Air pressure is the force exerted by air molecules pushing against everything around them. At sea level, the atmosphere presses down with a force of about 14.7 pounds per square inch, roughly 101,325 pascals, on every surface it touches, including your body. That might sound like a lot, and it is. The reason you don’t feel crushed is that the pressure inside your body pushes outward with equal force.
How Air Molecules Create Pressure
Air is made of tiny gas molecules (mostly nitrogen and oxygen) that are constantly moving in random directions at high speeds. As these molecules slam into surfaces, they transfer a small push of energy with each collision. Multiply that by the trillions of collisions happening every fraction of a second, and the combined force becomes substantial. NASA’s Glenn Research Center describes this as the linear momentum of the molecules: force divided by the area it acts on equals pressure.
Gravity plays a crucial role, too. Earth’s gravity pulls the atmosphere downward, stacking layers of air on top of each other. The air at ground level is compressed under the weight of all the air above it, which is why pressure is highest at sea level and drops as you go higher.
Standard Pressure at Sea Level
Scientists use a standard reference value for air pressure at sea level so that measurements around the world can be compared. That value is expressed in several different units depending on the field:
- 101,325 pascals (Pa), the standard SI unit
- 1,013.25 millibars (mb), commonly used in meteorology
- 29.92 inches of mercury (inHg), used in aviation and older weather reports
- 14.696 pounds per square inch (psi), familiar in everyday American engineering
- 1 atmosphere (atm), a convenient shorthand in chemistry
These all describe the same amount of pressure, just on different scales. Weather forecasters typically report in millibars or inches of mercury, while physics and chemistry classes lean on pascals or atmospheres.
Why Pressure Drops With Altitude
The higher you climb, the less air sits above you, so the pressure decreases. This relationship is exponential rather than linear, meaning pressure drops quickly at first and then more gradually. At roughly 18,000 feet (about 5,600 meters), pressure falls to about 500 millibars, half the sea-level value. By the time you reach the cruising altitude of a commercial jet, around 35,000 feet, pressure is only about a quarter of what it is on the ground.
Temperature complicates things. Warm air is less dense because its molecules move faster and spread farther apart. That means a column of warm air extends higher into the atmosphere before reaching a given pressure level, while a column of cold air is more compressed. NOAA notes that the actual elevation of the 500-millibar level will be higher in warm air than in cold air, which is why weather maps showing pressure at altitude reveal so much about temperature patterns below.
How Temperature Changes Air Pressure
Heating air transfers energy to its molecules, making them move faster and collide with surrounding surfaces more forcefully. In a sealed container, this directly raises the pressure. In the open atmosphere, the effect is more nuanced: warming a parcel of air causes it to expand and become less dense, which can lower the surface pressure beneath it as air rises and spreads out.
Cooling has the opposite effect. Cold air molecules move more slowly and pack more tightly together, increasing density near the surface. This is part of why cold, dry winter days often bring high barometer readings, while warm, humid conditions tend to lower them. Although these shifts happen too slowly to notice in real time, air pressure is almost always changing.
Air Pressure and Weather
Differences in air pressure from one place to another are the primary engine behind wind. The sun heats Earth’s surface unevenly (land warms faster than water, the equator receives more energy than the poles), creating regions of higher and lower pressure. Air naturally flows from high-pressure areas toward low-pressure areas, and the greater the difference over a given distance, the stronger the wind. Meteorologists call this difference the pressure gradient force.
High-pressure systems involve sinking air, which suppresses cloud formation and generally brings clear, calm weather. Low-pressure systems pull air inward and upward, causing it to cool and condense into clouds and precipitation. That’s why a falling barometer often signals approaching storms, while a rising barometer suggests fair skies ahead. On weather maps, the tightly packed lines (isobars) around a low-pressure center indicate steep pressure gradients and strong winds, like those in hurricanes or nor’easters.
How Scientists Measure Air Pressure
The earliest barometer, invented by Evangelista Torricelli in 1643, used a tube of mercury. Atmospheric pressure pushes down on a dish of mercury, forcing the liquid up into an inverted glass tube. The height of the mercury column, typically about 760 millimeters at sea level, gives a direct reading of pressure. This is where the unit “inches of mercury” comes from.
Most modern barometers are aneroid, meaning they contain no liquid. Instead, a small sealed metal capsule with most of the air removed flexes inward or outward as atmospheric pressure changes. That flexing is mechanically linked to a needle on a dial. A barograph takes this a step further, recording pressure changes continuously on a rotating chart, which is useful for tracking weather trends over hours or days.
One of the most dramatic demonstrations of air pressure’s force came in 1654, when Otto von Guericke, the mayor of Magdeburg, Germany, placed two hollow bronze hemispheres together and pumped the air out from between them. The atmospheric pressure holding the hemispheres together was so great that two teams of horses pulling in opposite directions could not separate them. When air was allowed back in, the hemispheres fell apart easily.
How Your Body Responds to Pressure Changes
You’ve likely felt air pressure change firsthand: that uncomfortable fullness in your ears during a flight’s takeoff or descent, or while driving through mountains. This happens because the air pressure in your middle ear and the pressure outside your eardrum are temporarily out of balance. Normally, a small channel called the eustachian tube connects your middle ear to the back of your throat and lets air flow in or out to equalize pressure automatically.
Swallowing, yawning, or chewing gum opens that tube, which is why those simple actions relieve the stuffiness. If the tube is blocked (from a cold, allergies, or sinus congestion), the pressure difference can cause ear pain, muffled hearing, dizziness, or in severe cases, nosebleeds. This condition, called ear barotrauma, is more likely during rapid altitude changes like scuba diving or flying with congestion.
At very high altitudes, low air pressure means each breath contains fewer oxygen molecules. Above about 8,000 feet, some people begin to experience altitude sickness: headaches, nausea, and fatigue as the body struggles to get enough oxygen. This is why airplane cabins are pressurized to simulate an altitude of roughly 6,000 to 8,000 feet rather than the 35,000 feet the plane is actually flying at.