The pressure of the Earth’s atmosphere, called the atmospheric (or barometric) pressure, is a force per unit area exerted by a column of air extending to the hypothetical top of the atmosphere. This is the entire body of air above the specific area.

As with any fluid pressure, atmospheric pressure changes with depth; it is higher at sea level than at the top of a high mountain. However, the Earth’s atmosphere is somewhat complicated because not only does the density of air vary greatly with altitude, but unlike a body of water, there is no distinct top surface to the atmosphere from which the height of an air column can be measured. However, with the following formula, the approximate difference in pressure between two altitudes can be calculated.

The pressure of the air at a given place varies slightly according to the weather. At sea level, the average atmospheric pressure is 1.013 × 105 Newtons per square meter (N/m2 also known as the unit Pascal or Pa) This value is used to define another unit of pressure in common use, the atmosphere (atm).

Another unit of pressure sometimes used in meteorology and on weather maps is the bar, where 1 bar is 1 × 105 Pa.

Thus, standard atmospheric pressure is slightly more than 1 bar. Other measurements of air pressure are used in various fields, as shown below.

## Conversion Factors Between Different Units of Pressure

In Terms of 1 Pa
=
1 N/m2
Related to 1 atm
1 atm
=
1.013 x 105 N/m2
1 atm
=
1.013 x 105 N/m2
=
1.013 x 105 Pa = 101.3 kPa
1 bar
=
1.000 x 105 N/m2
1 atm
=
1.013 bar
1 dyne/cm2
=
0.1 N/m2
1 atm
=
1.013 x 106 dyne/cm2
1 lb/in2
=
6.90 x 103 N/m2
1 atm
=
14.7 lb/in2
1 lb/ft2
=
47.9 N/m2
1 atm
=
2.12 x 103 lb/ft2
1 cm-Hg
=
1.33 x 103 N/m2
1 atm
=
76 cm-Hg
1 mm-Hg
=
133 N/m2
1 atm
=
760 mm-Hg
1 torr
=
133 N/m2
1 atm
=
760 torr
1 mm-H2O (4°C)
=
9.81 N/m2
1 atm
=
1.03 x 104 mm-H2O (4°C)