The atmospheric pressure around us is
caused by the collision of air molecules on any surface AND, quantitatively, by the weight of the gas
above you (note there are two contributions to atmospheric pressure).
So why does atmospheric pressure vary with
At very high altitudes there is little air, far
few collisions, less weight of gas above and so the pressure tends towards zero
The greater the height/depth of a gas, the
greater the weight of particles that gravity is pulling down to the Earth's
surface, hence the increase
in force per unit area the lower the level i.e. increase in pressure towards the
Earth's surface - where the atmospheric pressure will be the greatest.
As you increase in height above the Earth'
surface (increase in altitude) the atmospheric pressure decreases.
This is because the air is less dense and so
less collisions can take place in a given volume AND there is less weight of
molecules above a given altitude created by the downward force from the Earth's
Therefore the greatest atmospheric pressure
will be the greatest at the Earth's surface.
To express and explain the trend in another
The increase in pressure the nearer you are
to the Earth's surface, is due to the greater density - hence more collisions
in the same volume AND the greater the weight of air above you - the greater force per unit area.
weight of air above a certain height compresses the atmospheric gases below
that level and compression means increase in pressure (ignoring any
temperature differences) from more collisions between molecules.
Just as with liquid fluids discussed
above, gases are fluids and the weight of them acting downwards creates a
pressure in the same way AND acting in all directions.
At a given height above the Earth's surface, there is very
little variation in the density and pressure of the atmosphere.
weather systems are driven by regions of higher or lower pressure
compared to the average atmospheric pressure at that height.
If you look at
weather charts on the TV weather forecast you will see a 'high' (H) area
with a number like 1029 by it, conversely, a 'low' (L) might have a
number like 986 by it.
The average surface atmospheric pressure is ~1000
millibars (but don't worry about this unit, but 1 millibar = 0.1 kPa).
Barometers are used to measure
atmospheric pressure and can indicate weather changes e.g. rise in
barometric (fair weather, more sunny) or fall in barometric pressure (poorer
weather e.g. rain).
Since the atmosphere gets less dense (less
pressure) on breathing you take in less oxygen as you ascend to greater heights
above the Earth's surface.
This is why many early mountaineers
carried cylinders of oxygen to assist more efficient breathing.
Atmospheric pressure can be measured with a mercury barometer,
though this is being replaced by electronic pressure transducers.