|Is air pressure really "caused" by the weight
air above a body, or is it due to the kinetic
energy and of the air molecules at any particular
point in the atmosphere?|
|Question Date: 2013-01-16|
Great question. The answer is that both are
important in different ways. The air pressure
depends both on the temperature (or kinetic
energy of the air molecules) and on your
altitude (or the weight of the air above you),
so I would say that air pressure is caused by
Here's how it works. Think of the atmosphere
as giant mob of people trying to rush the stage
at a concert. Everyone wants to get to the
stage, but since there are so many people and
because everyone is pushing and shoving they
can't all get there. Still there will be a lot
of people close the stage and fewer far away.
Our atmosphere is just like this, gravity is
pulling all of the air molecules towards the
surface of the Earth but collisions between the
molecules keep them from getting too bunched up
near the surface of the Earth. This explains
why it's harder to breath at higher
As a simplified general model, we treat
pressure as the effect of molecules colliding
against a surface. (The kinetic energy of the
collision creates a force against a surface, and
given the number of collisions per time per unit
area, you can get the pressure by dividing the
force by the area.
Yes! It is the weight of the air above you
being pulled towards the earth due to gravity.
It is why the pressure goes down as you climb a
mountain. It is the same principle that explains
the high pressures experienced underwater (water
ways a lot more than air.) You suggested that it
has to due with kinetic energy, which is not
such a bad idea. The average kinetic energy of
molecules within a material is actually what
defines temperature! Of course, hot air and cold
air can have different densities, and thus
different weights, but it is the weight that we
experience. For instance, if you were to take
the exact atmosphere (same amount of air, same
temperature, etc.) and put it on the moon, the
smaller mass of the moon would exert less
gravitational force on the air and thus it would
weigh less, and we would experience less air
Both - pressure is a function of the average
kinetic energy of the air molecules (i.e.
temperature) and the density of said molecules.
The weight of the air above you compresses the
air to higher density, and thus higher pressure.
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