Basics
What Is Barometric Pressure? A Plain-English Guide
Barometric pressure is the weight of the air above you. Here is what the number means, the units it comes in, what is normal and why it changes with the weather.
Barometric pressure is the weight of the air above you. At sea level it averages 29.92 inches of mercury (inHg), or 1013.25 hectopascals (hPa). It rises and falls by a few percent as weather systems pass, and those changes are one of the best short-range weather signals there is.
The air has weight
Air feels like nothing, but the atmosphere above every square inch of ground weighs about 14.7 pounds. We do not feel it because the pressure pushes equally from every direction, including from inside our bodies.
A barometer measures that weight. The first ones, built in the 1640s, balanced the atmosphere against a column of mercury in a glass tube. The air could hold the mercury up to about 30 inches, which is why the US still reports pressure in inches of mercury.
The units you will see
| Unit | Standard sea-level value | Where it is used |
|---|---|---|
| inHg (inches of mercury) | 29.92 | US weather and aviation |
| hPa (hectopascals) | 1013.25 | Weather almost everywhere else |
| mb (millibars) | 1013.25 | Same as hPa, older name |
| mmHg (millimeters of mercury) | 760 | Some countries; also blood pressure |
| kPa (kilopascals) | 101.3 | Canada, engineering |
1 hPa is exactly 1 mb, so “1013 mb” and “1013 hPa” are the same reading. Convert any of them with the pressure converter.
Why pressure changes with the weather
The atmosphere is constantly stirred by uneven heating. Where air sinks, it piles up and pressure rises: that is a high-pressure system, usually with clear skies and light wind. Where air rises, it leaves less weight behind and pressure falls: a low-pressure system, where rising air cools, forms clouds and often rain.
So a falling barometer is an early sign of unsettled weather, and a rising one points to clearing. The size of the change matters: a drop of a few hundredths of an inch over a day is ordinary; a drop of 0.10 inHg (about 3.5 hPa) in three hours means a front or storm is arriving. See high vs low pressure.
Why pressure changes with height
Climb a mountain and there is less air above you, so pressure drops: roughly 1 inHg for every 1,000 feet near sea level. In Denver, at 5,280 ft, an uncorrected barometer reads around 24.6 inHg on a normal day.
That would make weather maps useless, so weather services correct every reading to sea level. The pressure in your forecast is therefore not what a sensor reads at your elevation but what it would read if your town sat at sea level. More in sea-level vs station pressure.
What range is normal?
At sea level, most readings fall between 29.60 and 30.30 inHg (1002–1026 hPa). Strong winter highs can exceed 30.70 inHg (1040 hPa); deep storms go below 29.00 inHg (982 hPa), and the strongest hurricanes far lower. Details in normal barometric pressure.
How to check it right now
The live barometer on our home page shows the sea-level pressure from the weather station nearest you, the change over the last three hours and a 24-hour graph. Or pick a city from pressure by city.