What Is Atmospheric Pressure?
Atmospheric pressure (also called barometric pressure or air pressure) is the weight of the air above a given point, pressing down on everything beneath it. The entire atmosphere, stretching roughly 100 km (62 mi) upward, has mass, and gravity pulls that mass toward the Earth’s surface. The result is a force distributed over every square centimetre of ground, ocean, and everything in between.
At sea level, the atmosphere exerts a force equivalent to about 10,330 kg per square metre (2,116 lb per square foot). You don’t feel it because your body pushes back with equal force, but it’s always there, and its variations drive nearly all weather.
Units of Measurement
Pressure is reported in several units depending on the context:
| Unit | Abbreviation | Used by |
|---|---|---|
| Hectopascals | hPa | Meteorological services worldwide |
| Millibars | mbar | Equivalent to hPa; older term still in common use |
| Inches of mercury | inHg | United States weather reports |
| Millimetres of mercury | mmHg | Some European countries, medical contexts |
| Atmospheres | atm | Scientific reference (1 atm = 1013.25 hPa) |
Hectopascals and millibars are numerically identical: 1 hPa = 1 mbar. Most weather services now use hPa, but you’ll still see mbar on older instruments and in casual use. In the US, pressure is typically given in inches of mercury (inHg), where the standard atmosphere is 29.92 inHg.
The standard atmosphere is defined as 1013.25 hPa (29.92 inHg) at sea level, at a temperature of 15°C (59°F). It serves as a baseline. Actual pressure at any given time and place will vary above or below this value.
How Pressure Varies with Altitude
The higher you go, the less atmosphere sits above you, so pressure drops. Near sea level, pressure decreases by roughly 1 hPa for every 8 metres (26 ft) of elevation gained. The relationship isn’t perfectly linear (air is compressible, so it’s denser near the surface), but the rule of thumb works well for moderate altitudes.
Some rough benchmarks:
| Altitude | Typical Pressure |
|---|---|
| Sea level | 1013 hPa (29.92 inHg) |
| 500 m (1,640 ft) | ~955 hPa (28.20 inHg) |
| 1,000 m (3,280 ft) | ~899 hPa (26.55 inHg) |
| 1,500 m (4,920 ft) | ~845 hPa (24.96 inHg) |
| 2,000 m (6,560 ft) | ~795 hPa (23.48 inHg) |
| 5,500 m (18,040 ft) | ~505 hPa (14.92 inHg) |
Because of this altitude effect, weather reports use mean sea level pressure (MSLP): the station’s measured pressure adjusted to what it would be at sea level. This allows meaningful comparison between locations at different elevations. Without this correction, a city at 1,500 m altitude would always appear to have much lower pressure than a coastal town, regardless of actual weather conditions.
High Pressure Systems
A high pressure system (also called an anticyclone) is a region where air is sinking from higher altitudes toward the surface. As the air descends, it compresses and warms, which discourages cloud formation. The result is typically:
- Clear or mostly clear skies
- Light winds
- Stable conditions that persist for days
- Warmer temperatures in summer, colder temperatures in winter (clear skies allow more solar heating by day and more radiative cooling at night)
In the Northern Hemisphere, winds around a high pressure system circulate clockwise; in the Southern Hemisphere, anticlockwise. This is a consequence of the Coriolis effect, the deflection caused by the Earth’s rotation.
High pressure readings generally fall above 1020 hPa (30.12 inHg), though the exact value depends on location and season.
Low Pressure Systems
A low pressure system (also called a cyclone or depression) is a region where air rises from the surface. As the air ascends, it cools, and the moisture it carries condenses into clouds and often precipitation. Low pressure systems bring:
- Increased cloud cover
- Rain, snow, or other precipitation
- Stronger winds (air rushes in to replace the rising air)
- Generally unsettled, changeable weather
Winds around a low pressure system circulate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Low pressure readings generally fall below 1000 hPa (29.53 inHg), though moderate lows might sit anywhere from 990–1010 hPa. Deep lows associated with powerful storms can drop to 960 hPa (28.35 inHg) or lower.
Pressure Ranges and Typical Weather
The table below gives a rough guide to what different sea level pressure readings tend to mean:
| Pressure (hPa) | Pressure (inHg) | Typical Conditions |
|---|---|---|
| 1040+ | 30.71+ | Very high; settled, clear, often cold in winter |
| 1020–1040 | 30.12–30.71 | High; generally fair weather |
| 1010–1020 | 29.83–30.12 | Average; conditions vary |
| 1000–1010 | 29.53–29.83 | Slightly low; increasing chance of cloud and rain |
| 980–1000 | 28.94–29.53 | Low; unsettled, windy, rain or snow likely |
| 960–980 | 28.35–28.94 | Very low; strong storms, gales |
| Below 960 | Below 28.35 | Extremely low; severe storms, hurricanes, typhoons |
These are broad generalisations. Context matters: a reading of 1005 hPa might bring perfectly fine weather in the tropics (where average pressure is lower) but suggest an approaching system in northern Europe.
Weather Fronts and Pressure
Fronts are the boundaries between air masses of different temperatures and humidity levels. They are closely tied to pressure systems, typically forming along the edges of low pressure areas.
A warm front occurs when a warm air mass advances and slides up over cooler air ahead of it. The warm air rises gradually, producing a broad area of cloud and steady, prolonged rain or drizzle. Pressure falls steadily as a warm front approaches.
