Why is atmospheric pressure in all directions

Air pressure

The atmosphere

The atmosphere is like a deep ocean of air that envelops the earth. In a sense, the atmosphere behaves like a liquid:

  • the pressure works in all directions.
  • the pressure drops when you go up in a balloon
    (because with increasing height there is less and less weight on your head).


Unlike a liquid, air can be compressed (squeezed). Therefore, the atmosphere near the ground is denser than, for example, on Mount Everest. The Earth's atmosphere extends hundreds of kilometers into space, but most of it is below about 10 kilometers.


Air pressure

At sea level, the atmospheric pressure is around 100 kPa (100,000 Newtons per square meter) - that's roughly the weight of ten junk cars stacked on top of one another in just one square meter. But a person is not crushed by this enormous pressure, since the pressure in the lungs and the pressure in the circulatory system are coordinated with it.



The mercury barometer

Instruments that measure atmospheric pressure are called barometers. The barometer on the right contains the liquid metal mercury. Atmospheric pressure can force the mercury into the tube because there is no air in the space at the top of the tube. There is a vacuum there.

At sea level, atmospheric pressure means that a column of mercury is 760 mm high on average. Because it's more convenient, scientists mostly refer to air pressure as "760 millimeters of mercury." On the other hand, it is very easy to convert these "760 mm of mercury" into pascals and other units, as you will see below.

The actual value of the atmospheric pressure will vary slightly depending on what kind of weather we are. Rain clouds form in areas with lower air pressure, so a drop in pressure when reading a barometer can mean that bad weather is on the way. However, the atmospheric pressure also decreases with the height above sea level. This process is used in the form of an altimeter. Such devices can be found in every aircraft to determine the flight altitude.


Normal air pressure

The pressure that pushes a column of mercury up 760.0 mm was formerly known as the physical atmosphere or 1 atm. The value in pascals can be determined by calculating the pressure due to such a column of mercury.

In a depth H a liquid with the density $ \ rho $ there is a pressure of:

$ \ rho \ \ cdot \ g \ \ cdot \ h $

in which:

$ \ mathrm {g \ = \ 9,807 \ \ tfrac {N} {kg}} $ (or 10 $ \ mathrm {\ tfrac {N} {kg}} $ if less accuracy is required).

Since the density of mercury is $ \ mathrm {13590 \ tfrac {kg} {m ^ 3}} $ and the height of the mercury column is 0.7600 m, the following applies:


$ \ mathrm {1 \ atm \ = \ \ rho \ \ cdot \ g \ \ cdot \ h \ = \ 13590 \ \ tfrac {kg} {m ^ 2} \ \ cdot \ 9.807 \ \ tfrac {N} {kg } \ \ cdot \ 0.7600 \ m \ = \ 101 300Pa} $


For the sake of simplicity, one can assume that the calculators:

1 atm = 100,000 Pa

In weather forecasting, millibars (mb) are usually used as the pressure unit for air pressure:

1 mb = 100 Pa

so that normal atmospheric pressure is around 1000 millibars.


The manometer

A pressure difference is measured with a manometer. The device at the top right is filled with mercury. By moving the mercury column, the pressure that the introduced gas has in addition to the air pressure is displayed. This extra pressure is called overpressure. To determine the actual pressure in the gas line, the air pressure is added to this overpressure.


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