To compare pressure conditions around the world, meteorologists recalculate the barometric pressure measured at weather stations to mean sea-level conditions.
Higher placed weather stations naturally have lower atmospheric pressure since the column of air above them is less high. At higher altitude the air pressure decreases. Since air pressure decrease due height above sea level is equivalent around the world, barometric pressure readings from weather stations located at any altitude can be equivalently converted into mean sea level pressure. Mean sea level pressure is the pressure your weather reporting station would have, if it was moved down to sea-level. Mean sea level pressure is generally higher than your measured pressure, which is called barometric pressure.
Your measured barometric pressure may read 28.62 inHg (969 mb) at an altitude of 1000 feet (305 m), but the published mean sea level pressure by your meteorologist will be 30.00 inHg (1016 mb) at this same location, because the meteorologist recalculated (transformed) your barometric pressure to mean sea-level conditions.
The standard sea-level pressure is 29.92 in Hg (1013 mb). Pressure conditions greater than 29.92 in Hg (1013 mb) are considered high pressure and less than 29.92 in Hg (1013 mb) are considered low pressure. In weather forecasts, high and low pressure regions are only relative terms as high pressure is generally considered a region of higher pressure than regions surrounding it and low pressure region has lower pressure than regions surrounding it.
Instructions for most weather stations recommend you to find the closest station run by a meteorologist to identify the offset you need to calculate the mean sea-level pressure. Our Mean sea-level pressure calculator calculates this offset for you.
Calculation is based on the following mean sea-level pressure transformation formula:
- Correction for atmospheric temperature
- Correction for humidity
- Correction for aspericity of the earth
- Correction for variation in gravity (the 4th therm) was not implemented as it makes a difference of only few 1/100ths of a pascal. For more information and table of variables please check the references below (point 5.)