Download Activity 2 A Thunderstorm Matures

Ch 2 Severe Weather
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4:59 PM
Page 77
Activity 2 A Thunderstorm Matures
Activity 2
A Thunderstorm Matures
Goals
In this activity you will:
• Investigate relationships between
air temperature and water vapor.
• Observe and record changes in the
shape and movement of clouds.
• Simulate cloud formation in a
bottle.
• Describe cloud formation in terms
of expansional cooling.
• Explain the conditions that cause
thunderstorms to mature.
• Learn about conditions that may
lead to the development of severe
weather.
Think about It
Thunderstorm clouds can build tens of thousands of feet up into
the atmosphere as they move over Earth’s surface.
• What causes towering rain clouds to form?
• What causes such clouds to move from place to place?
What do you think? Record your ideas about these questions in
your EarthComm notebook. Include a diagram showing towering
cumulus clouds and any relationships that you can make between
how they form and how they move. Be prepared to discuss your
responses with your small group and the class.
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Investigate
2. Look at the photograph below of an
intense thunderstorm that has
reached the mature stage. The
photograph was taken some distance
from the thunderstorm so that the
full height of the cloud can be seen.
Part A: Characteristics of Clouds
1. Examine the three photographs
shown below. The photographs are
of the same mass of clouds, taken
about 10 minutes apart. (Note that
the distance from the cloud mass is
increasing.)
a) In your notebook, draw a diagram
of the shape of the clouds. Using
arrows on the sketch, indicate the
directions of the wind.
b) Explain why you think the cloud
has a relatively flat top.
3. Spend some time outside observing
the sky.
a) In your notebook, record wind
direction, wind speed (light,
moderate, strong), temperature, and
precipitation. Also sketch the
different kinds of clouds you see
(if any).
b) Describe any changes in the shape
and location of clouds over time.
c) Describe any differences in the
appearance of clouds at different
altitudes.
a) What changes can you see in the
appearance of the clouds?
d) Do any of the clouds look like
they are producing precipitation?
How do you know?
b) How do you explain these changes?
c) Draw a diagram in your notebook
that shows how you think these
changes occurred.
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Activity 2 A Thunderstorm Matures
Part B: Observing the Behavior
of Water Vapor
1. Half fill a 500-mL beaker with roomtemperature water. Carefully observe
the outside of the beaker while
slowly adding ice cubes one by one.
Part C: Making Clouds
1. Fill a clean, transparent plastic
2-L bottle with very warm (but not
steaming hot) tap water. Pour the
water out until about two 2 cm of
water remain in the bottle.
2. Light a match and immediately blow
out the flame. Hold the bottle
sideways and place the smoking
match in the neck of the bottle to
allow a little smoke to enter the
bottle. Replace the cap on the bottle.
a) Record your observations in your
notebook.
b) Explain your observations.
Consider how adding ice changed
the situation.
2. Obtain two clean, transparent plastic
2-L bottles. Fill one bottle with very
hot tap water and the other with cold
tap water. Pour water out of both
bottles until about 3 cm of water
remains in each.
Place an ice cube on the top of the
open neck of each bottle. Observe the
bottles for several minutes.
a) Record your observations of each
bottle in your notebook. What
differences do you observe
between the two bottles?
3. Crush the bottle against the edge of
the table. Then make the bottle
return to its original shape.
a) Record your observations in your
notebook.
b) How can you explain the
difference?
b) Why was smoke from a smoldering
match introduced into the bottle?
Be careful picking up the beaker.
The sides may be wet and slippery.
Be careful with the hot water. It should
not be hot enough to scald. Use a funnel to put
the hot water into the bottle. Use insulated gloves
or pot holders to hold the bottle.
Wear safety goggles. Place burned
matches in a cup of water to soak before
placing them in the trash can.
Reflecting on the Activity and the Challenge
In this activity you noted and recorded
changes in the shape and movement of
clouds using images and outdoor
observations. You also explored the
relationship between air temperature
and water vapor. You explored some of
the conditions under which water vapor
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condenses into clouds. In order to
complete the Chapter Challenge, you
will need to understand why clouds
sometimes produce severe weather and
create hazards. To do so, you need to
understand the formation of clouds.
