Atmospheric pressure and its measurement

Air, a fluid that has weight

The Earth's atmosphere also exerts a pressure on every surface, called atmospheric pressure. At sea level, it averages 1013 hPa (about 101,300 Pa), a value often rounded to 1 bar.

Torricelli's barometer

In 1643, Torricelli measured atmospheric pressure using a tube filled with mercury and inverted in a mercury reservoir. The mercury drops in the tube until the pressure exerted by the remaining column exactly balances atmospheric pressure. At sea level, this column measures about 760 mm, hence the historical unit "mm of mercury" (mmHg).

Pressure decreases with altitude

The higher up you go, the less air there is above you, so the pressure decreases.

Altitude	Average pressure
0 m (sea level)	1013 hPa
3000 m	about 700 hPa
8848 m (Everest)	about 340 hPa

Common pitfall

Don't confuse the units: 1 bar = 1000 hPa = 100,000 Pa. A frequent mistake is writing "1013 Pa" instead of "1013 hPa," which gives a pressure 100 times too low. The Magdeburg hemispheres experiment (1654) spectacularly illustrates the strength of this pressure: two teams of horses could not separate two half-spheres emptied of air.