13.3 Tropical Storms MAIN Idea Normally peaceful, tropical oceans are capable of producing one of Earth’s most violent weather systems — the tropical cyclone.. Height km Height km He
Trang 1BIG Idea The exchange of
thermal energy in the
atmo-sphere sometimes occurs with
great violence that varies in
form, size, and duration.
13.1 Thunderstorms
MAIN Idea The intensity and
duration of thunderstorms
depend on the local conditions
that create them.
13.2 Severe Weather
MAIN Idea All thunderstorms
produce wind, rain, and
light-ning, which can have dangerous
and damaging effects under
cer-tain circumstances.
13.3 Tropical Storms
MAIN Idea Normally peaceful,
tropical oceans are capable of
producing one of Earth’s most
violent weather systems — the
tropical cyclone.
13.4 Recurrent Weather
MAIN Idea Even a relatively
mild weather system can
become destructive and
danger-ous if it persists for long periods
of time.
GeoFacts
• Hurricanes, tornadoes, and
every-day thunderstorms follow the
same life cycles.
• The largest hailstone measured
was nearly 18 cm in diameter.
• An F5 tornado can pack winds
that will flatten a building.
Flooding High winds
Storm surge
The Nature of Storms
(bkgd)Scientifica/NOAA/Visuals Unlimited, (t)Jim Reed/Photo Researchers, (cr)Radhika Chalasani/Getty Images, (b)Jim Reed/CORBIS
Trang 2Section 1 • XXXXXXXXXXXXXXXXXX 343
Start-Up Activities
Chapter 13 • The Nature of Storms 343
Visit glencoe.com to study entire chapters online;
explore animations:
• Interactive Time Lines
• Interactive Figures
• Interactive Tables access Web Links for more information, projects, and activities;
review content with the Interactive Tutor and take Self-Check Quizzes.
Why does lightning form?
You have probably felt the shock of static electricity
when you scuff your feet on a rug and then touch
a doorknob Your feet pick up additional electrons,
which are negatively charged These electrons are
attracted to the positively charged protons of the
doorknob metal, causing a small electrical current to
form The current causes you to feel a small shock
Procedure
1 Read and complete the safety lab form.
2 With a paper punch, create 10 paper
circles.
3 Place the circles in two piles of 5 on your desk.
4 Blow up a small balloon and mark one side
with an X
5 Rub the X side of the balloon on some
fabric.
6 Hold the X side of the balloon 2 cm above
one pile of paper circles.
7 Turn the balloon over, opposite the X, and
hold it 2 cm above the other pile of paper circles.
Analysis
1 Describe what happened to the paper
circles.
2 Explain what happened when you rubbed
the balloon on the fabric.
3 Infer how the static attracting the paper is
similar to the static electricity you produced
STEP 1 Make a 3-cm fold along the long side
of a sheet of paper and crease.
STEP 2 Fold the sheet into thirds.
STEP 3 Unfold the paper and draw lines along the fold lines Label
the columns Cumulus
Stage, Mature Stage, and Dissipation Stage.
As you read this section, diagram the air movement, and describe the conditions at each stage.
L
LA AU UNCH NCH Lab
Cumulus Stage MatureStage DissipationStage
Trang 3Hawaii
Puerto Rico National Climatic Data Center, NOAA
10 10
20 40
25 35 30
2520 40
25 50
50 65 60 45
35
50
60 50 454035 35 35
25 25 4045 6065
65 6570 75 80 80 8590 7085 807570 75 70 75
55 45
60 60
50 70 30
40
45 50
50 15
10 15
20 25
25 25 30
25 25
30 30 30 30
35
3535 35 30
30
25
25 20
20
1515
35
5 5 5 5 5 5 5
10 10
5 20 4050
More than 70
50 – 70
30 – 50
10 – 30 Under 10
Average Number of Thunderstorm Days Annually
344 Chapter 13 • The Nature of Storms
latent heat: stored energy in water
vapor that is not released to warm the
atmosphere until condensation occurs
MAIN Idea The intensity and duration of thunderstorms depend
on the local conditions that create them.