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Digging Deeper
HOW CLOUDS FORM
All air contains water vapor, which is water that exists in the atmosphere as
an invisible gas.Water vapor is not the same as clouds, fog, or steam, which
are composed of tiny, visible droplets of water suspended in the atmosphere.
There is an upper limit to the concentration of water vapor in air.When
that upper limit is reached, the air is saturated with water vapor.The upper
limit depends on temperature.Warm saturated air has more water vapor
than cool saturated air.When air is saturated with water vapor, some water
vapor condenses into tiny water droplets (or, if the temperature is very low,
forms tiny ice crystals) that are visible in the form of clouds.The most
common way clouds develop in the atmosphere is by cooling of air as it
rises in the atmosphere.
Ascending air cools because of expansion. A gas cools when it expands.
Consider a familiar example of expansional cooling. Air inside a bicycle tire is
under pressure.When you open the tire valve, the air that escapes is
relatively cool to the touch.The escaping air expands because it is entering
the atmosphere, where air pressure is much less than it is inside the tire. Air
does the work of expansion as it escapes the tire.Work requires energy, and
that energy is drawn from the internal heat energy of the air, causing a drop
in temperature.
Geo Words
air pressure: the cumulative
force of a multitude of air
molecules colliding with a unit
surface area of any object in
contact with air.
Air pressure is the cumulative force of a multitude of air molecules
colliding with a unit surface area of any object in contact with air.You can
think of air pressure as the weight of a column of air acting on a unit area at
the base of the column. Air pressure always decreases with increasing
altitude, because the mass of air above steadily diminishes (Figure 1). Air that
rises, as in an updraft, expands because it encounters progressively lower air
pressures. As air does the work of expansion, it cools.With sufficient
cooling, the air becomes saturated, and excess water vapor condenses into
droplets, which form clouds.This happens in the updraft of a thunderstorm.
Conversely, when a gas (or mixture of gases) is compressed, the environment
does work on the gas and its temperature rises. For example, as air is
pumped into a bicycle tire it is compressed, causing the air to warm and the
tire to become warm to the touch.Air descending in the atmosphere
encounters steadily increasing air pressure and is compressed and warms.
Compressional warming occurs in the downdraft of a thunderstorm. During
the dissipating stage of a thunderstorm, the downdraft spreads through the
thunderstorm cloud, and with compressional warming, the cloud vaporizes.
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34
standard sea-level pressure
30
26
altitude(km)
22
stratosphere
18
14
Geo Words
condensation nuclei: tiny
solid and liquid particles that
water vapor can condense on.
10
6
troposphere
2
0
200
600
400
pressure (mb)
800
Figure 1 Graph showing the
relationship between altitude and air
pressure.
1000
In addition to saturated air, cloud formation requires surfaces on which the
water vapor can condense. In Part B of the investigation, the outside of the
beaker provided a surface. In Part C of the investigation, smoke particles
provided surfaces. Earth’s atmosphere contains an abundance of tiny solid and
liquid particles that water vapor can condense on.The particles are called
condensation nuclei. Nuclei are products of many different natural and
human-related activities. Forest fires, volcanic eruptions, wind erosion of soil,
saltwater spray, motor vehicle exhaust, and various industrial emissions are all
sources of nuclei. Some nuclei promote condensation (called condensation
nuclei) and others cause formation of ice crystals (called ice-forming nuclei).
Types of Clouds
Meteorologists classify clouds into three broad categories based upon shape:
cirrus, stratus, and cumulus. Cirrus clouds are wispy, stratus clouds are
layered, and cumulus clouds are puffy (like cottonballs). Clouds are further
classified according to their altitude (high, middle, low, or clouds with vertical
development) and composition (water droplets or ice crystals), as shown in
the photographs on the following pages.
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High Clouds (cloud base above 6000 m, about 20,000 ft.)
Cirrus: Thin, wispy and feathery; composed
of ice crystals; also called “mares’ tails.”
Cirrocumulus: Puffy and patchy in
appearance; may form wave-like patterns,
which are sometimes called “mackerel sky.”
