How to read a barometric pressure chart
The number on a barometer says little. The trend, the rate of change and the daily rhythm say a lot. A practical guide to reading a pressure chart.
Tomas Hubalek · · 4 min read
A single pressure reading, say 1008 hPa, tells you very little. It might be the tail end of a storm or the start of one. The chart around that number is what carries the information. Here is how to read it.
Look at the direction first
The first question is always: which way is it going? Barometer Reborn answers it under the gauge, with Rising, Falling or Steady, and the exact change over the last three hours, for example −2.4 mBar over 3 hours.
Three hours is not an arbitrary window. It is the period weather services use for the pressure tendency, so the number is directly comparable with what a forecaster looks at.
Then look at the speed
How fast the pressure moves matters more than where it is. UK marine forecasts describe the three-hour change with a fixed vocabulary that is worth borrowing:
| Change in 3 hours | Described as |
|---|---|
| Less than 0.1 hPa | Steady |
| 0.1 to 1.5 hPa | Rising or falling slowly |
| 1.6 to 3.5 hPa | Rising or falling |
| 3.6 to 6.0 hPa | Rising or falling quickly |
| More than 6.0 hPa | Rising or falling very rapidly |
Most days sit in the first two rows. The last two are what people notice, and they are the reason the app’s ready-made alert rules watch for more than 5 hPa and more than 10 hPa within three hours.
Know the daily rhythm
Even on a perfectly calm day the pressure is not flat. The atmosphere has a tide: the pressure rises and falls twice a day, with highs around 10:00 and 22:00 local time and lows around 04:00 and 16:00. Near the equator the swing is a couple of hPa. In Central Europe it is usually below one, and it is easily drowned out by weather.
If your 24-hour chart shows a gentle double wave on a quiet day, that is the tide, not a fault.
Choose the window for the question
The history screen offers four windows, and each answers a different question:
- 24 hours: what is happening right now, and is the change speeding up?
- 3 days: a whole weather system passing. A front typically shows as a fall, a trough and a recovery within this window.
- 7 days: the rhythm of the week. Useful for comparing your own notes with the pressure.
- 30 days: the background. How unusual is today compared with the month?
Under the chart, the statistics give the minimum, maximum, average and total change for the window you picked.
What a front often looks like
A classic pattern in a mid-latitude climate:
- A steady or slowly falling pressure for a day.
- A faster fall over several hours as a low approaches.
- A trough: the lowest point, often with the most unsettled weather.
- A rise, sometimes sharp, as the system moves on.
- The chart flattens again.
Not every system follows the script, and a falling barometer does not guarantee rain. Barometer Reborn shows the change; it does not pretend to forecast. But once you have watched two or three fronts pass on your own chart, the shape becomes familiar.
Keep the chart clean
Two things spoil a pressure chart faster than any weather:
- Height. A lift ride or a drive into the hills changes the reading by more than a front does. Roughly 1 hPa per 8 metres. Use places so readings from different locations do not share one chart.
- Gaps. If background monitoring is off, the chart only has a point for each time you opened the app. Turn it on in Settings, and exclude the app from battery optimisation if your phone is aggressive about it.
Try it
Open the history screen tonight and switch to the 3 day window. Then check it again after the next change in the weather. The app is on Google Play.
