Download 1 Water in the Air

Name
Class
CHAPTER 16
Date
Understanding Weather
SECTION
1 Water in the Air
National Science
Education Standards
BEFORE YOU READ
After you read this section, you should be able to answer
these questions:
ES 1f, 1i
• What is weather?
• How does water in the air affect the weather?
What Is Weather?
Knowing about the weather is important in our daily
lives. Your plans to go outside can change if it rains.
Being prepared for extreme weather conditions, such as
hurricanes and tornadoes, can even save your life.
Weather is the condition of the atmosphere at a certain time and place. Weather depends a lot on the amount
of water in the air. Therefore, to understand weather, you
need to understand the water cycle.
STUDY TIP
Outline Before you read,
make an outline of this section using the questions in
bold. As you read, fill in the
main ideas of the chapter in
your outline.
READING CHECK
THE WATER CYCLE
The movement of water between the atmosphere, the
land, and the oceans is called the water cycle. The sun is
the main source of energy for the water cycle. The sun’s
energy heats Earth’s surface. This causes liquid water
to evaporate, or change into water vapor (a gas). When
the water vapor cools, it may change back into a liquid
and form clouds. This is called condensation. The liquid
water may fall as rain, snow, sleet, or hail on the land.
1. Define Write your own
definition for weather.
STANDARDS CHECK
Condensation is the process
by which water vapor changes
from a gas to a liquid. Clouds
form by this process.
Evaporation is the
process by which
liquid water changes
into water vapor,
which is a gas.
Precipitation is rain,
snow, sleet, or hail
that falls from clouds
to Earth’s surface.
ES 1i Clouds, formed by the
condensation of water vapor,
affect weather and climate.
Word Help: affect
to change; to act upon
2. Identify By what
process do clouds form?
Runoff is water that flows over
land and into rivers, streams,
and eventually the ocean.
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Water in the Air continued
What Is Humidity?
READING CHECK
3. Identify How does air
temperature affect how
much water vapor the air
can hold?
Water vapor makes up only a small fraction of the mass
of the atmosphere. However, this small amount of water
vapor has an important effect on weather and climate.
When the sun’s energy heats up Earth’s surface, water
in oceans and water bodies evaporates. The amount of
water vapor in the air is called humidity. Warmer air can
hold more water vapor than cooler air can.
!MOUNTOF7ATER6APOR4HAT!IR#AN
(OLDAT6ARIOUS4EMPERATURES
Math Focus
!MOUNTOFWATER
VAPORGM
4. Read a Graph How much
water vapor can air at 30°C
hold?
n n
4EMPERATUREŽ#
RELATIVE HUMIDITY
Math Focus
5. Calculate What is the
relative humidity of 25°C air
that contains 10 g/m3 of
water vapor? Show your work.
Scientists often describe the amount of water in the air
using relative humidity. Relative humidity is the ratio
of the amount of water vapor in the air to the greatest
amount the air can hold.
There are two steps to calculating relative humidity.
First, divide the amount of water in a volume of air by the
maximum amount of water that volume of air can hold.
Then, multiply by 100 to get a percentage. For example,
1 m3 of air at 25°C can hold up to about 23 g of water
vapor. If air at 25°C in a certain place contains only
18 g/m3 of water vapor, then the relative humidity is:
3
18 g/m
_______
⫻ 100 ⫽ 78% relative humidity
3
23 g/m
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Water in the Air continued
FACTORS AFFECTING RELATIVE HUMIDITY
Temperature and humidity can affect relative humidity.
As humidity increases, relative humidity increases if the
temperature stays the same. Relative humidity decreases
as temperature rises and increases as temperature drops
if the humidity stays the same.
Critical Thinking
6. Compare How is relative humidity different from
humidity?
MEASURING RELATIVE HUMIDITY
Scientists measure relative humidity using special
tools. One of these tools is called a psychrometer. A psychrometer contains two thermometers. The bulb of one
thermometer is covered with a wet cloth. This is called
a wet-bulb thermometer. The other thermometer bulb is
dry. This thermometer is a dry-bulb thermometer.