Real-World Reading Link Think about how an engine processes fuel to duce energy that powers an automobile Thunderstorms are atmospheric engines that use heat and moisture as fuel and expend their energy in the form of clouds, rain, lightning, and wind.
pro-Overview of Thunderstorms
At any given moment, nearly 2000 thunderstorms are in progress around the world Most do little more than provide welcome relief
on a muggy summer afternoon, or provide a spectacle of lightning
Some, however, grow into atmospheric monsters capable of ing hail the size of baseballs, swirling tornadoes, and surface winds
produc-of more than 160 km/h These severe thunderstorms can also vide the energy for nature’s most destructive storms — hurricanes
pro-These severe thunderstorms, regardless of intensity, have certain
characteristics in common Figure 13.1 shows which areas of the United States experience the most thunderstorms annually
How thunderstorms form In Chapter 11, you read that the stability of the air is determined by whether or not an air mass can lift Cooling air masses are stable and those that receive warming from the land or water below them are not Under the right condi-tions, convection can cause a cumulus cloud to grow into a cumu-lonimbus cloud The conditions that produce cumulonimbus clouds are the same conditions that produce thunderstorms For
a thunderstorm to form, three conditions must exist: a source of moisture, lifting of the air mass, and an unstable atmosphere
■ Figure 13.1 Both geography and air mass
movements make thunderstorms most common in
the southeastern United States.
Predict why the Pacific Coast has so few
thunderstorms and Florida has so many.
Trang 4Section 1 • Thunderstorms 345
Moisture First, for a thunderstorm to form, there must be an
abundant source of moisture in the lower levels of the atmosphere
Air masses that form over tropical oceans or large lakes become
more humid from water evaporating from the surface below
This humid air is less dense than the surrounding dry air and is
lifted The water vapor it contains condenses into the droplets that
constitute clouds Latent heat, which is released from the water
vapor during the process of condensation, warms the air causing it
to rise further, cool further, and condense more of its water vapor
Lifting Second, there must be some mechanism for condensing
moisture to release its latent heat This occurs when a warm air
mass is lifted into a cooler region of the atmosphere Dense, cold
air along a cold front can push warmer air upward, just like an air
mass does when moving up a mountainside Warm land areas, heat
islands such as cities, and bodies of water can also provide heat for
lifting an air mass Only when the water vapor condenses can it
release latent heat and keep the cloud rising
Stability Third, if the surrounding air remains cooler than the
rising air mass, the unstable conditions can produce clouds that
grow upward This releases more latent heat and allows continued
lifting However, when the density of the rising air mass and the
surrounding air are nearly the same, the cloud stops growing
Figure 13.2 shows a cumulus cloud that is on its way to becoming
a cumulonimbus cloud that can produce thunderstorms
Reading Check Describe the three conditions for thunderstorm growth.
Limits to thunderstorm growth The conditions that limit
thunderstorm growth are the same ones that form the storm
Conditions that create lift, condense water vapor, and release latent
heat keep the air mass warmer than the surrounding air The air
mass will continue to rise until it reaches a layer of equal density
that it cannot overcome Because the rate of condensation
dimin-ishes with height, most cumulonimbus clouds are limited to about
18,000 m Thunderstorms are also limited by duration and size
■ Figure 13.2 This cumulus cloud is growing as a result of unstable conditions
As the cloud continues to develop into a cumulonimbus cloud, a thunderstorm might develop.
Royalty-Free/CORBIS
Trang 5During the day, the temperature of land increases faster than the
temperature of water The warm air over land expands and rises,
and the colder air over the sea moves inland and replaces the
warm air These conditions can produce strong updrafts that
result in thunderstorms.
At night, conditions are reversed The land cools faster than water, so
the warmer sea air rises, and cooler air from above land moves over
the water and replaces it Nighttime conditions are considered stable.
