . Pressure falls as you climb — roughly half of the atmosphere lies within the first 5.6 kilometers. Differences in pressure set air moving, which is wind, and rising or sinking air inside high- and low-pressure systems shapes the fair or cloudy weather they generally bring.

Why this matters

Air pressure is the quiet driver behind most everyday weather. It is what a barometer reports in every forecast, what the H's and L's on weather maps mark, and what turns a calm morning into a windy afternoon: wind is air moving from higher toward lower pressure. Pressure also tells you whether the air overhead is likely to rise and form clouds or sink and clear the sky, which is why a falling barometer has warned of incoming storms for nearly two centuries. Because pressure is measurable and its patterns are visible on maps, it gives you a concrete way to watch the atmosphere at work and to understand the forecasts built on it.

The college version

The weight of the air above you

Atmospheric pressure is the force the atmosphere exerts on everything it touches. The air around you is made of countless molecules moving in random directions, and every time one strikes a surface it pushes on it; add up all those tiny collisions and you get pressure. At the surface, what we notice is mostly the weight of the air column above us. Gravity holds the atmosphere down, and the higher you go, the less air remains above you, so ground-level pressure is mainly dependent on how much atmosphere sits over that location. The scientific unit of pressure is the pascal, named for Blaise Pascal, but one pascal is tiny, so meteorology uses the , which equals 100 pascals. The millibar (mb) is numerically identical to the hectopascal, and the word bar comes from the Greek báros, meaning weight. In the United States pressure is also commonly reported in inches of mercury, the height of a mercury column the pressure can support.

The barometer and standard pressure

A barometer measures air pressure. In the classic mercury barometer, the atmosphere's push balances a column of mercury: at sea level, standard air pressure supports a column about 29.92 inches (760 mm) high, which is where the inches-of-mercury reading on weather broadcasts comes from. In metric units the same standard sea-level pressure is 1013.25 hectopascals, equal to 1013.25 millibars. Because pressure falls with height, raw readings from stations at different elevations cannot be compared directly, so every station converts its reading to the value it would show at sea level. That common denominator is what appears as the H's, L's, and isobars on weather maps. Pressure also responds to temperature and moisture: warm air is less dense than cool air because its molecules move faster and spread farther apart, and moist air is lighter than dry air because water molecules are lighter than the nitrogen and oxygen molecules they replace.

Pressure falls as you climb

As elevation increases, fewer air molecules remain above you, density drops, and pressure decreases. The decrease is steepest near the ground and gentler higher up, because air is compressible. A useful benchmark from NOAA: the atmosphere reaches hundreds of miles upward, yet about half of all its molecules are contained within the first 18,000 feet (about 5.6 kilometers) above sea level. That is why the 500-millibar pressure level, roughly half of the surface value near 1000 millibars, sits at an average altitude of about 18,000 feet. It is also why climbers on high mountains feel thin air: less atmosphere presses down, and each breath carries fewer oxygen molecules.

Pressure differences drive the wind

Wind is air in motion, and its engine is the . Where pressure changes quickly over a short distance the gradient is steep; where it changes slowly the gradient is gentle. The tries to equalize the difference, pushing air from higher toward lower pressure. If it were the only force acting, air would flow straight from high to low. Earth's rotation adds the Coriolis force, named for Gaspard-Gustave Coriolis, the French scientist who described it mathematically in 1835. The deflects anything moving long distances: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Because of it, winds curve instead of traveling straight, flowing clockwise around high-pressure centers and counter-clockwise around low-pressure centers in the Northern Hemisphere. Friction with the rough surface adds a final influence, slowing the air and letting it spiral outward from highs and inward toward lows.

