Barometer

What it does

Shows live air pressure and your altitude, read from your iPhone's built-in barometer.

What you need

• An iPhone. This tool is iOS only.
• No network needed when the phone has a barometer.
• Without a barometer (for example in a simulator, or on an older iPhone), it falls back to your location: altitude comes from GPS and pressure from the weather service. In that case you are asked to set your location first, and it needs a network connection.

How to use it

1. Open the tool. Pressure and Altitude update on their own.
2. To change the pressure unit, tap the info icon on the card and follow Settings > Units. Pressure can be shown in inHg (the default), hPa, or mmHg. Altitude uses your Imperial or Metric setting (ft or m).
3. To calibrate (barometer phones only), tap Calibrate, choose Calibrate by Altitude or Pressure, enter your current altitude or the local sea-level pressure, and tap Save. Once set, the link reads Calibrated · Recalibrate, and Reset to Standard removes your calibration.

Reading the results

• Pressure is the reading at your location. When it comes from the weather service instead, it is labeled Sea-Level Pressure with the source named, because it is a different quantity.
• Altitude comes from the best source available: a calibrated barometer first, then GPS (typically within 10 to 20 m), then an uncalibrated barometer. A note under the values says which one is in use.
• With no GPS reading and no calibration, altitude assumes standard sea-level pressure (1013.25 hPa) and will drift as the weather changes.

Tips

• Calibrate at a spot where you know the elevation.
• Calibration can't be saved until a live pressure reading arrives.