If barometric pressure changes with the weather, how can it still be accurate for altitude measurement?

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If barometric pressure changes with the weather, how can it still be accurate for altitude measurement?

In: Physics

In airplanes that use a barometric altimeter, you calibrate the altimeter before you take off. The altitude of the airport is well known, and you set the altimeter to read that altitude while you are still on the ground.

Even so, such altimeters are not that accurate. When it really matters, such as trying to land when visibility is low, airplanes use other techniques to ensure the reach the ground at the right time, while avoiding mountains or other terrain.

The changes in barometric pressure due to weather may impact your altitude measurements by a few hundred meters. However this is long term effects. If you have a way to calibrate your barometric pressure altimeter it will give you very fast and accurate readings for hours afterwards. This is why a barometric altimeter is often combined with a GPS sensor which is slow and inaccurate so that the altimeter is constantly calibrated to account for weather. For example if you compare 10 minutes of GPS data to the corresponding pressure data then you will see that even though they might vary by 20m over time the average relation stays pretty consistent which means you can use this moving average as an offset for your barometric altimeter to account for the change in pressure due to the weather. Another system used by airplanes is that they use a standard pressure, your exact altitude in the air is not that important as long as everyone calibrates their systems the same. And when you get close to landing the airports will measure the pressure and send this to the airplanes as part of their weather updates so that the pilots can calibrate their altimeter for the pressure at the airport. You might sometimes notice when the pilot is doing this when the autopilot is engaged as the autopilot will try to adjust to the new altitude calibration.

If you listen to ATC for people flying at lower altitudes you will here ATC is frequently letting pilots know what the pressure is at a specific location so you can adjust your altimiter. It really is just adjusting a dial

Aircraft altimeters and any other altitude measuring devices can be calibrated for the current barometric pressure. In the case of aircraft, the barometric pressure is everywhere. It’s in every weather report, and you get it almost every time you first check in with an air traffic control center, you also must read the setting back to confirm that, or state that you have the current weather information.

It fluctuates but not often to significant levels. If it does fluctuate significantly, air traffic control will be alerting the frequency of the change, and new special weather reports will be issued.

The field elevation for many/all registered airports is also readily available on maps and in information books, and in a pinch a pilot can set the altimeter to show the field elevation while on the ground.

That’s the aviation side of it (source: private pilot). Hope that helps!