You are probably most familiar with dry-bulb
thermometers. Wet-bulb thermometers work differently
than dry-bulb thermometers. As air passes through the
cloth on a wet-bulb thermometer, some of the water in
the cloth evaporates. As the water evaporates, the cloth
cools. The wet-bulb thermometer shows the temperature
of the cloth.
If humidity is low, the water evaporates more quickly.
Therefore, the temperature reading on the wet-bulb thermometer is much lower than the reading on the dry-bulb
thermometer. If the humidity is high, less water evaporates. Therefore, the temperature changes very little.
Dry-bulb thermometer
Wet-bulb thermometer
TAKE A LOOK
7. Identify What are two
parts of a psychrometer?
Scientists can use psychrometers like
this one to measure relative humidity.
The difference in temperature readings between the
dry-bulb and wet-bulb thermometers is a measure of the
relative humidity. The larger the difference between the
readings, the lower the relative humidity.
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Water in the Air continued
USING A RELATIVE-HUMIDITY TABLE
Scientists use tables like the one below to determine
relative humidity. Use the table to work through the following example.
The dry-bulb thermometer on a psychrometer reads
10ºC. The wet-bulb thermometer reads 7ºC. Therefore, the
difference between the thermometer readings is 3ºC. In
the first column of the table, find the row head for 10ºC,
the dry-bulb reading. Then, find the column head for 3ºC,
the difference between the readings. Find the place where
the row and column meet. The number in the table at this
point is 66, so the relative humidity is 66%.
Relative Humidity (%)
Dry-bulb
Difference between wet-bulb reading and
reading (ºC) dry-bulb reading (ºC)
TAKE A LOOK
8. Apply Concepts The
dry-bulb reading on a
psychrometer is 8ºC. The
wet-bulb reading is 7ºC.
What is the relative humidity?
1
2
3
4
5
6
7
8
0
81
64
46
29
13
2
84
68
52
37
22
7
4
85
71
57
43
29
16
6
86
73
60
48
35
24
11
8
87
75
63
51
40
29
19
8
10
88
77
66
55
44
34
24
15
12
59
78
68
58
48
39
29
21
14
90
79
70
60
51
42
34
26
16
90
81
71
63
54
46
38
30
18
91
82
73
65
57
49
41
34
20
91
83
74
66
59
51
44
37
What Is Dew Point?
READING CHECK
9. Explain What happens
when the temperature of air
is below its dew point?
What happens when relative humidity reaches 100%?
At this point, the air is saturated with water vapor. The
temperature at which this happens is the dew point. At
temperatures below the dew point, liquid water droplets
can form from the water vapor in the air.
Condensation happens when air is saturated with
water vapor. Air can become saturated if water evaporates and enters the air as water vapor. Air can also
become saturated when it cools below its dew point.
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Water in the Air continued
AN EVERYDAY EXAMPLE
You have probably seen air become saturated because
of a temperature decrease. For example, when you add
ice cubes to a glass of juice, the temperatures of the juice
and the glass decrease. The glass absorbs heat from the
air, so the temperature of the air near the glass decreases.
When the air’s temperature drops below its dew point,
water vapor condenses on the glass. The condensed
water forms droplets on the glass.
The glass absorbs heat from the air.
The air cools to below its dew point.
Water vapor condenses onto the side
of the glass.
Critical Thinking
10. Apply Concepts People
who wear glasses may notice
that their glasses fog up
when they come indoors on
a cold day. Why does this
happen?
TAKE A LOOK
11. Describe Where did the
liquid water on the outside
of the glass come from?
How Do Clouds Form?
A cloud is a group of millions of tiny water droplets or
ice crystals. Clouds form as air rises and cools. When air
cools below the dew point, water droplets or ice crystals
form. Water droplets form when water condenses above
0ºC. Ice crystals form when water condenses below 0ºC.
