The Polar products measure altitude by using an air pressure sensor. Air pressure changes constantly due to varying weather conditions.
Therefore, measured altitudes in the same position may vary. Pressure variations due to weather conditions, or indoor air-conditioning, may affect altitude readings. Therefore it is recommend to calibrate the altitude value before exercising.
For example, a thunderstorm can cause a pressure change of up to 80 hPa /2.36 inHg, meaning a change of approximately 700 m / 2000 ft in calculated altitude.
Note: If the current air temperature differs from the normal (see the table below), there are errors in altitude calculation, because the altitude calculation is based on the barometric pressure at a normal temperature. When climbing in higher than normal temperature the actual altitude value is higher compared to the value shown at the wrist unit's display. If the temperature is lower than normal temperature the actual altitude value is lower than the value shown on the wrist unit.
The International Standard Atmosphere Values
Altitude (above sea level) Temperature