- Air has mass and pressure due to gravity pressing down on its mass. Denser air has more pressure. - Atmospheric pressure can be measured using barometers like mercury or aneroid barometers. Weather reports use units like inches of mercury or millibars to indicate pressure. - Air pressure decreases with increasing altitude and elevation due to there being less air above pressing down. The higher the altitude, the less dense the air becomes.

1. Air Pressure
Notes

2. Properties of Air
Oxygen (O2)
Nitrogen (N2)
CO2, H2O,
and
others

3. Properties of Air
• Because air is made up of atoms of nitrogen,
oxygen, carbon, and many others, it has mass.
Because air has mass, it also must have
density.
• Remember, density is the amount of mass in a
certain volume (or how tightly packed particles
are).
• Density = mass/volume m
d v
or

4. Properties of Air
• Also because air has mass, it has pressure.
Pressure is the force pushing on an area or a
surface.
• Air has pressure because air’s mass has weight
due to gravity. It may not seem like a lot, but it is
there. This weight presses down.
• Remember. mass x gravity = weight
• Air Pressure is the pressing down of the weight
of air on an area. Denser air has more air
pressure then less dense air.

5. The weight of all
of this air
presses down
onto the surface,
creating air
pressure.

6. Measuring
Atmospheric
Pressure
• A barometer is an instrument that measures air
pressure.
• One kind of barometer is a mercury barometer.
A mercury barometer is made up of a glass
tube that is open at the bottom end. The open
end of the tube sits in a dish of mercury. As the
air pressure increases on the dish of mercury,
the level in the tube will rise.

8. Measuring
Atmospheric
Pressure
• An aneroid barometer has a closed
space surrounded by metal. Air
pressure pushes the metal in and out.
As the shape of the space changes, a
needle moves to show what the air
pressure is.
• If you have a barometer in your house,
hanging on the wall, this is the same
type of barometer.

10. Measuring
Atmospheric
Pressure
• Weather reports use several different units for
atmospheric pressure. Most weather reports for the
general public use inches of mercury (inHg). For
example, if the column of mercury in a mercury
barometer is 30 inches high, the air pressure is 30
inHg.
• Some systems use mmHg, or millimeters of Hg, as
well.
• Hg is the chemical symbol for mercury.
• National Weather Service maps indicate air pressure
in millibars (mb). One inHg is approximately
33.87 mb, so 30 inHg is approximately equal to
1,016 mb.

12. Altitude and
Atmospheric
Pressure
• Altitude is the distance above sea
level. Altitude is also known as
elevation.
• Elevation is usually used if the object
being referenced is still touching the
ground.
• Sea level is the average level of the
ocean’s surface.

13. Altitude and
Atmospheric
Pressure
• Air at sea level carries the weight of all
of the air above it. As you go higher, the
air has less air on top of it, and
therefore it has less weight. So, air
pressure at sea level is greater than the
air at any place above it.

14. Sea Level
Pressure will decrease if air is
rising for any reason.
Pressure will increase if air is
sinking.
More on this later.

15. Notice how
the weight
would
decrease as
the pyramid
goes higher
and higher.

16. Altitude and
Atmospheric
Pressure
• Altitude will also affect the density of the
air.
• Because gravity is affected by both
mass AND distance, gravity can more
easily hold gases near the surface due
to their low masses.
• As you go higher, the air becomes less
and less dense. As you get closer to
sea level, the air becomes more and
more dense.

18. Altitude and
Atmospheric
Pressure
• Because the air is less dense, there are
fewer molecules of nitrogen and oxygen
in a given area.
• This is why, at the top of a mountain, it
becomes very difficult to breathe and air
tanks are required.