Article
How to read a barometer
To read a barometer, read two things: the level the needle points to and the direction it is moving — and the direction tells you more. The level only lines up with the forecast once the instrument has been set to an official sea-level reading; an unset barometer, or one showing raw station pressure, is the usual reason the two disagree.
The dial: two scales and two hands
A wall aneroid barometer is usually marked in two scales: inches of mercury (inHg) and millibars (mb), which are the same as hectopascals (hPa). University of Kentucky agricultural meteorology guidance describes the dial as marked in inches and millibars, with roughly 34 mb to an inch. Standard sea-level pressure is 29.92 inHg, or 1013.25 hPa (what inHg means). The site converts at 1 hPa = 0.02953 inHg. Older dials may show inches only.
Most dials have two pointers. One follows the pressure automatically. The other, usually called the reference, set or second hand, stays where you leave it. To use it:
- Tap the glass lightly, since the needle can stick.
- Turn the knob so the reference hand sits directly over the pressure needle.
- Come back in a few hours and see which way the pressure needle has moved away from it.
The gap between the two hands is the change. That is the useful reading: a barometer can only measure pressure, and the trend tells you more than the single value.
What the dial words are worth
Many dials print words such as “Stormy,” “Rain,” “Change,” “Fair” and “Very Dry.” Treat them as a rough rule only. Where one word ends and the next begins differs from maker to maker, and we found no authoritative table that fixes them, so none is given here. Low readings tend to go with unsettled weather and high readings with settled weather, but the numbers are a better guide than the words.
For the numbers, use this site’s five fixed pressure zones. They are sea-level values, the same for every city. They are conventions, and the exact cut points vary between sources.
| Zone | inHg | hPa | Typical association |
|---|---|---|---|
| Very low | below 29.38 | below 995 | Deep low; storm conditions |
| Low | 29.38–29.68 | 995–1005 | Unsettled; cloud and precipitation more likely |
| Normal | 29.68–30.12 | 1005–1020 | Ordinary conditions |
| High | 30.12–30.42 | 1020–1030 | Settled, drier, often clear |
| Very high | above 30.42 | above 1030 | Strong anticyclone |
These bands only apply to a sea-level reading. For a worked example of a single number, see 29.95 inHg, and for the range in more detail, normal barometric pressure and high vs. low pressure.
Setting it to sea level
A barometer measures the pressure where it hangs. Forecasts, TV and radio report pressure reduced to sea level, so that places at different elevations can be compared. To make your dial agree with them, set it once to an official sea-level figure:
- Get the current sea-level pressure from the nearest official observation, such as a local weather office or airport. Use an observation, not a forecast. The Royal Meteorological Society suggests the latest hourly observation from the nearest station.
- Choose a calm day when pressure is fairly steady over a wide area, as the Royal Meteorological Society advises. The University of Kentucky suggests a high-pressure spell, for the same reason: pressure then varies less from place to place.
- Find the small adjustment screw on the back. With a small screwdriver, turn it in small steps until the pointer matches the official figure. Per the University of Kentucky, clockwise raises the pointer and counter-clockwise lowers it. Tap the glass between turns.
- Recheck against an official reading occasionally.
Digital weather stations do the same job through a menu. Depending on the model, you enter your elevation or set the relative (sea-level) pressure to an official reading. Follow your manual. One vendor, Ambient Weather, explains that its consoles carry two readings: absolute (measured where you are) and relative (corrected to sea level). Its example is 28.62 inHg absolute against 30.00 inHg relative at about 1,000 feet.
As for which type is more accurate: this guide does not rank them. Aneroid or digital, each is only as good as its setting against an official reading.
Why it disagrees with the weather report
Almost always, one of four things:
- Station vs. sea-level pressure. According to the National Weather Service, station pressure is the true pressure at your elevation, and for every thousand feet of elevation gain it drops about 1 inHg. At altitude, a raw reading will sit far below the number in the report.
