Sea Level Pressure Correction Calculator

Barometric pressure falls by roughly 1 hPa for every 8 meters of ascent near sea level, which means raw station readings from observatories at different elevations cannot be compared or contoured on a synoptic chart. Reduction to sea level answers a hypothetical question: what would the barometer read if a column of air extended from the station down to mean sea level. The hypsometric relation P_sea = P_station times exp(g h / (R Tv_mean)) supplies the answer, with g near 9.80665 m/s2, the specific gas constant for dry air R equal to 287.05 J per kg per K, h the station elevation in meters, and Tv_mean the mean virtual temperature of the fictitious column in kelvin. Virtual temperature rather than actual temperature is used because water vapor is less dense than dry air, and the correction reaches about 3 K in warm humid conditions. Sea level pressure at the surface normally ranges from about 980 to 1040 hPa, against a standard atmosphere value of 1013.25 hPa. The extremes on record are 870 hPa in Typhoon Tip in 1979 and 1083.8 hPa at Agata in Siberia in 1968. The reduction is reliable for low-lying stations but becomes increasingly artificial with altitude, since the temperature of a column of air that does not exist must be guessed. Standard practice, following WMO guidance, estimates it from the current station temperature and the value twelve hours earlier. Above roughly 500 meters the resulting uncertainty can exceed 1 hPa, so high-altitude stations often report altimeter setting instead.

The problem is as old as the barometer. Evangelista Torricelli built the mercury barometer in 1643, and in 1648 Blaise Pascal arranged for his brother-in-law Florin Perier to carry one up the Puy de Dome in the Auvergne, observing a fall of about 3 inches of mercury over 1000 meters and establishing that atmospheric pressure decreases with height. Edmond Halley published a relation between pressure and altitude in 1686, and Pierre-Simon Laplace refined the barometric formula in the early nineteenth century into the form still recognizable today, incorporating temperature and, later, humidity. The practical need for reduction arose with the synoptic weather map. After the loss of the French warship Henri IV in a storm at Balaklava in 1854, Urbain Le Verrier organized a telegraphic network of observing stations across Europe, and Robert FitzRoy established the British Meteorological Office in the same decade. Drawing isobars across stations at different elevations required a common datum, so national services adopted reduction procedures during the 1860s and 1870s. The International Meteorological Organization, founded in 1873 and reconstituted as the World Meteorological Organization in 1950, progressively standardized the method, and the twelve-hour mean temperature convention became the widely used approach. The standard atmosphere reference of 1013.25 hPa was fixed by ICAO in 1952.