Isobars, highs, and lows

Weather maps draw the geometry of pressure with isobars, lines of equal pressure: iso means equal, and bar is a unit of pressure. Around a high, each concentric closer to the center carries a higher value; around a low, each line closer to the center is lower. Where isobars are packed tightly, pressure changes quickly, the gradient is steep, and winds tend to be strong; where they are far apart, winds tend to be light. The blue H's and red L's locate the centers of the systems. Vertical air motion links pressure to the weather: rising air cools as it climbs, often reaching the point where water vapor condenses into clouds and precipitation, while sinking air warms and dries, clearing the sky. So, as a general tendency, high-pressure systems are associated with fair, dry weather and low-pressure systems with cloudy, rainy weather. These are typical associations rather than promises — a high's edges can still host clouds, and a low can have a briefly clear center. The same logic builds the planet's broad pressure belts: warm air rising near the equator produces a persistent band of low pressure, and cold air sinking at the poles creates the polar highs.

Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Air pressure is how hard the air above you pushes down. The atmosphere is a deep ocean of air, and you live at the bottom of it, carrying the weight of everything above. A barometer measures that push, and at sea level the standard reading is 1013.25 hectopascals, or about 29.92 inches of mercury. Climb a mountain and the pressure drops, because less air is above you — about half of the whole atmosphere lies within the first 5.6 kilometers. The push is not the same everywhere. Where the air presses harder we draw an H for high pressure; where it presses less, an L for low. Air flows from high to low, and that flow is wind. Because Earth spins, the moving air curves instead of traveling straight. On weather maps, lines called isobars connect places with equal pressure: packed-together lines mean pressure changes fast and winds blow strong. Rising air makes clouds; sinking air clears them. That is why lows often bring rain and highs bring fair weather.

Picture it like this

Picture the atmosphere as a tall stack of blankets on a bed. The bottom blanket carries the weight of every blanket above it, so it feels the most pressure. You live near the bottom of the stack, under the whole pile, and that is why pressure at sea level is high. Climb high and you are near the top of the stack, with only a few blankets above you and less pressure. Where the blanket pile is uneven, air slides off the high lumps toward the hollows, and that sliding is the wind.

Where the picture stops working

A blanket stack sits still, but air is a compressible fluid that heats, cools, gains and loses moisture, and moves in all three dimensions. It presses on things from every direction, not just downward, and its flow carries the clouds and storms the blankets cannot show. The stack captures the weight; the weather needs the motion.

Worked example

On a Tuesday morning, a barometer at a coastal station at sea level reads 1013 hPa, right at the standard value. The same hour's weather map shows isobars packed tightly across the Midwest, where pressure falls from 1016 to 1004 hPa over a short distance, while isobars far to the west sit far apart. The tight packing marks a steep pressure gradient, and because wind speed grows with the pressure gradient, forecasters expect strong winds over the Midwest and light winds out west. By Thursday, a low-pressure center approaches: the barometer falls, air converges and rises near the center, clouds thicken, and rain begins — the general signature of a passing low. Meanwhile, a passenger flying at about 5.6 km (18,000 ft) would read roughly half the sea-level pressure, since about half of the atmosphere lies below that height.

Key takeaway

Air pressure is the weight of the atmosphere above a point; measured with a barometer, it falls with altitude, and differences in pressure drive the winds, while rising or sinking air in high- and low-pressure systems shapes the fair or stormy weather they generally bring.

Quick check

3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.

Question 1 of 3foundational

What is atmospheric pressure?

Choose an answer, then check it.
Question 2 of 3foundational

According to NOAA, what is the standard sea-level air pressure?

Choose an answer, then check it.
Question 3 of 3intermediate

A weather map shows isobars packed very close together across a region. What does the pattern most directly indicate?

Choose an answer, then check it.
Practice all 5

Keep learning

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Practice this lesson
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related

You’ll learn to

  • Define air pressure and explain how the weight of the air above a point creates it.
  • State the standard sea-level pressure in hectopascals and in inches of mercury, and describe how a barometer measures pressure.
  • Explain how air pressure decreases with altitude, including the rule that about half of the atmosphere lies within the first 18,000 feet (5.6 km).
  • Explain how the pressure gradient drives wind and how the Coriolis effect deflects moving air.
  • Distinguish high- and low-pressure systems by their circulation and their general weather associations.
  • Interpret isobars on a weather map to judge where pressure changes quickly and winds are likely strong.

