Name ____________________________ Date ____________________ Class ____________
The Atmosphere
Guided Reading and Study
■
Air Pressure
This section describes several properties of air, including density and air pressure. The
section also explains how air pressure is measured and how it changes with altitude.
Use Target Reading Skills
As you read about the properties of air, fill in the detail boxes that explain the main
idea in the graphic organizer below.
Main Idea
Because air has mass, it also . . .
Detail
Detail
b.
a.
Introduction
1.
Suppose that you are not carrying anything on your back. Why do your
shoulders still have pressure on them?
________________________________________________________________________
________________________________________________________________________
Properties of Air
2.
Circle the letter of each sentence that is true about air.
a. Air has mass because it is composed of atoms and molecules.
b. Because air has mass, it has density and pressure.
c. The more molecules in a given volume of air, the greater its density.
d. The greater the density of air, the less pressure it exerts.
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Name ____________________________ Date ____________________ Class ____________
The Atmosphere
3.
■
Guided Reading and Study
Complete the cause-and-effect table to show the relationship among
mass, volume, and density.
EFFECT
If mass
and volume
then density
increases
stays the same,
a.
b.
stays the same,
decreases.
stays the same
decreases,
c.
stays the same
d.
decreases.
The Atmosphere
CAUSE
e. Use the information in the table to write one or two sentences about
the relationship among mass, volume, and density.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
Measuring Air Pressure
4.
An instrument that is used to measure air pressure is a(n) ________________________.
5.
What is the difference between how air pressure is indicated in a
mercury barometer and an aneroid barometer?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
6.
Draw a line on the glass tube to show where the
level of the mercury might be if the air pressure fell.
Mercury
Air pressure
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Air pressure
Name ____________________________ Date ____________________ Class ____________
The Atmosphere
■
Guided Reading and Study
7. Two different units used to measure air pressure are ________________________
and ________________________.
8.
If the air pressure is 30 inches, how many millibars of air pressure are there?
_________________________________________________________________________
________________________________________________________________________
Altitude and the Properties of Air
9. Another word for elevation, or distance above sea level, is _____________________.
10. Is the following sentence true or false? Air pressure increases as altitude
increases. ________________________
11. Is the following sentence true or false? As air pressure decreases, so does
air density. ________________________
12. Why is air pressure greater at sea level than at the top of a mountain?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
13. Is the following sentence true or false? As altitude increases, so does air
density. ________________________
14. Explain why mountain climbers sometimes bring tanks of oxygen along
with them on their climbs.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
15. Circle the letter of the sentence that helps explain why you would have
more difficulty breathing at high altitudes than at sea level.
a. Air pressure is higher at high altitudes.
b. Density of the air is greater at high altitudes.
c. The percentage of oxygen in the air is lower at high altitudes.
d. The amount of oxygen in each breath is less at high altitudes.
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Name ____________________________ Date ____________________ Class ____________
The Atmosphere
■
Section Summary
Air Pressure
Key Concepts
What are some of the properties of air?
■
What instruments are used to measure air pressure?
■
How does increasing altitude affect air pressure and density?
Air consists of atoms and molecules that have mass. Therefore, air has
mass. Because air has mass, it also has other properties, including density
and pressure. The amount of mass in a given volume of air is its density. The
force pushing on an area or surface is called pressure. Air pressure is the
result of the weight of a column of air pushing down on an area. The
molecules in air push in all directions. This is why air pressure doesn’t crush
objects.
A barometer is an instrument that is used to measure air pressure. Two
common kinds of barometers are mercury barometers and aneroid
barometers. A mercury barometer consists of a glass tube open at the
bottom end and partially filled with mercury. The open end of the tube rests
in a dish of mercury, and the space above the mercury in the tube contains
almost no air. The air pressure pushing down on the surface of the mercury
in the dish is equal to the weight of the column of mercury in the tube. At sea
level, the mercury column is about 76 centimeters high, on average. An
aneroid barometer has an airtight metal chamber that is sensitive to changes
in air pressure. The thin walls of the chamber flex in and out as air pressure
changes, and the movements are recorded on a dial.
In weather reports, air pressure usually is given in inches of mercury.
National Weather Service maps indicate air pressure in millibars. One inch
of mercury equals 33.87 millibars.
Altitude, or elevation, is the distance above sea level. Air pressure
decreases as altitude increases. As air pressure decreases, so does density.
Sea-level air has the weight of the whole atmosphere pressing on it, so air
pressure is highest at sea level. Air pressure is much lower at the tops of
mountains. There the low density of air can make it hard to breathe because
there is less oxygen in each cubic meter of air.
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The Atmosphere
■
Name ____________________________ Date ____________________ Class ____________
The Atmosphere
Review and Reinforce
■
Air Pressure
Altitude is greater at point _______.
Air pressure is greater at point _______.
A
Density of the air is greater at point _______.
