What does barometric pressure mean?

The weight of the column of air above a point, measured by a barometer — Torricelli's 1643 mercury-tube instrument gave it the name. The sea-level standard is 1013.25 hPa, or 29.92 inches of mercury.

Are inHg and mb the same thing?

They measure the same pressure in different units. Millibars (identical to hectopascals — 1 mb = 1 hPa) are the metric scale; inches of mercury are the US weather-report scale. One inHg equals 33.8639 hPa, so 29.92 inHg and 1013.25 hPa describe the same air.

Why does my barometer show two numbers?

The dial carries two unit scales for one measurement — typically inHg on the outer ring and hPa/mb on the inner. The pointer crosses both at once; read whichever scale your forecast source uses.

What does the set needle on a barometer do?

It's a manual bookmark. Align it with the live pointer, and the gap that opens over the next hours is your pressure trend — the 3-hour change forecasters actually watch, visible at a glance.

How accurate is a phone barometer?

The chip is good at relative change — small enough shifts to sense a stairwell climb — but it reads raw station pressure at the phone's location, uncorrected to sea level. Compare trends on it; take absolute values from a corrected source like an airport observation.

How do I read barometric pressure at altitude?

Use sea-level-corrected values, which is what published readings already are. True station pressure drops about 1 inHg per 1,000 ft, so Denver's instrument-level reading sits near 24.6 inHg — the correction is what makes readings comparable between cities at different elevations.

What is a normal barometric pressure reading?

The sea-level standard is 1013.25 hPa (29.92 inHg), and everyday corrected readings move within a band around it. The trend matters more than the level: a fall of 0.06 inHg in 3 hours is coded "falling rapidly" by the NWS and usually signals an approaching system.

How to Read Barometric Pressure

Barometric pressure is the weight of the atmosphere pressing down on where you stand, and reading it well takes exactly two skills: knowing what the units mean, and watching which way the number is moving. The value itself — 29.92 inHg, 1013.25 hPa — matters less than most people expect.

This guide covers the number from the ground up: what a barometer actually measures, how the five common units convert, how to read an aneroid dial (including what the second needle is for), why your reading is corrected to sea level, and why forecasters watch the 3-hour trend instead of the raw value.

One atmosphere, five numbers

Every bar is the same physical pressure — the sea-level standard — expressed in a different unit. Knowing which scale a chart uses is the first reading skill.

Hectopascals (= millibars)1013.25 hPa

Millimeters of mercury760 mmHg

Inches of mercury29.92 inHg

Pounds per square inch14.696 psi

Atmospheres1 atm

The sea-level standard atmosphere in each unit: 1013.25 hPa = 760 mmHg = 29.92 inHg = 14.696 psi = 1 atm (U.S. Standard Atmosphere, 1976 — NOAA/NASA/USAF; ICAO uses the same reference). Bar length shows each unit’s numeric magnitude for the identical pressure; 1 hPa = 1 mb by definition and 1 inHg = 33.8639 hPa.

Barometric, defined: weighing the air

The word traces to the instrument. In 1643, Evangelista Torricelli inverted a mercury-filled glass tube into a dish and found the atmosphere would hold the column about 30 inches high — the first barometer, and the reason two of our pressure units are literally lengths of mercury. When the air above grew heavier, the column rose; when it lightened, the column fell.

Nothing fundamental has changed since. A barometer reads the weight of the entire column of air above it, from the instrument to the top of the atmosphere. Denser, sinking air masses press harder and read high; rising, storm-building air presses less and reads low. That single physical fact is what makes the instrument a forecasting tool rather than a curiosity.

The units: one pressure, five numbers

The standard atmosphere at sea level — the reference every scale is anchored to — is 1013.25 hectopascals, which is the same air as 29.92 inches of mercury, 760 millimeters of mercury, 14.696 pounds per square inch, and exactly 1 atmosphere (NOAA/NASA U.S. Standard Atmosphere, 1976; ICAO uses the same figure).

Two translation notes untangle most confusion. First, hectopascals and millibars are the same unit with two names — 1 hPa equals 1 mb by definition, so a 1013 mb reading and a 1013 hPa reading are identical. Second, US weather reports run on inches of mercury while scientific charts run metric: 1 inHg equals 33.8639 hPa, which is why 29.92 inHg and 1013.25 hPa keep appearing together.

Reading an aneroid dial — and the set-needle trick

A traditional wall barometer is an aneroid: a sealed, springy metal capsule that compresses as pressure rises and relaxes as it falls, driving the pointer through a lever chain. The dial usually carries both unit scales, inHg on one ring and hPa or mb on the other — two scales, one measurement.

The second, manually-turned needle is the whole trick of the instrument. Rotate the knob until the set needle sits exactly over the live pointer, then walk away. Hours later, the gap between the two needles is your pressure change — the trend, read at a glance, no logbook required. A live pointer that has climbed away from the set needle means building pressure; one that has dropped below it means the atmosphere is losing weight over your house.

Station pressure vs sea-level pressure

Pressure falls fast with elevation — roughly 1 inHg for every 1,000 ft of altitude near the surface, by the NWS rule of thumb. Left uncorrected, that swamps the weather signal: Denver, at about 5,280 ft, has a true station pressure near 24.6 inHg on an ordinary day — a value that would read as an apocalyptic storm at sea level.

So reported values are standardized. Station pressure is what the instrument physically measures; sea-level pressure is that value corrected to what it would be at elevation zero, which is what weather maps, METARs, and this site's city pages display; the altimeter setting is aviation's variant of the same correction. The correction is why Denver and New Orleans can both report near 29.92 inHg and mean the same weather, and why a home barometer fresh out of the box should be calibrated against the nearest airport's reported value before you trust it.

