How-to
How to Read a Barometer (and Forecast the Weather)
How to read a dial barometer or barometer app, set it correctly for your elevation and use the rising or falling trend to forecast the next 12–24 hours.
To read a barometer, note the pressure, then watch which way it moves. A steady rise means fair weather is on the way; a steady fall means clouds, wind and rain; a fall of more than about 0.10 inHg (3.5 hPa) in three hours means a storm is close.
1. Read the number
A dial barometer has one or two scales. In the US the outer scale is usually inches of mercury (28 to 31); the inner scale is millibars / hectopascals (950 to 1050). Read the main needle like a clock hand. A digital barometer or app just shows the number.
The words on the dial — Stormy, Rain, Change, Fair, Very Dry — are rough labels for typical weather at that pressure. Treat them as hints, not forecasts.
2. Use the set hand
Most dial barometers have a second, usually brass, needle that you move with a knob on the glass. That is the set hand:
- Tap the glass lightly so the main needle settles.
- Turn the knob until the set hand covers the main needle.
- Check back in a few hours. If the main needle has moved right of the set hand, pressure is rising; if left, it is falling.
Our live barometer draws the same thing: the thin brass hand marks where the pressure was three hours ago.
3. Turn the trend into a forecast
| What the barometer does | What usually follows |
|---|---|
| Rising slowly, over a day or more | Settled, fair weather for some time |
| Rising quickly after rain | Clearing, often windy and cooler; may not last |
| Steady and high | Fair weather continues |
| Steady and low | Unsettled, dull weather continues |
| Falling slowly | Cloud increasing, rain possible within a day |
| Falling quickly (> 3.5 hPa in 3 h) | Wind and rain soon |
| Falling very rapidly (> 6 hPa in 3 h) | Storm or gale approaching |
Combine it with the wind: in the Northern Hemisphere, a falling barometer with wind backing from west to south or east is a classic sign of an approaching low.
4. Set it up correctly
A barometer measures the pressure where it is. Weather reports give pressure corrected to sea level. If your home is 800 ft above sea level, an uncorrected barometer reads about 0.85 inHg lower than the forecast and its needle will sit near “Rain” on a sunny day.
To fix it, look up your current sea-level pressure (find your city on pressure by city) and turn the adjustment screw on the back of the barometer until it matches. Do this on a calm, settled day. From then on, it reads in sea-level terms like the forecast.
5. Common mistakes
- Reading once a day. One reading tells you little. Compare readings 3–6 hours apart.
- Hanging it in the sun or over a radiator. Aneroid barometers are temperature-compensated, but direct heat still adds error.
- Expecting precision. Household dial barometers are good to a few millibars. The trend is what matters.
Why it works
Pressure falls when air is rising over your area, which is what low-pressure systems and fronts do. More in high vs low pressure and how a barometer works.