The Air
Around You
Earth’s Atmosphere
• Earth’s atmosphere is the envelope of gasses that
surround the planet.
• Nitrogen makes up 78% of our air
• Oxygen makes up 21% of our air
• The remaining 1 % are called trace gasses.
• How many more times Nitrogen than Oxygen in
our air?
• Read about nitrogen and oxygen on page 7-8 in
your book and answer questions 4-8 in your
notes.
Water Vapor
• Water in the form of a
gas is called water
vapor
• Water vapor is not the
same as steam
because steam is
warm air with tiny
droplets of water in it.
• What role does water
vapor play in Earth’s
weather?
How does Earth’s atmosphere make
conditions on Earth suitable for living things?
• Oxygen and other gases
needed for life
• Constantly moving in and
out of living things
• Warmth
• Liquid water
• Protects from radiation
and meteoroid
Do the next page in your notes
(pg. 50) on your own as review.
What you don’t get done is
homework.
When you are finished, begin to read
section two (Air Pressure).
Properties of Air
• Weight of the atmosphere is constantly
pushing on your body
• Air has mass because it is composed of
atoms and molecules.
• Because air has mass, it has density and
pressure
• The more molecules in a given volume of
air, the greater its density.
Cause and Effect
• If mass increases and volume stays the same
then density increases
• If mass decreases and volume stays the same
then density decreases
• If mass stays the same and volume decreases
then density increases
• If mass stays the same and volume increases
then density decreases
Measuring Air Pressure
• An instrument used to measure air
pressure is a barometer.
• Mercury barometers have liquid mercury
forced up a column when air pressure
increases
• Aneroid barometer has no liquid and has
thin walls and air tight metal chamber that
bulges when air pressure increases.
Mercury Barometer
Aneroid Barometer
1644- Evangelista Torricelli
1843- Lucien Vidie
Read pp. 12-13
Aneroid = “without fluid”
Units of Air Pressure
• Most weather reports for the general
public use inches of mercury. For
example, if the column of mercury in a
mercury barometer is 30 inches high, the
air pressure is 30 in.
• National Weather Service maps indicate air
pressure in millibars. One inch of mercury
is approximately 33.87 millibars.
Isobars are lines on maps that join places that have the
same air pressure.
Altitude
• Another word for elevation, or distance
above sea level
• At the top of a mountain the air pressure is
less than the air pressure at sea level.
• Air pressure decreases as altitude increases.
• As air pressure decreases, so does density.
• Read “Altitude Affects Air Pressure” on pg.
13 to answer why air pressure is greater at
sea level than at the top of a mountain.
Altitude also affects density
• As you go up through the atmosphere the
density of air decreases.
• The gas molecules that make up the
atmosphere are farther apart at higher
altitudes.
• If you were near the top of a tall mountain
and began to run, you would run out of
breath more quickly than at sea level.
Why?
– The air contains 21% Oxygen at any level, but
since the air is less dense at higher altitudes
there are fewer oxygen molecules to breath in
each cubic meter if air than at sea level.
• Explain why mountain climbers sometimes
bring tanks of oxygen along with them on
their climbs.
Do page 57 and 58 for homework
**Do not give up on page 58! Be sure to read the
directions first. They will help you find the right
answers!**