Sea Level Pressure (SLP)

Sea-level pressure is atmospheric pressure reduced to mean sea level, removing the strong dependence on station altitude so that horizontal pressure patterns can be compared and analyzed on synoptic charts. The reduction uses the hypsometric relation with an assumed temperature profile for the air column between the station and sea level. Reported in hectopascals, the global mean is near 1013 hPa; mid-latitude highs exceed 1040 hPa and deep tropical cyclones fall below 900 hPa. Isobars of SLP define the highs, lows, troughs, and ridges, and their spacing gives the pressure gradient that, with the Coriolis force, sets the geostrophic wind.