|Air is surrounding the earth, forming the atmosphere (see image).|
On each square meter of the earth's surface (approx. 5·108 km ² and/or 5·1014 m²) the air column weighs about 10 tons.
The pressure (energy per surface) at sea level is about 101,000 N/m², corresponding to 101,000 Pa or 1010 hPa.
|Air pressure decreases with altitude. |
It is easy to imagine that the air at the bottom, close to the ground, is compressed by the air molecules above it and therefore more dense (see image).
In the upper layers, the pressure is lower and decreases with the altitude.
|Since the air pressure always decreases with the atitude and never is equal in two different altitudes, one can imagine the atmosphere also as a pile of layers with equivalent air pressure (isobar layers). |
In the picture such levels are with the printing pressure p0, P1, p2…. p6 represented.
Isobar means lines of "same barometric" pressure.
Source: E. Parlow (2008).