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What is barometric pressure?
Barometric pressure is the pressure exerted by the atmosphere — in effect, the weight of the column of air above you — and it is exactly the same quantity as atmospheric pressure or air pressure. At sea level it averages 29.92 inHg (1013.25 hPa), and it rises and falls as weather systems move through.
One quantity, three names
“Barometric pressure”, “atmospheric pressure” and “air pressure” are synonyms. Atmospheric pressure is the pressure within Earth’s atmosphere; “barometric” simply names it after the instrument that measures it, the barometer. If a weather app says one and your watch says another, they are describing the same thing — though, as below, not always measured at the same height.
The reference value is the standard atmosphere: 29.92 inHg (1013.25 hPa), the average pressure at mean sea level in the International Standard Atmosphere. It is a yardstick, not a target. Real sea-level pressure wanders above and below it every day, and most of that wandering is ordinary.
Where it comes from is gravity. The planet pulls on the gases of the atmosphere, and the air above any point presses down on it. How much depends on the mass of the planet, the radius of the surface, and the amount and composition of the gases — then rotation, wind, and temperature-driven differences in air density modify it locally. That last group is what makes the number move from day to day.
Why pressure goes up and down
The amount of air over you is constantly changing, so its weight — the pressure — is constantly changing too. NOAA’s JetStream puts it plainly: these changes in air pressure are indications of changes in the weather.
The pattern is qualitative, and it is the same one behind the “typical association” column of this site’s pressure zones. Falling pressure is associated with an approaching low-pressure system or front: cloud, wind, unsettled weather. Rising pressure is associated with a high building in behind it: drier, more settled, often clearer conditions. Temperature plays its part too: it changes the density of the air, so the character of the air mass overhead shifts the reading as well.
What this does not give you is a timetable. The direction and the size of the change are the useful parts.
How large do those changes get? In this site’s ten-year dataset of 50 US cities, the steepest single 24-hour fall belongs to Boston: 1.52 inHg (51.5 hPa). Boston sits at 19 m of elevation, so that is weather, not an altitude artifact. At the other end, every city’s long-run average lands inside the normal band — see below.
The units, in one table
Pressure is quoted in at least six units. This site shows inHg first and hPa beside it, and converts with one fixed factor: 1 hPa = 0.02953 inHg. Hectopascals and millibars are the same unit under two names.
| Unit | Symbol | Standard atmosphere | 1 hPa equals |
|---|---|---|---|
| Inches of mercury | inHg | 29.92 | 0.02953 |
| Hectopascals | hPa | 1013.25 | 1 |
| Millibars | mb | 1013.25 | 1 |
| Millimeters of mercury (≈ torr) | mmHg | 760 | 0.750 |
| Kilopascals | kPa | 101.325 | 0.1 |
| Pounds per square inch | psi | 14.696 | 0.0145 |
Standard-atmosphere values per the International Standard Atmosphere (source). Per-hPa figures for mmHg and psi are derived from those values and rounded.
Two things worth carrying away. First, a change that looks tiny in inHg is not tiny: 0.30 inHg is 10 hPa, which is a large day-to-day move. Second, when you compare two readings, make sure they are in the same unit and the same kind of pressure — which is the next section.
Sea-level pressure vs. station pressure
Pressure falls as you climb, because there is less air above you. Near sea level the rate is about 0.35 inHg (12 hPa) for every 100 meters. A few hundred meters of elevation therefore moves the raw reading by more than most weather systems do.
That would make cities impossible to compare, so weather reports quote mean sea-level pressure: the station’s reading reduced to what it would be at sea level. This site does the same — every comparative figure is sea-level-reduced, as the methodology explains.
A barometer showing raw station pressure does no such reduction, and at altitude the gap is large. Denver sits at 1,615 m and Colorado Springs at 1,832 m; a raw reading there sits far below the sea-level number in the forecast, by much more than any storm moves it. Each city page lists its mean station pressure beside the sea-level figures so you can tell which one your device is showing. The normal barometric pressure article walks through this altitude trap in more detail.
The reduction has a cost of its own. Reducing a high-altitude reading to sea level amplifies temperature-driven variation, which is part of why Denver and Colorado Springs record the most swing days in the set — 281.6 and 279.6 a year, against a national median of 180.7.
How barometric pressure is measured
Aneroid barometers
An aneroid (“dry”) barometer is built around a sealed metal cell whose volume is very sensitive to air pressure: it expands and contracts as the pressure changes, and a lever attached to the cell moves the needle. NOAA credits the idea of a dry barometer to Gottfried Wilhelm Leibniz, around 1700.
Phone and watch sensors
Phones and smartwatches use tiny MEMS pressure sensors. These read the raw pressure where the device is, and they are not sea-level-corrected the way an official report is. That is the usual answer to “why does my watch disagree with the weather app?”: one is station pressure, the other sea-level pressure. Unless your device lets you apply a sea-level or altimeter correction, its number and the forecast’s will not match — and the gap grows with elevation.
Reanalysis vs. station data
The historical numbers on this site are not from any single barometer. They come from Open-Meteo’s ERA5 hourly reanalysis, 2015–2024 — 87,672 hourly sea-level values per city. Reanalysis is a physically consistent model reconstruction constrained by observations. It makes cities directly comparable, at the cost of smoothing the sharpest, very local excursions a station’s barograph would catch.
What a typical reading looks like
Averaged over ten years, sea-level pressure in the 50 cities spans a narrow range: from 29.89 inHg (1012.3 hPa) in Phoenix to 30.08 inHg (1018.5 hPa) in Atlanta. Denver averages 29.97 inHg (1014.8 hPa); Colorado Springs, 29.97 inHg (1015.0 hPa). Every one of them sits inside the site’s “normal” band of 29.68–30.12 inHg (1005–1020 hPa).
So the average tells you very little about a place. Two cities with near-identical means can live very different barometric lives: Denver and Colorado Springs swing past the threshold on most days of the year — compare them with the Miami page. What separates places is how often and how sharply the pressure moves, not where it settles.
For scale beyond these cities: the highest sea-level pressures on Earth occur in Siberia, with records close to 32.04 inHg (1085 hPa), and the lowest ever measured at sea level was 25.69 inHg (870 hPa), during Typhoon Tip on October 12, 1979 (source). Those are global records, not figures from this site’s dataset.
What it means for you
If you want to read a single number, the pressure zones are the quickest way: five fixed sea-level bands, from very low to very high, the same in every city. For a specific reading, the 29.95 inHg article shows where values from 29.50 to 30.50 inHg fall.
The outlook calendar on each city page bins the 24-hour pressure change, not the level, as the methodology explains. Pressure change has been statistically associated with migraine onset in some people and not in others, and the calendar carries 30 days of history, so you can line up your own bad days against what the barometer actually did. Start with your city on the city list, or check any US location if yours is not one of the 50.
Pressure change has been statistically associated with migraine onset in some people and not in others; effect sizes are modest and individual sensitivity varies widely. Nothing on this page predicts a headache, diagnoses anything, or is medical advice. If migraine is affecting your life, that is a conversation for a clinician.