DIFFERENT KINDS OF CLOUDS
Scientists classify clouds by shape and altitude. The
three main cloud shapes are stratus clouds, cumulus
clouds, and cirrus clouds. The three altitude groups are
low clouds, middle clouds, and high clouds. The figure on
the next page shows these different cloud types.
READING CHECK
12. Explain How are clouds
classified?
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Water in the Air continued
Say It
Observe and Describe Look
at the clouds every day for a
week. Each day, write down
the weather and what the
clouds looked like. At the
end of the week, share your
observations with a small
group. How was the weather
related to the kinds of clouds
you saw each day?
(IGHCLOUDSAREFOUNDABOVEM
4HETEMPERATUREATTHISHEIGHTISVERYLOW
4HEREFOREHIGHCLOUDSAREMADEOFICE
CRYSTALS4HEPREFIXCIRROISUSEDTO
INDICATEHIGHCLOUDS
#IRRUSCLOUDSARETHINFEATHERY
LOOKINGCLOUDSMADEOFICE
CRYSTALS4HEYFORMATHIGH
ALTITUDESWHENTHEWINDISSTRONG
M
#IRRUS
#IRROCUMULUS
#IRROSTRATUS
M
#UMULONIMBUS
!LTOSTRATUS
-IDDLECLOUDSAREFOUNDBETWEENM
ANDM-IDDLECLOUDSCANBEMADE
OFWATERDROPLETSORICECRYSTALS4HEPREFIX
ALTOISUSEDTOINDICATEMIDDLECLOUDS
M
4HEPREFIXNIMBOANDTHE
SUFFIXNIMBUSAREUSEDTO
INDICATECLOUDSTHATARE
PRODUCINGPRECIPITATION
#UMULUSCLOUDSLOOKPUFFYLIKE
COTTONBALLS4HEYFORMWHENWARM
AIRRISES,ARGECUMULONIMBUS
CLOUDSCANPRODUCETHUNDERSTORMS
!LTOCUMULUS
#UMULUS
M
TAKE A LOOK
13. Compare How is a
nimbostratus cloud different
from a stratus cloud?
.IMBOSTRATUS
,OWCLOUDSAREFOUNDBELOWM
4HEYAREMADEOFONLYWATERDROPLETS
4HEREISNOPREFIXTHATISUSEDTO
INDICATELOWCLOUDS
3TRATUS
3TRATUSCLOUDSARELAYEREDCLOUDS
THATSTRETCHACROSSTHESKY4HEY
FORMWHENALARGEBODYOFAIR
RISES.IMBOSTRATUSCLOUDS
USUALLYPRODUCECONTINUOUSRAIN
What Is Precipitation?
READING CHECK
Water in the air can return to Earth’s surface through
precipitation. Precipitation is solid or liquid water that
falls to Earth’s surface from clouds. There are four main
kinds of precipitation: rain, snow, sleet, and hail. Rain
and snow are the most common kinds of precipitation.
Sleet and hail are less common.
14. Define What is
precipitation?
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Water in the Air continued
RAIN
Water droplets in clouds are very tiny. Each droplet is
smaller than the period at the end of this sentence. These
tiny droplets can combine with each other. As the droplets combine, they become larger. When a droplet reaches
a certain size, it can fall to Earth’s surface as rain.
SLEET
Sleet forms when rain falls through a layer of very cold
air. If the air is cold enough, the rain freezes in the air and
becomes falling ice. Sleet can make roads very slippery.
When it lands on objects, sleet can coat the objects in ice.
READING CHECK
15. Explain What happens
to water droplets in clouds
when they combine?
SNOW
Snow forms when temperatures are so low that water
vapor turns directly into a solid. That is, the water vapor in
the cloud turns into an ice crystal without becoming a liquid
first. Snow can fall as single ice crystals. In many cases, the
crystals join together to form larger snowflakes.
READING CHECK
16. Identify What is a
snowflake?
HAIL
Balls or lumps of ice that fall from clouds are called
hail. Hail forms in cumulonimbus clouds. Hail can
become very large. Hail grows larger in a cycle, as shown
in the chart below.
