How to read a pressure trend
The single number matters less than the direction it is moving and how fast. Weather services describe the change over the last three hours, and the UK Met Office uses these bands in its station reports:
| Description | Change in 3 hours |
|---|---|
| Rising or falling slowly | 0.1 to 1.5 hPa |
| Rising or falling | 1.6 to 3.5 hPa |
| Rising or falling quickly | 3.6 to 6.0 hPa |
| Rising or falling very rapidly | more than 6.0 hPa |
On the chart above, a gentle slope over a whole day is ordinary weather. A line that drops steeply over a few hours, especially overnight, is the kind of change people with weather-sensitive symptoms often want to know about. A rise after a deep low usually means the system is moving away.
What the numbers mean
Standard sea-level pressure is 1013.25 hPa, which is the same as 1013.25 millibars and 29.92 inches of mercury. Day-to-day readings wander above and below that figure as high- and low-pressure systems pass. The extremes are far outside everyday weather: the World Meteorological Organization lists 870 hPa in the eye of Typhoon Tip in October 1979 as the lowest sea-level pressure on record, and 1083.8 hPa at Agata in Siberia on 31 December 1968 as the highest below 750 m.
Storms that deepen quickly get their own name. NOAA calls it bombogenesis when the pressure at a mid-latitude storm's centre falls by at least 24 millibars in 24 hours at 60 degrees latitude, or about 17.8 millibars at the latitude of New York. That is the storm's centre; the fall you see at home is usually smaller.
For a full table of conversions between hPa, inHg and kPa, see normal barometric pressure and units.
Why the chart shades falls of 6 hPa
Any threshold is a choice, so here is ours. Barometric Pressure Tracker treats a change of 6 hPa or more within 6 hours as a major change, and this chart uses the same 6 hPa over a looser 24-hour window so that slower, sustained drops are visible too. Research does not point to one magic number: a Japanese study found migraine attacks clustered when pressure was 6 to 10 hPa below the 1013 hPa standard, and another linked more frequent migraine to day-to-day falls of more than 5 hPa.
A shaded stretch is a large swing worth noticing. It is not a warning that you will feel anything, and an unshaded day is not a promise that you won't. If you want to know whether big falls matter for you, compare them with a log of your symptoms; our guide to barometric pressure and headaches explains how.
Why your own barometer may read differently
This chart shows sea-level pressure, the figure weather services publish. It takes the pressure measured at a station and adjusts it to what it would be at sea level, so that a city in the mountains and a city on the coast can be compared on the same map. A barometer on your wall or in your phone measures the air where it is, and pressure falls with height: roughly 1 hPa for every 8 metres near sea level. In Denver, about 1,600 metres up, the standard atmosphere gives a station pressure of roughly 835 hPa even on a day when the sea-level figure is a perfectly normal 1013 hPa.
So if your phone shows a lower number than this page, that is expected. What matters for spotting a change is that both lines move the same way at the same time.
Where the data comes from
Readings and forecasts come from Apple Weather, hourly. When you use your location, the browser asks your permission first, and the coordinates are rounded to one decimal place, about 11 km, before they are sent, which is plenty for pressure because it changes smoothly over distance. They are used only to look up pressure. Choosing a city sends that city's coordinates instead and needs no permission at all.