I'm no scientist, but I'm certain a
barometer measures
atmospheric pressure only. Altitude can be extrapolated from atmospheric pressure, but none too accurately, because atmospheric pressure changes? I think that's what
@NEEpps is referring to with "current weather conditions" - but I'm not sure because 'weather' and 'atmospheric pressure' are actually different things - at least that's what they taught me at school?
GPS, by referencing your position by satellite in relation to the earth's crust, is surely the best measure of
altitude.
Therefore, the ideal must be to
combine a barometric measure of atmospheric pressure with a GPS reading of altitude - exactly what my phone does. Calibration then becomes unnecessary, because you do not have what
@Whiteburn calls a "barometric altimeter". It can be configured to additionally give all barometer readings
adjusted to if they were at sea level. This is invaluable for weather forecasting on the hill, because it removes the effects of your own changes in altitude as you ascend and descend over the course of a hike, without which you would not be comparing like with like, and an accurate indication of fall/rise in atmospheric pressure is the single most important factor in short-term weather forecasting. I check this several times daily, and it has never failed to prove an accurate indicator of how the weather will change over the coming 12-24 hours, just what's needed when deciding on the advisability of a route or camp.
Of course it's important to check the available online weather forecasts before setting out, and during a hike if accessible, but I scratch my head a bit when I read of hikers equipped with smartphones unable to forecast weather because of no internet signal. They have a potent microclimatic weather forecasting tool in their pocket/pack if they know how to access and use it.