One hectopascal is four steps on a staircase

Walk up to the second floor, and the pressure drops as much as a weather change can. That's why the app can't do without knowing how high up you are.

Sit down in the armchair on the ground floor with your phone next to you. Then take it up to the second floor. Without a single low-pressure system passing through, without a front going by, your phone's barometer has just recorded a pressure drop that rivals part of what would happen if the weather actually changed.

That's not a fault in the sensor. It's physics, and it's unforgiving.

The floor that looks like a front

The rule of thumb is: one hectopascal corresponds to about 8.3 metres of height near sea level. A typical floor-to-floor distance in an apartment building falls somewhere in that range. That means the distance between your living room and your bedroom on the first floor can cost you as many hectopascals as the air loses when a real low-pressure system approaches over a couple of hours.

A sensor that just measures pressure, without knowing how high above the ground it is, can't tell the two things apart. It sees a drop. It doesn't know whether that's the weather, or you taking the lift.

The trend can be trusted. The number can't

That's also why the absolute pressure from a phone can never stand alone. Compare two phones side by side, and they rarely show exactly the same number — partly because each sensor has its own small deviation, partly because even a metre's difference in height shifts the figure. What research into crowdsourced phone measurements has shown is that the trend — whether pressure is rising or falling — is extremely reliable, while the exact values are not, precisely because of height differences and sensor bias between units.

To use the trend properly, you first have to strip the height out of it. Otherwise you end up logging a trip up the stairs as if it were a change in the weather.

Your location never leaves the phone

That's why Aneroid asks for your location — not to know where you are, but to know how high up you are standing. The calculation happens on the phone, and the readings stay there. No server collects them. The one exception is if you turn on the weather forecast in the app yourself — then your approximate location is sent to Apple's weather service, because that requires a network connection. Everything else, including the pressure reading itself and the height correction, stays on the device.

Without the height, the app wouldn't be measuring the weather. It would be measuring your floor.