"Doug" wrote in message
ups.com...
The local altimeter measures the weight of the air above it. When
temperature is higher, the air expands (less density), but it's the
same amount of air in a taller column. The atmosphere expands you see
and is not bound tightly by a blocade above (it is bound by gravity
actually). Anyway another item of interest is what you are setting the
altimeter to is "sealevel" barometric pressure. Imagine a hole in the
ground and the altimeter is lowered to the bottom of the hole (at your
airport). Of course they dont actually have a hole, they have another
way of adjusting it. Standard barometric pressure at sealevel is about
29.92. If you fly an airplane with a manifold pressure guage, it gives
the local "absolute" pressure (non-sealevel adjusted). Anyway consider
these principals and your knowledge will expand.

Standard pressure at sea level is 29.92 at 15 degrees C.

The weight of the air contrary to what you wrote above does vary hence high
and low pressure weather systems.

With low pressure air rises up the column diverges at the top and then
descends giving a high pressure somewhere else.

with a high pressure, the air converges high up, descends and then diverges
when it gets to the ground.

The principle of the altimeter is that it works on pressure, and pressure is
the force exerted by the air and one common force is weight.

When the air is less dense there are fewer molecules, the mass of air is
less and therefore the column of air weighs less, (mass x gravity) hence
the lower pressure.

when you understand these principles then you will understand a bit more
about altimetry, weather systems, high pressure, low pressure and winds all
of which are about the forces of air.