Air pressure is the weight of the air above you, so the higher you stand, the less air is left overhead and the lower the reading. Near sea level it falls by about one hectopascal for every 8.4 metres you climb. That one fact explains most of the confusion people have about their own barometers.
It also means a raw reading tells you more about where a place is than about its weather. Mexico City is not having a record storm. It is well over two kilometres up.
What is normal at your height
| Elevation | Pressure | inHg | Water boils at |
|---|---|---|---|
| Sea level | 1013.25 hPa | 29.92 | 100 °C |
| 200 m | 989 hPa | 29.22 | 99 °C |
| 500 m | 955 hPa | 28.19 | 98 °C |
| 1,000 m | 899 hPa | 26.54 | 97 °C |
| 1,500 m | 846 hPa | 24.97 | 95 °C |
| 2,000 m | 795 hPa | 23.47 | 93 °C |
| 2,500 m | 747 hPa | 22.05 | 92 °C |
| 3,000 m | 701 hPa | 20.70 | 90 °C |
These are reference figures for an average atmosphere, so expect the real reading to sit a little either side.
Two things stand out. The fall is not quite a straight line: it takes 8.4 metres to lose a hectopascal near the ground, and about 10.4 metres at 2,200 m, where the air is already thinner. And height beats weather easily. A week of dramatic weather might move the pressure 30 hPa; walking up 250 metres does the same.
Two numbers for a high city
Every city page here has a switch, and always says which figure is showing:
- The sea-level figure, corrected to what it would be at sea level. This is what forecasts quote, and what makes two cities comparable.
- The reading where you stand, which is what a barometer in the room shows.
Near the coast the two are almost the same. In Mexico City, 2,238 m up, a barometer reads about 786 while the published sea-level figure is around 1019: a gap of 233 hPa that says nothing at all about the weather. Above 1,000 m this site shows the local reading alongside the headline whether you asked for it or not.
| City | Elevation | What a barometer read | Sea-level figure at the same hour |
|---|---|---|---|
| Madrid | 651 m | 942 hPa | 1015 hPa |
| Denver | 1,615 m | 845 hPa | 1018 hPa |
| Johannesburg | 1,749 m | 840 hPa | 1025 hPa |
| Mexico City | 2,238 m | 788 hPa | 1021 hPa |
| Bogotá | 2,582 m | 755 hPa | 1020 hPa |
| La Paz | 3,767 m | 652 hPa | 1023 hPa |
Both figures in each row were read from our own server at the same hour, on the morning of 21 August 2026. Height carries the barometer column down through hundreds of hectopascals, while every sea-level figure stays close to ordinary. That is the correction doing its job: with altitude taken out, what remains is weather.
Setting a barometer high up
Most home barometers and weather stations arrive set to show sea-level pressure, which is why they ask for your altitude when you set them up. If you live high up you have a choice:
- Leave it on sea level and it agrees with the forecast. In Mexico City it will read around 1019 while the air in the room is nearer 786.
- Set it to the local reading and it shows the air you are actually standing in. Compare it against the local figure on your city page rather than the headline.
Neither is wrong. The trouble comes from not knowing which one your instrument is doing, and in a year you will not remember, so write it down.
Boiling and breathing
Water boils when its vapour pressure matches the air around it, so thinner air means a lower boiling point. That is the last column in the table, and it is why "simmer for ten minutes" means something different in Denver than in London. Food cooked in water takes longer, because it is cooking cooler.
Thin air is also why altitude affects breathing, and it is worth seeing the scale: climbing from sea level to Mexico City removes about 230 hPa, while a deep Atlantic storm removes perhaps 30.
