The sensor you didn't know you had

There's a barometer in your iPhone. It was put there to count stairs — and meteorologists have written papers on what else it can be used for.

The small barometer inside your iPhone wasn't built to measure the weather. It was built to count floors. Walk up a flight of stairs, and air pressure drops a fraction — enough for the phone to work out whether you just took the lift or climbed it yourself.

The sensor has been in the phone since the iPhone 6, and today it's used for elevation readings in the Health app and for finer positioning when the GPS signal is weak indoors. It wasn't put there for your sake as someone interested in weather. It was put there because a barometer is the cheapest way to measure changes in altitude without satellites — a small flexible membrane that compresses or expands with air pressure, and a circuit that translates the flexing into a number.

The question from 2014

The same year the barometer arrived in the phone, two American meteorologists asked a question right in the title of their paper: could pressure readings from ordinary smartphones be a potential revolution for high-resolution weather prediction? The question mark was deliberate. No one had yet tried it at scale.

The idea was simple, and a little mad. An official network of weather stations is expensive to run, and the stations are typically spread many kilometres apart. An instrument already sitting in hundreds of millions of pockets could, in theory, form a far denser network — without anyone building a single new mast. Soon after, other researchers began testing the idea in practice: could pressure data from ordinary phones actually make a forecast better? The answer was yes, at least for the short-range warnings, the hours right ahead of you.

What Aneroid uses it for

Aneroid logs pressure from the same sensor, but uses it for something more modest than a revolution in weather forecasting. The app finds your own link between changes in the weather and how you feel. It measures in one place — wherever the phone is, usually in your pocket — and saves one value every five minutes, both when you have the app open and in the background while the phone sits in your pocket.

That's far less often than the sensor could deliver data. It's also plenty. Air pressure changes over hours, not seconds, and a change in the weather never lasts just five minutes. A reading every five minutes catches any real change in good time, without using a level of detail no one needs — and without draining the battery for it.

A sensor, not an official weather station

There's a catch, and it's worth saying up front: a barometer in a phone isn't calibrated like an official weather station. Later research has shown that the exact pressure reading from your particular phone rarely stands on its own — it depends on where the phone is, and on differences between individual devices. The change the sensor records over time, on the other hand, is extremely reliable. That's a distinction important enough to deserve an article of its own.

Other researchers have since looked at the question from a different angle: can a single smartphone even function as its own small weather station for the exact place it happens to be? The answer depends on what you're asking it to answer — the absolute figure, or what happens to it over time.

The stair-counting sensor turned into a weather instrument, without anyone changing a single chip.