March 29, 2023

Things you should know about atmospheric pressure, some will surprise you

Jose Miguel Vinas Jose Miguel Vinas Weather Spain 7 minutes
aneroid barometer
aneroid barometer. This instrument makes it possible to measure atmospheric pressure thanks to the contractions and expansions of a capsule in which a semi-vacuum has been created, contained within it.

In this short article we will offer some brief notes on atmospheric pressure. The first person to deduce its existence and understand the true nature of air was the Italian physicist Evangelista Torricelli., who in 1643 performed a series of experiments that culminated in the invention of the mercury barometer, also named the Torricelli barometer in his honor. His contribution was one of the greatest milestones in the history of Meteorology, as it overturned the false idea that void of horror considered by Aristotle two millennia ago.

Thanks to Torricelli’s barometer, it was possible to see how the pressure decreases with height, which can also be seen as you climb a mountain. This reduction and the existence of a void where the atmosphere ends was theorized by the French mathematician, physicist and philosopher Blaise Pascal. and was proved experimentally by his brother-in-law Florin Périer and a group of companions who, following Pascal’s instructions, climbed the Puy de Dôme, in central France, on September 19, 1648, taking pressure readings with a Torricelli barometer at different times during the ascent, taking lower and lower prices as they rose.

Atmospheric pressure and altitude
Curve representing the decrease in pressure with altitude. Source:

Atmospheric pressure – like air density – decreases with altitude and does so at a much faster rate near the Earth’s surface than at higher atmospheric levels, due to the compressibility exerted by the air itself in the lower layers. Half of the atmospheric mass is concentrated in the first 5 km of the atmosphere. this figure increases to 90% if we consider the first 11-22 km, which mark the top of the troposphere in the mid-latitudes.

Although the pressure drop follows a parabolic curve (see figure above), in this low part of the atmosphere and to a first approximation we can consider a linear behavior, in such a way that for every 9 meters of ascent the pressure decreases by one ectopascal (hPa).

In the standard atmosphere (ISA), the mean sea level (MSL) pressure value is 1013.25 hPa. This pressure marks the theoretical boundary between high and low pressures. Until relatively recently, atmospheric pressure was expressed in millibars (mb), but this unit fell into disuse with the introduction of the Metric Decimal System. In this system, pressure is measured in pascals (Pa). A Pascal is one Newton (unit of force) per square meter (unit of area). One centopascal (100 pascals) is equal to one millibar.

World high and low pressure records

While the decrease in pressure in the vertical follows, as we have seen, a well-defined curve, in the horizontal plane the pressure fluctuations are constant and capricious, which is dictated by the atmospheric dynamics themselves, which in turn depend on the temperature differences between some regions and others. The general circulation of the atmosphere, despite its variability, allows us to understand the spatial distribution of large anticyclones and low pressure areas in summer and winter.

Average pressure maps
Average sea level pressure values ​​on a global scale in the months of January and July. Source: © Encyclopædia Britannica, Inc.

The values ​​that sea level pressure can reach vary over a wide numerical range. Taking the extreme weather values ​​compiled by the World Meteorological Organization (WMO) as a reference, we found some incredible data. The highest value of reduced sea level pressure at a place less than 750 m above sea level is 083.8 hPa measured at the Russian site of Agatha, Siberia, on 31 December 1968. In places where the altitude is higher than the aforementioned 750 m, the record is 1089.1 hPa measured at Tosontegel, Mongolia on 30 December 2004. In both cases, there were very strong winter anticyclones.

    The lowest atmospheric pressure values ​​are reached in the most intense tornadoes that form on Earth. The problem is that no immediate organic measures can be taken due to the destructive power of these dangerous eddies, which sweep away everything in their path. tornadoes apart, the WMO sets the lowest sea level pressure value at 870 hPa at the center of the eye of Typhoon Tip, which was reached on 12 October 1979as it crossed the western Pacific, southern Japan and the eastern Philippines.

    The pressure dropped to sea level

    In the previous section, reference was made to reduced pressure at sea level, which is a theoretical concept that requires a brief explanation. Ground weather stations (thousands of which are spread around the world) measure different meteorological variables, including local atmospheric pressure. To make an isobaric map, with the pressure field at sea level, the pressure values ​​measured at different meteorological stations or observatories must be recalculatedassuming each of these locations is at sea level.

    Reduced sea level pressure
    Schematic figure explaining the procedure used to estimate the reduced sea level pressure of a weather station at a given elevation. Source: The Pennsylvania State University (

    The pressure drop at sea level, mentioned above, is the theoretical value resulting from the fact that pressure varies linearly with altitude, at a constant rate (1 hPa for every 9 meters, as mentioned earlier), so that the altitude of the observatory corresponds to a certain number of ectopascals, which are added to the local pressure value measured by the barometer. These ectopascals are the result of the (theoretical) difference between the pressure at mean sea level and that measured at the observatory, assuming there is an air column between the two levels.

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