Some modern smart mobile devices, including the Apple Watch, among other things, offer an altimeter function based not only on GPS data, but also on information from a special sensor. Most regular users hardly need height determination, but the feature is there. Recent days have shown that at least the Apple Watch Series 6 and Watch SE have problems determining altitude in certain weather conditions.
In particular, in Germany, which has been in a low-pressure area in recent days, some users of this watch have found that, according to their devices, they are at an altitude of 200-300 meters. It is unclear how, in addition to resetting the settings, to recalibrate the altimeter, but it is likely that this problem may occur in other regions.