The 3-hour trend is the signal, not the number

Official observations don't just record the pressure — they record its 3-hour tendency, because the direction and speed of change carry the forecast information. A barometer sitting at a modest value while rising tells a calmer story than a high value in free fall. Falling pressure generally means air is rising and weather is building; rising pressure means air is sinking and skies tend to clear.

Speed sets the stakes. The NWS observing handbook codes a change of 0.06 inHg or more in 3 hours — about 2 hPa — as rising or falling "rapidly," and a rapid fall is the classic signature of an approaching storm system. On a home instrument, that is the set-needle gap growing noticeably in a single afternoon.

The instruments: mercury, aneroid, digital

Three families of barometer read the same atmosphere. Mercury barometers — Torricelli's design, refined — remained the reference standard for centuries and are now mostly retired in favor of instruments without the liquid metal. Aneroid barometers put the sealed-capsule mechanism in everything from wall dials to aircraft altimeters. Digital barometers use a tiny pressure-sensitive silicon element and now ship inside most smartphones, which is how a phone can estimate which floor of a building it is on.

A phone's chip measures station pressure at wherever the phone is — pocket, 40th floor, airplane cabin — so its raw number will not match the sea-level-corrected value in a weather app. For weather reading, the corrected value is the one to use; the raw chip shines at measuring short-term change, which, as the previous section argues, is the part that matters.

Station vs sea-level vs altimeter

Three versions of the same measurement — only the sea-level-corrected one compares between cities.

Station pressureSea-level pressureAltimeter setting

What it isThe raw instrument readingCorrected to elevation zeroAviation's correction

Altitude effectFalls ~1 inHg / 1,000 ftRemovedRemoved

Where you see itThe sensor itselfWeather maps, city pagesMETARs, cockpit dials

Denver (~5,280 ft) reads~24.6 inHgNear 29.92 inHgNear 29.92 inHg

Station pressure vs sea-level pressure vs altimeter setting, per NWS pressure-reporting definitions. The Denver row applies the NWS rule of thumb of ~1 inHg lost per 1,000 ft to its ~5,280 ft elevation — approximate by construction.

Reading the barometer, by the numbers

29.92inHg

the sea-level standard atmosphere (1013.25 hPa)

U.S. Standard Atmosphere 1976

1inHg

of pressure lost per 1,000 ft of elevation — why readings are sea-level corrected

NWS

0.06inHg / 3 hr

the change the NWS codes as rising or falling “rapidly”

NWS Observing Handbook No. 1

1643

Torricelli's mercury barometer — the origin of the inHg and mmHg units

NOAA / NWS

Check your local barometer

The most dramatic barometer traces in the US follow the storm tracks — the nor’easter lane up the East Coast and the Alberta-clipper lane across the northern plains — while trade-wind and Pacific-high metros barely move the needle. These pages show each city’s live pressure and trend, or look up any US city on the barometric pressure hub:

Boston, MA barometric pressure — Nor'easters ride the East Coast storm track past it — some of the deepest, fastest pressure falls in the Lower 48.
Buffalo, NY barometric pressure — Great Lakes lows and clipper systems make its winter barometer trace a sawtooth.
Minneapolis, MN barometric pressure — Sits in the Alberta-clipper lane, where fast-moving lows swing the barometer several times a week in winter.
Fargo, ND barometric pressure — Northern-plains position puts it under both clippers and deepening Colorado lows.
Anchorage, AK barometric pressure — Downstream of the Aleutian storm factory — home to some of the lowest sea-level pressures ever measured in the US.
Denver, CO barometric pressure — At about 5,280 ft, the textbook case for why station pressure and sea-level pressure are two different numbers.

Frequently asked

What does barometric pressure mean?: The weight of the column of air above a point, measured by a barometer — Torricelli's 1643 mercury-tube instrument gave it the name. The sea-level standard is 1013.25 hPa, or 29.92 inches of mercury.
Are inHg and mb the same thing?: They measure the same pressure in different units. Millibars (identical to hectopascals — 1 mb = 1 hPa) are the metric scale; inches of mercury are the US weather-report scale. One inHg equals 33.8639 hPa, so 29.92 inHg and 1013.25 hPa describe the same air.
Why does my barometer show two numbers?: The dial carries two unit scales for one measurement — typically inHg on the outer ring and hPa/mb on the inner. The pointer crosses both at once; read whichever scale your forecast source uses.
What does the set needle on a barometer do?: It's a manual bookmark. Align it with the live pointer, and the gap that opens over the next hours is your pressure trend — the 3-hour change forecasters actually watch, visible at a glance.
How accurate is a phone barometer?: The chip is good at relative change — small enough shifts to sense a stairwell climb — but it reads raw station pressure at the phone's location, uncorrected to sea level. Compare trends on it; take absolute values from a corrected source like an airport observation.
How do I read barometric pressure at altitude?: Use sea-level-corrected values, which is what published readings already are. True station pressure drops about 1 inHg per 1,000 ft, so Denver's instrument-level reading sits near 24.6 inHg — the correction is what makes readings comparable between cities at different elevations.
What is a normal barometric pressure reading?: The sea-level standard is 1013.25 hPa (29.92 inHg), and everyday corrected readings move within a band around it. The trend matters more than the level: a fall of 0.06 inHg in 3 hours is coded "falling rapidly" by the NWS and usually signals an approaching system.