TESA WS2900C-PRO Operation Manual page 23

3) Viewing Pressure History
Press the [PRESSURE -] button to view the 12 hour, 24 hour, 48 hour
and 72 hour pressure average.
4) Relative Pressure Calibration Discussion
To compare pressure conditions from one location to another,
meteorologists correct the pressure to sea-level conditions. Because
the air pressure decreases as you rise in altitude, the sea-level
corrected pressure (the pressure your location would be at if located at
sea-level) is generally higher than your measured pressure.
Thus, your absolute pressure may read 969.18 hPa (28.62 inHg) at an
altitude of 305m, but the relative pressure is 1015.92hpa (30.00 inHg).
The standard sea-level pressure is 1013.21 hPa (29.92 inHg). This is
the average sea-level pressure around the world. Relative pressure
measurements greater than 1013.21 hPa (29.92 inHg) are considered
high pressure and relative pressure measurements less than 1013.21
hPa (29.92 inHg) are considered low pressure.
To determine the relative pressure for your location, locate an official
reporting station near you.The internet is the best source for real time
barometer conditions, such as Weather.com or Wunderground.com),
and set your weather station to match the official reporting station.
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