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I really love wearing an altimeter watch while in the mountains. I know that it's affected by weather, but unfortunately I don't actually know how it is affected. This usually causes me to ignore the altitude readings entirely when the weather is unstable (and thus altimeter readings are inconsistent), rather than use what I have with some kind of mental/watch correction.

So, what types of weather patterns impact the altimeter reading?

To still estimate the altitude, can I do a mental re-calibration and/or what features of the watch could I use to mitigate any changes due to weather patterns?

Felix
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3 Answers3

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In simplest terms, as you climb, the air pressure around you decreases and your watch subsequently registers an increase in elevation.

However, thunderstorms are often associated with a low-pressure weather system moving in, which will also register on your barometer. This can erroneously be interpreted as an increase in elevation. Similarly, high pressure (clear weather) systems moving in can make your watch register a decrease in elevation.

Some modern watches have accelerometers in them that can sense when you are moving. Algorithms then determine whether a change in pressure is likely due to your climbing (moving, and in an upward trending motion) or due to weather (no movement). These calculations can increase accuracy significantly, but you'll still need to calibrate on a regular basis.

Some GPS watches like Garmin's Fenix 3 (which has an acceleromter) also can auto-calibrate elevation based on GPS information -- though GPS elevation is notoriously bad.

More accurately, the Fenix 3 can also use your smartphone connection to look up your elevation based on USGS survey data of your current location (given by the GPS). This is not automated, but can allow you to more precisely calibrate the watch as often as you feel necessary.

Without super-sensitive tech to register these pressure changes, you'll be hard-pressed to track this on your own in any meaningful way.

Finally, temperature does have some (albeit slight) effect on mercury-based barometers as the the density of the liquid changes causing variation in the readings (low pressure = more volume). Modern non-mercury barometers are not influenced by this to any significant amount.

Lost
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In a nutshell:

  • when the weather goes bad, pressure drops, your perceived altitude increases
  • when the weather clears, pressure rises, your perceived altitude decreases

Using a precise topographic map of your hike, re-calibrate your altitude watch whenever possible.

(At the start, at a summit, at a crossing,...)

njzk2
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As you climb the average rate of change in atmospheric pressure over the first 5 kilometers above sea level is − 78.694 millibars per 1000m climb. But weather can change the air pressure we have all seen the big H and L on weather maps signifying high and low pressures.

Standard air pressure at sea level is 1013.25 mb. The highest air pressure recorded was 1084 mb in Siberia. The lowest air pressure, 870 mb, was recorded in a typhoon in the Pacific Ocean. So that record high air would make your readings 101m higher and that record low would make your readings 1,820m higher. But that would be rare. On today's North Atlantic weather map the lowest low is 987 and the highest high is 1025 so an air pressure altimeter would read 333 meters lower in the low and 149m higher in the high.

Or you could load a GPS altimeter app on to your phone.

Stephen Wordsworth
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