2. Low temperatures at the
top of the cloud cause the
raindrops to freeze, forming
tiny pieces of hail.
1. Rising air in a cloud carries raindrops to the top of
the cloud.
3. The hail falls through the
cloud, and more raindrops
collect on it.
This process
may repeat
many times.
4. Another body of rising air
carries the hail into the top
of the cloud again. There, the
rain freezes to the hail, making the hail larger.
TAKE A LOOK
17. Identify When does hail
fall to the ground?
5. Eventually, the hail
becomes too heavy to be
carried by the rising air. It
falls to Earth’s surface.
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Section 1 Review
NSES
ES 1f, 1i
SECTION VOCABULARY
cloud a collection of small water droplets
or ice crystals suspended in the air, which
forms when the air is cooled and
condensation occurs
condensation the change of state from a gas to
a liquid
humidity the amount of water vapor in the air
precipitation any form of water that falls to
Earth’s surface from the clouds
relative humidity the ratio of the amount of
water vapor in the air to the amount of water
vapor needed to reach saturation at a given
temperature
weather the short-term state of the atmosphere,
including temperature, humidity, precipitation,
wind, and visibility
1. Identify Relationships How is dew point related to condensation?
2. Identify What is the main source of energy for the water cycle?
3. Explain How do clouds form?
4. Compare What is the difference between sleet and snow?
5. Apply Concepts Fill in the spaces in the table to describe different kinds of clouds.
Name
Altitude
Cirrostratus
high
Altocumulus
Shape
Precipitation?
no
puffy
Nimbostratus
Cumulonimbus
low to middle
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Earth Science Answer Key continued
9. toward the west
10. upper troposphere, lower stratosphere
11. The jet stream is in different places. The pilot
8. Ozone in the stratosphere blocks UV light,
which can be harmful to humans.
9.
would want to catch or avoid the jet stream.
12. Global winds blow in one direction, but
local winds can blow in any direction.
13. An L should be on the arrow end of each
wind path; winds blow from H to L.
Ozone near the
ground
Forms naturally
from human activity
Is not a pollutant
a pollutant
Protects Earth from
UV rays
harmful to living things
10. Short-term effects happen quickly and go
Review
1. Arrows can go clockwise or counterclock-
away once the pollution is gone. Long-term
effects develop over a long time and do not
go away easily.
11. The electricity to run them must be generated. Many electrical power plants burn fossil fuels to generate electricity. This causes
pollution in the areas near, and downwind
of, the power plants.
wise; arrows should point from H to L;
sinking air should be labeled “C”; rising air
should be labeled “W.”
2. westerlies
3. During the day, the mountains warm up
and the air above them warms and rises.
At night, as the mountains cool off, the air
cools down and sinks, producing winds.
4. No. Winds are caused by differences in air
pressure, which are caused by differences in
temperature.
Review
1. Burning fossil fuels puts primary pollutants
into the air, causing air pollution. Some of
these pollutants can combine with water in
the atmosphere to make acid precipitation.
Vehicle exhaust combines with sunlight and
forms secondary pollutants such as ozone.
SECTION 4 AIR POLLUTION
1. Ozone forms when other pollutants react
2.
with one another and with air in the presence of sunlight.
2.
Ozone in the
stratosphere
Pollutant
Source
Negative
effects
Solutions
CFCs
air conditioners
and aerosol
sprays
holes in the
ozone layer
ban on
CFCs
Ozone
vehicle
exhaust
that reacts
with air
lung
damage
driving
limits,
vehicle
emission
controls
Sulfur
dioxide
burning of
fossil fuels
acid rain
less fossil
fuel use
Pollutant
Primary
pollutant or
secondary
pollutant?
Natural or
caused by
people?