A cubic meter of air has less mass at point _______.
The percentage of oxygen in the air at point A is about
B
_______ percent.
6. State three properties of air.
______________________________________________
7. Why doesn’t air pressure crush objects such as
your desk?
_________________________________________________________________________
1.
2.
3.
4.
5.
8. What two units of air pressure are commonly used in weather reports?
_________________________________________________________________________
Building Vocabulary
Match each term with its definition by writing the letter of the correct definition on
the line beside the term.
____
9. air pressure
a. the amount of mass in a given volume of air
____ 10. altitude
b. force pushing on an area or surface
____ 11. aneroid barometer
c. the result of the weight of a column of air
pushing down on an area
____ 12. barometer
d. any instrument that measures air pressure
____ 13. density
e. instrument that measures air pressure using
liquid mercury
____ 14. mercury barometer
____ 15. pressure
f. the distance above sea level
g. instrument that measures air pressure without using a liquid
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The Atmosphere
Understanding Main Ideas
Study the figure below, and then complete the following statements.
Name ____________________________ Date ____________________ Class ____________
The Atmosphere
■
Enrich
Isobars and Air Pressure
Air pressure is an important factor affecting weather. Changes in air
pressure help weather forecasters predict how the weather will change.
Falling air pressure usually indicates stormy weather. Rising air pressure
means that the weather is clearing. Air pressure readings from barometers
are shown on weather maps, like the one below, with lines called isobars.
Isobars are drawn to connect areas that have the same air pressure.
1016
1012
992
New York
1000
0
1004
1016
Los Angeles
eles
1008
8
1004
1008
1012
1012
Refer to the map to complete the following statements.
1.Each isobar differs from the next isobar by ________________________ millibars.
2.The lowest air pressure reading on the map is ________________________ millibars.
3.Where this low pressure occurs, the weather is likely to be _____________________.
4.The highest air pressure reading on the map is ________________________ millibars.
5.This high-pressure area is likely to be experiencing ________________________
weather.
6.An area of ________________________ air pressure is centered northwest of Chicago.
7.The air pressure in Chicago is about ________________________ millibars.
8.The air pressure in Los Angeles is about ________________________ millibars.
9.The air pressure in New York City is about ________________________
millibars.
10.Most of Florida has a barometric pressure between about 1008 millibars
and ________________________ millibars.
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Name ____________________________ Date ____________________ Class ____________
The Atmosphere
Skills Lab
■
Working Under Pressure
Problem
How can a barometer detect changes in air pressure?
Materials
modeling clay
white glue
pencil
metric ruler
large rubber balloon
drinking straw, 12–15 cm long
cardboard strip, 10 cm × 25 cm
scissors
tape
wide-mouthed glass jar
rubber band
Procedure
Review the safety guidelines in Appendix A.
1. Cut off the narrow opening of the balloon.
2. Fold the edges of the balloon outward. Carefully stretch the balloon over the open
end of the glass jar. Use a rubber band to hold the balloon on the rim of the glass jar.
3. Place a small amount of glue on the center of the balloon top. Attach one end of the
straw to the glue. Allow the other end to extend several centimeters beyond the edge
of the glass jar. This is your pointer.
4. While the glue dries, fold the cardboard strip lengthwise and draw a scale along the
edge with marks 0.5 cm apart. Write “High pressure” at the top of your scale and
“Low pressure” at the bottom.
5. After the glue dries, add a pea-sized piece of modeling clay to the end of the pointer.
Place your barometer and its scale in a location that is as free from temperature
changes as possible. Arrange the scale and the barometer as shown in the diagram in
your textbook. Note that the pointer of the straw must just reach the cardboard strip.
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The Atmosphere
Measuring
Air pressure changes are related to changing weather conditions. In this lab,
you will build and use your own barometer to measure air pressure.
Name ____________________________ Date ____________________ Class ____________
The Atmosphere
Skills Lab
■
Working Under Pressure (continued)
6. Tape both the scale and the barometer to a surface so they do not move during your
experiment.
7. In your notebook, make a data table, or use the one below. Record the date and time.
Note the level of the straw on the cardboard strip.
8. Check the barometer twice a day. Record your observations in your data table.
9. Record the weather conditions for at least three days.
Data Table
Date and Time
Air Pressure
Weather Conditions
Analyze and Conclude
Write your answers on a separate sheet of paper.
1. Interpreting Data What change in atmospheric conditions must occur to cause the
free end of the straw to rise? What change must occur for it to fall?
2. Drawing Conclusions Based on your observations, what kind of weather is usually
associated with high air pressure? With low air pressure?
3. Communicating Write a paragraph in which you discuss what effect, if any, a large
temperature change might have on the accuracy of your barometer.
More to Explore
Compare your pressure readings with high and low air pressure readings
shown on newspaper weather maps during the same period. How do your
readings compare with those in the newspapers?
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