Cirrostratus: Light gray or white; thin
sheets of ice crystals that usually cover
much of the sky; may create a halo around
the Sun or moon.
Middle Clouds (cloud base 2000–6000 m, about 6500–20,000 ft.)
Altostratus: Uniformly gray or bluish
white layers composed mostly of water
droplets; covers most of the sky in a thin
sheet; Sun or moon may shine through as
a bright spot as if viewed through frosted
glass.
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Altocumulus: Roll-like puffy, patchy clouds
composed mostly of water droplets, and
grouped in large sheets; also called “sheep’s
back” clouds.
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Low Clouds (cloud base below 2000 m, about 6500 ft.)
Stratus: Light or dark gray, low cloud; covers
most of the sky; may produce drizzle; fog is a
stratus cloud in contact with the ground.
Nimbostratus: Thick dark gray cloud
producing rain or snow; often with a ragged
base.
Stratocumulus: Irregular masses of
clouds, often rolling or puffy in appearance.
Vertically Developed Clouds (cloud thickness from 500 to 18,000 m,
or about 1600–60,000 ft.)
Cumulus: puffy white clouds resembling
cotton balls, popcorn, or cauliflower heads
floating in the sky; usually have almost flat
bottoms; occur individually or in groups.
Cumulonimbus: thunderstorm clouds; tall,
billowing towers of puffy clouds with flat bases;
can have sharp, well-defined edges or anvil shape
at the top; often produces rain and sometimes
hail, strong winds, or tornadoes.
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The Mature and Dissipating Stages of a Thunderstorm
Geo Words
troposphere: the portion of
the atmosphere next to the
Earth’s surface, in which
temperature generally
decreases rapidly with altitude.
tropopause: the top
boundary of the troposphere.
stratosphere: the outer layer
of the atmosphere overlying
the troposphere. The air
temperature is at first constant
with altitude and then
increases with altitude.
120
110
100
In the previous activity you learned that a thunderstorm begins when a
cumulus cloud develops in an updraft of air. The more humid the air, the
better the chance that cumulus clouds will form. If conditions in the
atmosphere favor thunderstorm development, cumulus clouds will
continue to billow upward and eventually the system will reach its mature
stage (when precipitation begins).
In general, the more vigorous the updraft, the greater the altitude to which a
thunderstorm cloud builds, and the more likely that the thunderstorm will
produce severe weather.To recognize how high a thundercloud can build,
consider the four different layers of the Earth’s atmosphere (listed from
lowest to highest): troposphere, stratosphere, mesosphere and
thermosphere.The boundaries
between these layers are defined
by air temperature, that is, how
air temperature varies with
altitude in each layer, as shown
thermosphere
in Figure 2.
90
altitude (km)
80
mesopause
70
mesosphere
60
50
stratopause
40
30
stratosphere
20
10
tropopause
troposphere
0
-100
-80
-60
-40
-20
0
20
temperature (˚C)
40
60
Figure 2 The layers of the atmosphere are
defined on the basis of how air temperature
varies with altitude.
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We live in the troposphere. On
average, air temperature drops
with increasing altitude up to the
top boundary of the troposphere,
called the tropopause.Air
temperatures are usually lower in
mountainous terrain than at sea
level.The next layer up is the
stratosphere, in which the air
temperature is at first constant
with altitude and then increases
with altitude.A thunderstorm
cloud that pushes above the
tropopause and into the lower
stratosphere will be colder and
denser than the surrounding air
and will sink back down into the
troposphere. For that reason,
even very intense thunderstorms
cannot build much higher than
the tropopause. Usually,
cumulonimbus clouds that billow
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Activity 2 A Thunderstorm Matures
up to the tropopause spread laterally and develop the characteristic flat, anvilshaped top as shown in Figure 3.An anvil top is most commonly displayed by
an intense thunderstorm during its mature stage.
Thunderstorm precipitation may be in the form of rain, snow, or hail.
