Pressure altimeter

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An altimeter measures how high you are by sensing air pressure. The higher you go, the lower the pressure. A pressure or barometric altimeter uses a calibration to display altitude and is the type found in most airplanes. Skydivers often wear wrist altimeters, and hikers use wrist or handheld models along with maps, compasses, or GPS.

Altitude readings come from a formula that links pressure, temperature, and sea-level pressure. The constant depends on gravity and the air’s properties. Because readings are tied to air density, weather changes (like a cold front) can cause the indicated altitude to jump by hundreds of feet even if you haven’t moved.

World-wide, the common calibration unit is hectopascals (hPa); in North America and Japan, inches of mercury (inHg) are used. To get an accurate height in feet or meters, the local barometric pressure must be set correctly using the barometric formula.

The idea of the pressure altimeter goes back to 1772, when Rev. Alexander Bryce showed barometer readings could measure height. A barometric altimeter, used with a topographic map, can help verify your location and can be more reliable than GPS in some situations.

Hikers recalibrate when they reach known spots, such as trail junctions or peaks. Altimeters are crucial in skydiving and many jumpers use two or more devices: options include a mechanical analog, an in-helmet audible warning, and a digital display on the arm. The setup depends on the jumper’s experience and the jump type.

In aircraft, an aneroid altimeter measures outside air pressure. Pressure falls with altitude, roughly 100 hPa per 800 meters or about 1 inHg per 1000 feet near sea level. The instrument is calibrated to show altitude above mean sea level according to the International Standard Atmosphere.

Older analogs used a single needle, then multiple needles, then drum-type displays. Today’s transport aircraft mostly use drum-type or electronic displays. Modern “sensitive” altimeters let pilots set the reference pressure (QNH) or field setting (QFE). Temperature differences from the standard atmosphere can cause height errors.

Air data computers now combine altitude with speed and temperature for automatic flight guidance. Pilots perform preflight checks by setting the local altimeter and confirming the readout matches the airport’s elevation; if it’s off by more than about 75 feet, recalibration is required.

Some cars, including a few luxury models from the past, also featured altimeters for navigation.