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					Properties of Air

What is Air?

Air is made up of
78% Nitrogen
21% Oxygen
1% Other gases
              Properties of Air
• Air is a mixture of
    elements, compounds
    and molecules . Air has
    mass, volume, and
    therefore density.
•   The more molecules we
    put in a given volume of
    air, the more mass it will
    have. So this air will be
    more dense.
• Pressure is a force that acts over a certain
•   Liquids and gases are fluids. Fluids are any
    material that is able to flow. Fluids exert
    pressure because of the motion of their
•   Pressure will always move from a high
    pressure to a low pressure area. The
    pressure will always try to equalize. You see
    this when you get a hole in your bicycle tire.
 Inflating example
• Air inside a ball
  pushes against the
  sides. The more air
  we put in a ball, the
  more the molecules
  push. So the
  pressure increases
  as we add air.

• If we increase the force, what should
  happen to the pressure?
• It should increase.
• If we increase the area over which we
  apply a force, what should happen to the
• It should decrease!
Air Pressure
• Air pressure is the
    result of the column
    of air that is above
•   There is so much air
    above you that at
    sea level you have
    14.7 lbs/in2 pushing
    on you.
Why are we not crushed by air
• Air pressure is equal
    in all directions.
•   So air pushes equally
    in all sides of us.
    The forces are
Measuring Air Pressure
• We use barometers to
    measure air pressure.
•   Mercury barometer: a
    glass tube sealed at the
    top partially filled with
•   Air pressure pushing on
    the mercury in the dish
    causes the mercury to
    rise and fall in the tube.
Aneroid Barometers
 • Aneroid Barometer:
  Works without
  liquid. Has an
  airtight metal can
  that is sensitive to
  changes in air
  pressure. This
  chamber is
  connected to a dial.
Units of Air Pressure
• TV weather stations
  and aviation use inches
  of mercury.
• Meteorologists (and the
  NWS) use millibars, an
  SI unit. A unit of
  pressure equal to one-
  thousandth of a bar or
  100 pascals, most
  commonly used to
  measure atmospheric
Altitude and Pressure
• Altitude (or
    elevation) is the
    distance above sea
•   The higher the
    altitude, the less air
    there is above you.
    So as altitude
    increases, air
    pressure decreases.
Pressure and Boiling Points

• As the air pressure decreases, it takes
 less energy to cause water to boil.
 There is less force pushing down on the
 water at higher altitudes, so it is easier
 for the water molecules to escape into
 the air. If you go high enough you could
 boil water at room temperature. It
 would not burn you, and would not cook
 your food!
Pressure analogy
• Imagine a stack of
 books. The bottom
 book feels all the
 weight of the all the
 books stacked above
 it, while the second
 one up feels slightly
 less weight. The
 higher in the stack,
 the less pressure
 one would feel.
Altitude and Density
• As the air pressure
    decreases, the density of
    the air decreases. The
    air particles are not
    squashed together as
    tightly the higher one
•   The air at sea level and
    at 6km has the same 21%
    oxygen, but at 6km there
    are fewer molecules, so
    you take in less oxygen
    with each breath.
Madgeburg Hemispheres
• Air pressure is
  surprisingly strong.
  If we have two
  hollow metal spheres
  full of air, the
  pressures are equal
  so they will easily
Madgeburg Hemispheres
• But if we remove the air
  from the hollow spheres,
  there will be much lower
  pressure on the inside.
  The higher pressure
  outside will push in on the
  spheres. If the spheres
  are small, with a diameter
  of 4 inches, it will take
  over 180 pounds of force
  to pull them apart.
Pressure Demo
• If we attached half
    of the sphere to the
    ceiling and removed
    the air, Mr. Brown
    would be able to
    hang from it.
•   This is also why the
    soda can was so
    easily crushed by
    the air pressure in
    the room.
Layers of the Atmosphere
• Four layers
• classified by
  changes in
Temperature and the Troposphere

• Troposphere:
    temperature at
    surface is warmed by
    the earth absorbing
    energy from the sun.
•   The air cools by about
    6.5°C for every 1-km
    above the ground.
Temperature and the
• Ozone absorbs
 ultraviolet radiation
 from the sun,
 causing the
 temperature to
Temperature and the
• This layer does not
  absorb energy from
  the sun, so it starts
  to cool again.
Temperature and the
• Solar radiation first hits
  this layer, so the few
  particles that are here
  can gain lots of energy.
  They move rapidly, so they
  have a very high
• But the air is so thin here
  that it takes special
  instruments to measure
  the temperature
The Troposphere
•   Lowest (inner) layer
•   weather occurs here
•   we live in it.
•   “tropo” means turning
    or changing conditions
•   depth varies from 9km
    above the poles to
    16km at the equator
•   shallowest layer, but
    contains most of the
The Stratosphere
• “strato” means layer
    or spreading out
•   Contains the ozone
•   Temperature
The Mesosphere
• Drop in temperature
    marks beginning of
•   “Meso” means middle
•   Most meteors burn
    up here
The Thermosphere
• Very top layer
• Air is very, very thin, about 1/1000th as dense
    as the air at sea level
•   “Thermo” means heat
•   Extends from 80km to space
•   No definite outer edge
•   Very hot (1800°C), but since air is so thin it
    would not feel warm at all.
•   Divided into two parts, the ionosphere and
    the exosphere
The Ionosphere
• Energy from sun strips the
    electrons from the gas
    molecules creating charged
    particles called ions.
•   Radio waves can bounce off of
    ions, allowing radio waves to
    travel great distances.
•   The aurora borealis (Northern
    Lights) occur here
The Exosphere
• “Exo” means outer
• Extends for 1000’s
    of miles
•   Satellites orbit here

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