Every reading and forecast on this site comes from one model, run by the European Centre for Medium-Range Weather Forecasts. New figures arrive four times a day, and every page prints the local hour its reading belongs to.
We use one model rather than blending several, so the number you see is a number a real forecast produced.
What the graph shows
The line runs from a week ago through now into the week ahead, and both halves come from the model.
The part behind you is close to a record: each of those hours was calculated from real observations, including real barometers, so it tracks what actually happened. The part ahead is a forecast, and it is firmer near the start than at the far end. A storm can arrive half a day late, or turn up deeper or shallower than drawn.
Sea level, and the reading where you stand
Air pressure falls as you go up, by roughly 1 hPa for every 8 to 9 metres. A raw reading therefore tells you more about how high a city sits than about its weather, so weather services correct readings to what they would be at sea level. That is what makes two cities comparable, and it is what this site shows by default.
Every page also has a switch to the pressure at the city's own height, which is what a barometer in the room reads, and always says which of the two is on screen. In Mexico City, 2,238 m up, those two figures were 786 and 1019 at the same moment. The whole gap is altitude.
Above 1,000 m the local reading is always shown alongside the headline, because that high up the sea-level figure needs the context. Readers in Russia see the local reading by default, because that is what Russian forecasts publish.
Why the change is measured over 24 hours
Pressure rises and falls twice a day on its own, driven by the sun warming the air. In Europe this is a small ripple. Near the equator it is the largest thing on the graph, two to four hectopascals a day with no weather behind it at all.
That is why the headline here is the change since the same time yesterday. Over a full day the daily rhythm cancels out, so the figure means the same thing in Jakarta as in Warsaw. The three-hour change most weather services quote would, at the equator, be almost entirely time of day.
Where that daily rhythm is strong enough to dominate the graph, the page says so and draws a thin dotted line showing the weather underneath it.
The sentence under the number
It is built from the same figures the graph draws, by fixed rules: which way the pressure has gone in the last 24 hours, how far, and where the line turns next. It is not written by hand for each city and not written by a language model.
The words for size, such as "slowly" or "quickly", sit on ranges that shift with latitude, because a fall of nine hectopascals is ordinary in Reykjavik and a major event in Jakarta. The figure itself is always printed next to the word, so you can ignore the adjective.
What can be wrong
- The grid point is not your street. The model is computed on a grid of about 9 km, but the freely published feed we use carries one value per cell of about 28 km. This matters far less for pressure than for rain, but a coastal or mountain city can have a grid point at a different height from the city centre.
- A system can be late, or the wrong depth. More likely towards the far end of the graph.
- The far end can be older. Two of the four daily model runs stop at six days, so the last day or two may come from a slightly older run.
- The line is smoother than the real air. The feed carries a value every three hours, six at the far end of the graph, and the hours between are filled in.
- Pages are up to half an hour old. Every page prints the hour its figures belong to.
No warnings, no health forecasts
Storm warnings belong to your national weather service, which has the observations and the responsibility. A falling barometer here tells you something is coming and leaves the warning to them.
There is no headache forecast or pain scale either.
Where the data comes from
Forecast data from ECMWF (CC BY 4.0), processed by Open-Meteo and by us; ECMWF is not responsible for what we have made of it. The five-year pressure figures in city descriptions contain modified Copernicus Climate Change Service information (ERA5, 2021 to 2025); neither the European Union nor ECMWF is responsible for our use of it. City names, coordinates and populations from GeoNames (CC BY 4.0). Elevations from the Copernicus digital elevation model. Map tiles built from OpenStreetMap data (ODbL) and served from our own server. Weather icons by the Norwegian Meteorological Institute and NRK (CC BY 4.0). Flags use the Twemoji Country Flags font (CC BY 4.0). The wording for three-hour changes follows the Met Office marine glossary.
