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Our earth’s atmosphere is filled with air. Air is simply the mixture of many gases present in the atmosphere like oxygen, carbon dioxide, hydrogen etc.
Earth’s atmospheric pressure at sea level is 1 bar. (1.01325 bar exactly)
1.01325 bar or 1 atm.

=

101.325 KN/m2 or Kpa


=

101325 N/m2 or pa


=

760 mm of mercury


=

10.34 meter of water column

Bar:
Bar is one of the units of pressure. Pressure is actually the force acting per unit area.
Here is the conversion,
1 N/m2 = 1 Pascal
100,000 Pascal = 1 bar
Understanding atmospheric air pressure:
Atmospheric pressure is simply the weight of air molecules present above the measuring point. Air molecules have mass.
Look at this concept picture which explains atmospheric pressure. At left side of the picture you can see that the total mass present above the point 1 is 5 kg in other words 5 kg mass acts on this point. This point is at the very bottom of the arrangement.
Now take a look at the right side of the picture where point 2 is placed at higher altitude. You can see that the total mass present above this point is 2 kg in other words 2 kg mass acts on this point.
So as you increase the altitude the mass reduces and vice versa. Now just take the concept and apply it to real atmosphere.
On the lest side image the point 1 is placed at earth's surface and on the right side image point 2 is placed at certain higher altitude.
The amount of air molecules above the point 1 is very high compared to point 2. Since air molecules have mass, the total weight of molecules above point 1 is also high compared to point 2. Hence atmospheric pressure is high at sea level and low at mountain.
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