- The absolute/relative setting. If a digital console has been switched to absolute (ABS) rather than relative (REL), it will not match internet sites, which report relative pressure (Ambient Weather).
- A calibration offset. If the dial was never set, or was set against a different figure, it will be off by a constant amount.
- Timing and definition. The report is an observation from a particular hour, and pressure moves. The NWS also notes that sea-level pressure and the altimeter setting heard in broadcasts “are similar, but they are not identical” (NWS Central Region, The Front, 2002), so a small difference can remain even when your instrument is right.
How big the altitude gap is: Denver and Miami
The site’s ten-year Open-Meteo ERA5 hourly reanalysis (2015–2024; reanalysis, not a station barograph) records both the sea-level figure and the surface pressure for each city.
| City (elevation) | Mean sea-level pressure | Mean surface pressure | Gap |
|---|---|---|---|
| Denver (1,615 m) | 29.97 inHg (1014.8 hPa) | 24.74 inHg (837.7 hPa) | 5.23 inHg (177.1 hPa) |
| Miami (6 m) | 30.03 inHg (1017.1 hPa) | 30.01 inHg (1016.4 hPa) | 0.02 inHg (0.7 hPa) |
A Denver barometer left on raw station pressure would average about 24.7 inHg, roughly 4.6 inHg below the bottom edge of the very low zone (29.38 inHg). The same instrument in Miami would average within 0.02 inHg (0.7 hPa) of the sea-level figure. Neither instrument would be broken; the Denver one is just reporting a different quantity. The full story is in pressure at sea level.
One caveat: the sea-level reduction itself is an estimate. The NWS says it depends on temperature assumptions and should be used with caution at high elevations. It is also why this site notes that sea-level reduction inflates apparent pressure variability at altitude, in Denver and Colorado Springs. Denver is used here as an altitude example, not a volatility ranking.
Phone and watch sensors
A sensor in a phone or watch measures the pressure where the device is. That makes it good for tracking change over hours and less useful for the absolute level. Garmin’s manual for one watch says its barometer readings reflect changes in ambient pressure and depend on the calibrated altitude. Some apps estimate a sea-level figure on top of the raw reading; that figure is the app’s estimate, not the sensor’s. If it disagrees with the forecast, the section above applies.
Reading the change
Describe what you see in three words: falling, rising or steady. Falling pressure usually means a low or front is approaching and rising pressure a high building (falling and rising pressure). Readings taken at intervals, together with clouds, temperature and wind, can help with short-term judgments, but a barometer alone does not forecast the weather.
For magnitude, this site uses 24-hour changes: a swing is a change of at least 0.18 inHg (6 hPa) within 24 hours and a big swing is at least 0.30 inHg (10 hPa), both defined in the methodology. For a faster yardstick, the National Weather Service adds a “pressure falling rapidly” remark to its aviation weather reports when pressure falls at 0.06 inHg (about 2 hPa) per hour or more. That is a reporting threshold for aviation, not a health threshold.
For scale, a 24-hour fall of 0.18 inHg (6 hPa) or more is a drop day. Denver averages 177.5 drop days a year in this dataset, and Miami 6.9. Denver’s count is inflated in part by the sea-level reduction described above.
If you keep a symptom log
Some people write down a reading, and which way it is moving, next to how they feel. A log like this is a way to check your own pattern against pressure change, which studies associate with symptoms in some people (see what barometric pressure is linked to headaches). It works best with readings taken the same way each time.
Pressure change has been associated with symptoms such as migraine in some people, and not in others. Effect sizes are modest and sensitivity varies widely. A reading is a description of the weather, not a prediction about you, a diagnosis or medical advice. If symptoms are affecting your life, talk to a clinician.
Where to go from here
Once your barometer is set, compare it with the live chart on your city page for the long-run context: the ten-year range, the swing-day counts and the zones. If your town is not one of the 50, check any US location.