Air Pressure
Air consists of atoms
And molecules
Which both have mass.
Because air has mass, it also has
other properties, including
density and pressure.
Air Pressure
 Density is the amount of mass in a
given volume.
 You can calculate density by
dividing mass by volume.
Air Pressure
 Pressure is the force pushing on an
area or surface.
 A denser substance has more mass
than a less dense one.
 So denser air exerts more pressure
than less dense air.
Air Pressure
Air pressure is the result of the
weight of a column of air pushing down
on an area.
There is a column of air
above you all the time.
The weight of the air in
the atmosphere causes
air pressure.
Air Pressure
 Why doesn’t air pressure
crush your desk?
The molecules in air push in all directions. So the
air pushing down on the top of your desk is
balanced by the air pushing up on the bottom.
Measuring Air Pressure
Have you ever heard a
weather report say that
the air pressure is falling?
Measuring Air Pressure
Falling air pressure usually
indicates that a storm is
approaching.
Rising air pressure usually means
that the weather is clearing.
Measuring Air Pressure
A barometer is an instrument
that is used to measure changes
in air pressure.
 There are two kinds of
barometers:
mercury barometers
aneroid barometers
Measuring Air Pressure
 A mercury barometer consists of a glass tube open at
the bottom end and partially filled with mercury.
Measuring Air Pressure
 The word aneroid means “without
liquid.”
 An aneroid barometer has an
airtight metal chamber.
 The metal chamber is sensitive to
changes in air pressure.
 As the shape of the chamber
changes, the needle on the dial
moves.
Measuring Air Pressure
Increasing Altitude
 Altitude, or elevation, is the
distance above sea level.
 Sea level is the average level of the
surface of the oceans.
 Air pressure decreases as altitude
increases.
 As air pressure decreases, so
does density.
Altitude Affects Air Pressure
 Imagine a stack of ten books. Which book has more
weight on it, the second book from the top or the
book at the bottom?
 Air at sea level is like the bottom book.
 Air near the top of the atmosphere is like the second
book from the top.
Altitude Also Affects Density
 If you were near the top of a mountain and tried to
run, would you get out of breath quickly?
 Why would you have difficulty breathing at high
altitudes?
 As you go up through the atmosphere, air pressure
decreases
 As air pressure decreases, the density of air
decreases
Pressure and Density based on Elevation