Common mistakes

  • Treating the high/low weather association as a guarantee.

    High pressure usually means fair, dry weather and low pressure usually means cloudy, rainy weather, but these are general tendencies, not promises. Local conditions such as terrain and moisture can produce different weather under either system.

  • Expecting wind to blow straight from high to low pressure.

    The pressure gradient force starts air moving from high to low, but Earth's rotation (the Coriolis effect) turns the flow, so winds spiral: clockwise around highs and counter-clockwise around lows in the Northern Hemisphere.

  • Comparing raw barometer readings from places at different elevations.

    Pressure falls with altitude, so a mountain town naturally reads lower than a coastal city. Stations convert readings to sea-level values precisely so that maps and forecasts can be compared fairly.

  • Reading isobars as temperature or cloud lines.

    Isobars connect points of equal air pressure only. Temperature and cloud cover are shown with other symbols, and fronts have their own colored lines.

  • Assuming warmer air always means higher pressure.

    Warm air is less dense than cool air, so warming a column of air can lower its pressure. What matters is the total weight of air above the point, which depends on temperature, moisture, and altitude together.

Easily confused

High-pressure system vs. Low-pressure system

Air generally sinks in a high and rises in a low; highs are usually associated with fair, dry weather, lows with cloudy, rainy weather.

Hectopascals vs. Inches of mercury

Two units for the same quantity: standard sea-level pressure is 1013.25 hPa and about 29.92 inHg; broadcasts often give the inches reading while maps use hPa or millibars.

Pressure gradient force vs. Coriolis effect

The pressure gradient force pushes air from high to low pressure and gives wind its strength; the Coriolis effect turns the moving air (right in the Northern Hemisphere), giving wind its curved direction.

Isobars vs. Fronts

Isobars connect points of equal pressure and show the pressure pattern; fronts mark boundaries between air masses with their own symbols, a topic of its own lesson.

Key vocabulary

Air pressure
The force the atmosphere exerts on a surface, produced by the weight of the air above that surface.
Barometer
An instrument that measures atmospheric pressure, from classic mercury columns to modern digital gauges.
Hectopascal (hPa)
A unit of pressure equal to 100 pascals; standard sea-level pressure is 1013.25 hPa, the same value as 1013.25 millibars.
Inches of mercury
A unit of pressure equal to the height of a column of mercury the atmosphere can support; standard sea-level pressure is about 29.92 inches.
Isobar
A line on a weather map connecting points with equal atmospheric pressure.
Pressure gradient
The rate at which pressure changes over a given distance; a steep gradient means pressure changes quickly.
Pressure gradient force
The force that pushes air from regions of higher pressure toward regions of lower pressure.
Coriolis effect
The apparent deflection of long-distance moving objects, including air, caused by Earth's rotation: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
High-pressure system
A region where pressure is higher than its surroundings, where air generally sinks and fair, dry weather is common.
Low-pressure system
A region where pressure is lower than its surroundings, where air generally rises and clouds and precipitation are common.

Sources & references

  1. JetStream: Air Pressure — NOAA National Weather Service JetStream (Online School for Weather)
  2. JetStream: Origin of Wind — NOAA National Weather Service JetStream (Online School for Weather)
  3. Global Atmospheric Circulations — NOAA National Weather Service, JetStream Online School for Weather
  4. SciJinks: How to Read a Weather Map — NOAA / NESDIS SciJinks (It's All About Weather)
  5. SciJinks: What Is the Coriolis Effect? — NOAA / NESDIS SciJinks (It's All About Weather)

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Researched 2026-08-21

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