# Forecast the weather using a barometric altimeter

Compare the measured altitude (barometer, watch) with the actual altitude (e.g. stated on maps, signposts, huts).
Which value is the greater?

## Causes of inaccurate measurements

You are travelling in the mountains and at the beginning of the tour you specifically calibrated the barometric altimeter on your watch (or another device). But after a short while, the measured altitude readings no longer correspond to the values stated on your map or on the huts. Is your watch broken?

No, probably not. If your watch features a barometric altimeter, it measures the altitude by using the air pressure (more information here). So the barometric altimeter functions according to the assumption, that the air pressure does not change during the day. And this is the cause of these measurement errors, since air pressure often does change.

## Forecasting weather using conflicting altitude data

However, these measurement errors which occur with changing air pressure can actually be very useful. If you find yourself at a point, where you know the exact height (summits, huts, signposts, towns etc), you can actually turn the situation to your advantage and use the altitude data on your watch to come up with your own weather forecast!

If your watch displays a higher altitude than the actual one, then that means that the air pressure has dropped during your tour. Since at the last calibration the air pressure, which the watch measures, corresponded to a few meters higher. And since the watch works on the assumption that air pressure remains stable, it falsely calculates that you are still (or already) somewhat higher.

On the other hand a lower altitude means that the air pressure has risen. If your watch shows the correct altitude then the weather conditions are stable, since the air pressure has not changed.

Predicting the weather using barometric altimeters
Difference in the altitude measurement Air pressure Forecast
measured value < actual value higher weather improving
measured value = actual value stable no change in weather
measured value > actual value lower weather worsening

## Practical applications

The air pressure usually changes before changes are actually visible in the sky. So, with help from your watch you can make an early weather forecast. This can be very helpful for allowing you to prepare for upcoming weather changes and storms in good time.

What’s more, this doesn’t just work in the mountains, it’s also useful for hiking tours on which you don’t encounter major differences in altitude. When you are hiking at one consistent level, but your watch is displaying a lower altitude, this is a strong indication that an area of low pressure is approaching.

Though you don’t need to issue a storm warning when the difference in altitude is only a matter of a few metres. The air pressure can also fluctuate without the weather changing. On top of this, any measuring device can also have some degree of measurement inaccuracy. And don’t forget: The more often the altimeter is calibrated, the higher its precision!

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