Precipitation falls through the updraft, weakening it and eventually dragging
air downward, producing a downdraft alongside the updraft. The downdraft
leaves the base of the cloud and flows along the ground ahead of the shaft
of precipitation. The leading edge of this rain-cooled gusty air is like a
miniature cold front and is known as a gust front. (See Figure 3.) This
explains why a relatively cool and gusty wind often precedes an
approaching thunderstorm.
Geo Words
gust front: the leading edge
of the rain-cooled gusty air
preceding a thunderstorm.
overshooting gap
storm movement
anvil top
cold
downdraft
warm
updraft
Check Your
Understanding
new cloud
formation
rain
gust front
Figure 3 Profile of a thunderhead.The arrows indicate the directions of air movement,
as either updrafts or downdrafts.
In an intense thunderstorm, a downdraft may strike the ground with wind
speeds in excess of 100 km/h (60 mph). An intense downdraft is known as
a downburst. A downburst pushes ahead of the thunderstorm and can be
strong enough to uproot trees and damage buildings.
As precipitation spreads throughout the thunderstorm cloud, so does the
downdraft. Eventually the downdraft overpowers the updraft.The updraft
dies, precipitation comes to an end, and the cloud slowly vaporizes.This is
the final, or dissipating, stage of the thunderstorm.
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1. Describe what happens
to the temperature of
a gas when the volume
of the gas increases.
2. Describe what happens
to the temperature of
a gas when the gas is
compressed.
3. In your own words,
define what is meant
by air pressure.
4. Describe what causes a
downburst.
5. Describe the vertical
motion of air in a
thunderstorm that has
reached its mature
stage.
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Understanding and Applying What You Have Learned
1. What hazards do cumulonimbus
clouds pose to aircraft?
2. Approximately how high can a
cumulonimbus cloud build? What
limits a cloud’s height?
3. Have you ever seen “steam” rising
from a pond on a cold day?
Explain why this happens.
8. On a piece of paper, draw the
vertical profile of a prominent
mountain range. Assume that a
humid wind blowing from left to
right encounters the mountain
range. Because the wind cannot go
through rock, it is forced to go up
and over the mountain range.
a) Using a broad smooth arrow,
draw the wind flowing up the
mountain on one side and down
the mountain on the other. In
pencil, sketch clouds on the side
of the mountain where you
think clouds and thunderstorms
are most likely to form.
b) Share your drawing with
classmates in your group, make
any modifications in your
drawing, and discuss why
clouds and precipitation are
more likely on one side of the
mountain than on the other.
c) On which slope of the mountain
range is a rain forest more likely
to be found? On which slope of
the mountain is a desert more
likely to be found?
4. In the investigation, smoke from
the match acted as condensation
nuclei needed to form a cloud.
What are some natural and
artificial sources of condensation
nuclei?
5. How does lowering the
temperature of humid air affect the
likelihood of cloud formation?
6. Are thunderstorms more likely to
form in humid or dry air? Justify
your response.
7. Why does an updraft produce
clouds and why does a downdraft
cause clouds to vaporize?
Preparing for the Chapter Challenge
Working with the other members of
your small group, prepare a table that
summarizes the characteristics of each
of the three stages in a thunderstorm’s
life cycle. Using that table and the
other components of this activity,
prepare a list of observations that
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might warn people of the approach of
a thunderstorm that has reached its
mature stage. You need to add this
information to your report for the
entertainment company. Present your
list of warning signs during a
classroom discussion.
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Inquiring Further
the type of hazard that caused
the damage.
c) Could any of the damage in
your community have been
prevented? If so, how?
1. Observing storm damage
a) Make a list of all of the types of
thunderstorm-related hazards
you can think of. Divide your
list into three categories:
2. Earth Systems connections
• Hazards that may exist as a
thunderstorm approaches.
a) Think about what you have
learned about condensation
nuclei and cloud formation.
Predict the type of weather
changes that might occur when
a volcano erupts and releases
particles into the atmosphere.
b) After you have made your
prediction, conduct research on
the relationship between
volcanism and weather.
• Hazards that may exist during
a thunderstorm (during the
mature stage when
precipitation is falling).
• Hazards that may exist as a
thunderstorm is dissipating.
b) List all of the types of
thunderstorm-related damage
that you have observed in your
community. If possible, identify
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