Car exhaust
primary
human-caused
Dust
primary
natural or
human-caused
Ozone
secondary
human-caused
Paint chemicals
primary
human-caused
Pollen
primary
natural
3. Answers include: soil, water, plants, animals,
Sea salt
primary
natural
Volcanic ash
primary
natural
buildings
4. The thinning ozone layer lets harmful UV
radiation reach Earth. UV can cause skin
cancer.
3. vehicle exhaust
4. motor vehicles
5. If there is not enough ventilation, pollutants
Chapter 16 Understanding
Weather
can get trapped inside.
6. Answers will vary.
7. People burn coal for energy.
Pollutants are released.
Pollutants combine with water in the air.
Acid rain falls in the lake.
Fish die.
SECTION 1 WATER IN THE AIR
1. Weather is a description of what is going on
in the atmosphere at a certain time and place.
2. condensation
3. Warmer air can hold more water vapor.
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Earth Science
Earth Science Answer Key continued
4. about 30 g/cm3
5. 10 g/m3 23 g/m3 0.43
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
5.
0.43 × 100 43% relative humidity
Humidity is the absolute amount of water
vapor in the air. Relative humidity is a measure
of how saturated with water vapor the air is.
wet-bulb and dry-bulb thermometers
about 87%
Water vapor condenses into liquid water.
The glasses are as cold as the air outside.
They absorb heat from the air inside. As a
result, the air inside cools below its dew
point. Water condenses on the glasses.
water vapor in the air
by shape and by altitude
A nimbostratus cloud is producing
precipitation.
water that falls to Earth from clouds
They get bigger.
several ice crystals joined together
when it is too heavy to be carried by
updrafts
How it affects weather
cP from northern
Canada
very cold winter weather and
cool, dry summer weather
mP from the North
Pacific Ocean
rain and snow in the Pacific
in the winter, fog in the
summer
mT from the Gulf of
Mexico
heat, humidity, hurricanes,
thunderstorms in East Coast
and Midwest
cT from the deserts
clear, dry, hot weather in the
summer
6. a place where two or more air masses meet
7. It rises because the cold air pushes it up.
8. a place where a warm air mass moves over a
9.
10.
11.
12.
Review
1. Possible answer: Water can condense from
cold air mass
The cold air masses push it out of the way.
Stationary fronts do not move, so the
weather they bring stays in one place.
Cyclones have lower pressures than surrounding areas, but anticyclones have higher
pressures. Cyclones bring rainy or stormy
weather, but anticyclones bring dry, clear
weather.
anticyclone to cyclone
Review
1. A front is a place where two air masses meet.
2. First row, from left to right: rises and cools;
air that is below its dew point.
2. the sun
3. Air rises and cools. When it cools below its
dew point, water vapor condenses to form
liquid water droplets or ice crystals. These
droplets or ice crystals form a cloud.
4. Sleet forms when liquid water freezes in clouds
and falls to the ground as ice. Snow forms
when water vapor turns directly into a solid.
5. Name
Altitude Shape
Precipitation?
cloudy, rainy, and stormy
Second row, from left to right: ... higher than
surrounding pressure; dry and clear
3. cold, warm, occluded, stationary
4. Pacific Ocean, Atlantic Ocean, Gulf of
Mexico
5. cT
SECTION 3 SEVERE WEATHER
Cirro-stratus
high
layered
no
1. It must contain enough water vapor to form
Altocumulus
middle
puffy
no
Nimbostratus
low
layered
yes
Cumulonimbus
low to
middle
puffy
yes
clouds.
Electricity travels between an area with a
positive charge and an area with a negative
charge.
sound produced by vibrations in expanding
air that was superheated by lightning
The center of the tornado has low pressure.
when it touches the ground
74 mph to 112 mph
Ocean water is too cold there.
the Coriolis effect
the center of the hurricane that contains
warm, fairly calm air with low pressure
2.
3.
4.
5.
6.
7.
8.
9.
SECTION 2 AIR MASSES AND FRONTS
1.
2.
3.
4.
Air mass
by water content and temperature of the air
dry, warm
northern Canada
Warm air masses form over warm water or
land, and the North Atlantic and Pacific are
cold.
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