A cold front occurs when a cold air mass pushes under warmer air, forcing it up sharply. Cold fronts tend to produce a narrower band of more intense weather: heavy showers, thunderstorms, and sometimes gusty winds. Pressure drops as the front approaches, then rises sharply once it passes.
An occluded front forms when a cold front catches up with a warm front, lifting the warm air entirely off the surface. These produce mixed conditions and are common in mature low pressure systems.
Understanding fronts helps explain why pressure changes are as informative as the pressure itself.
Reading a Barometer
A barometer is an instrument that measures atmospheric pressure. Traditional mercury barometers use a column of mercury in a glass tube; modern ones are digital or use aneroid (mechanical) mechanisms. Regardless of the type, the principle for reading weather trends is the same:
- Rising pressure generally means improving conditions: skies clearing, winds calming, drier weather ahead.
- Falling pressure generally means deteriorating conditions: increasing cloud, wind, and a higher chance of rain or storms.
- Steady pressure means conditions are unlikely to change much in the near term.
- Rapidly falling pressure is a strong signal of an approaching storm. A drop of more than 5–6 hPa in three hours is considered a rapid fall and warrants attention.
The rate of change matters as much as the absolute value. A slow, steady rise from 1005 to 1015 hPa over a day suggests a gradual improvement. The same 10 hPa change in a few hours would signal a dramatic shift.
Barometers on older instruments often include markings like “Fair,” “Change,” and “Stormy.” These are rough guides based on the principle above, but they don’t account for your specific location, altitude, or the broader weather pattern.
Pressure in Numerical Weather Prediction
Numerical weather prediction (NWP) models treat pressure as one of their core variables. Pressure fields define the large-scale structure of the atmosphere: where highs and lows sit, how strong they are, and how air flows between them.
Models calculate pressure at multiple levels of the atmosphere, not just the surface. Upper-level pressure patterns (particularly at 500 hPa, roughly 5,500 m / 18,000 ft altitude) are critical for forecasting because they steer surface weather systems. A deep trough in the 500 hPa pattern typically brings unsettled weather at the surface below it; a ridge brings settled conditions.
Surface pressure observations are among the most important inputs for initialising models. They are relatively easy to measure accurately and provide a dense, reliable network of data. When models assimilate new observations every few hours, surface pressure measurements help anchor the entire atmospheric state.
Ensemble forecasting, where models are run many times with slightly varied starting conditions, often reveals uncertainty through the spread of predicted pressure fields. When ensemble members agree on the position and intensity of a low pressure system, forecasters have high confidence. When they disagree, the forecast is less certain.
Pressure and Health
Changes in atmospheric pressure can affect the human body, though the mechanisms are not always fully understood.
Headaches and migraines. Some people report that falling pressure triggers headaches or migraines. Research has found associations between pressure drops and migraine onset, though individual sensitivity varies widely. The prevailing theory is that changes in external pressure affect sinus cavities and blood vessels in the head.
Joint and muscle pain. Many people with arthritis or old injuries notice increased pain when pressure drops. Studies have produced mixed results, some supporting the association and others finding no clear link. One hypothesis is that lower external pressure allows soft tissues to expand slightly, putting more pressure on nerves in and around joints. Regardless of the precise mechanism, the anecdotal pattern is widespread enough that it’s worth noting.
Altitude-related effects. At high altitudes, the reduced pressure means less oxygen per breath. Above roughly 2,500 m (8,200 ft), some people develop acute mountain sickness (AMS), with symptoms including headache, nausea, and fatigue. This is a direct consequence of lower atmospheric pressure reducing the partial pressure of oxygen.
Ear discomfort. Rapid pressure changes, such as during flights or when driving through mountains, can cause discomfort or pain in the ears as the pressure inside the middle ear takes time to equalise with the surrounding atmosphere.
Diurnal Pressure Variations
Even on a calm, settled day, atmospheric pressure follows a subtle daily cycle called the atmospheric tide. Pressure peaks at around 10:00 and 22:00 local time, and dips at around 04:00 and 16:00. The variation is small, typically 1–2 hPa in the tropics and even less at higher latitudes, but it’s consistent enough that meteorologists account for it when analysing pressure trends.
This cycle is driven primarily by solar heating of the atmosphere, which causes a thermal expansion wave that propagates around the globe.
Tips for Using Pressure Information
- Watch the trend, not just the number. A single pressure reading tells you less than how pressure has changed over the past few hours. A steady drop is more informative than the absolute value.
- Combine with other data. Pressure alone doesn’t tell the whole story. Pair it with cloud cover, wind direction, and precipitation forecasts for a more complete picture.
- Know your local baseline. Average pressure varies by location and season. What counts as “high” or “low” in your area may differ from the textbook numbers.
- Rapid drops deserve attention. If pressure falls more than 5–6 hPa in three hours, expect a significant change in weather. Secure loose outdoor items and check the forecast.
- Consider altitude. If you live at elevation, your station pressure will always be lower than sea level values. Weather apps and forecasts typically show MSLP, but a home barometer will display station pressure unless it’s been calibrated.
How Airpult Shows Atmospheric Pressure
On Airpult, atmospheric pressure is shown on the forecast page as a sea level-adjusted value in hPa. You can see both the current reading and the trend, making it easy to spot whether conditions are improving or deteriorating. Use the explore page to search for any location and check its pressure alongside temperature, wind, and other conditions.