346 Chapter 13 • The Nature of Storms
Types of Thunderstorms
Thunderstorms are often classified according to the mechanism that causes the air mass that formed them to rise There are two main types of thunderstorms: air-mass and frontal
Air-mass thunderstorms When air rises because of unequal heating of Earth’s surface within one air mass, the thunderstorm is called an
Earth’s surface reaches its maximum during afternoon, so it is common for air-mass thunder-storms, also called pop-up storms, to occur
mid-There are two kinds of air-mass thunderstorms
mass rises by orographic lifting, which involves air
moving up the side of a mountain Sea-breeze
that occur because land and water store and release thermal energy differently Sea-breeze thunder-storms are common along coastal areas during the summer, especially in the tropics and subtropics
Because land heats and cools faster than water, temperature differences can develop between the air over coastal land and the air over water, as shown in Figure 13.3.
Frontal thunderstorms The second main
type is frontal thunderstorms, which are produced
by advancing cold fronts and, more rarely, warm fronts In a cold front, dense, cold air pushes under warm air, which is less dense, rapidly lifting it up a steep cold-front boundary This rapid upward motion can produce a thin line of thunderstorms, sometimes hundreds of kilometers long, along the leading edge of the cold front Cold-front thunder-storms get their initial lift from the push of the cold air Because they are not dependent on daytime heating for their initial lift, cold-front thunder-storms can persist long into the night Flooding from soil saturation is common with these storms
Floods are the main cause of thunderstorm-related deaths in the United States each year
Less frequently, thunderstorms can develop along the advancing edge of a warm front In a warm-front storm, a warm air mass slides up and over a gently sloping cold air mass If the warm air behind the warm front is unstable and mois-ture levels are sufficiently high, a relatively mild thunderstorm can develop
■ Figure 13.3 Temperature differences exist over land and
water and vary with the time of day
Infer why water is warmer than the land at night.
Trang 6Height (km) Height (km) Height (km)
Section 1 • Thunderstorms 347
Thunderstorm Development
A thunderstorm usually has three stages: the cumulus stage, the
mature stage, and the dissipation stage The stages are classified
according to the direction the air is moving
Cumulus stage In the cumulus stage, air starts to rise vertically,
as shown in Figure 13.4. The updrafts are relatively localized and
cover an area of about 5–8 km This creates updrafts, which
trans-port water vapor to the cooler, upper regions of the cloud The water
vapor condenses into visible cloud droplets and releases latent heat
As the cloud droplets coalesce, they become larger and heavier until
the updrafts can no longer sustain them and they fall to Earth as
precipitation This begins the mature stage of a thunderstorm
Mature stage In the mature stage, updrafts and downdrafts
exist side by side in the cumulonimbus cloud Precipitation,
com-posed of water droplets that formed at high, cool levels of the
atmo-sphere, cools the air as it falls The newly cooled air is more dense
than the surrounding air, so it sinks rapidly to the ground along with
the precipitation This creates downdrafts As Figure 13.4 shows,
the updrafts and downdrafts form a convection cell which produces
the surface winds associated with thunderstorms The average area
covered by a thunderstorm in its mature stage is 8–15 km
Dissipation stage The convection cell can exist only if there is
a steady supply of warm, moist air at Earth’s surface Once that supply
is depleted, the updrafts slow down and eventually stop In a
thunder-storm, the cool downdrafts spread in all directions when they reach
Earth’s surface This cools the areas from which the storm draws its
energy, the updrafts cease, and clouds can no longer form The storm
is then in the dissipation stage shown in Figure 13.4. This stage will
last until all of the previously formed raindrops have fallen
■ Figure 13.4 The cumulus stage of a thunderstorm is characterized mainly by updrafts The mature
stage is characterized by strong updrafts and downdrafts The storm loses energy in the dissipation stage.
Interactive Figure To see an animation of the thunderstorm development, visit glencoe.com.
Incorporate information from this section into your Foldable.
Trang 7+ + + + + +
+ + –
+
+ + + +
– – –
– –
– – – – –
– – –
+ + + + + +
+ +
– –
air is called a stepped leader, and it generally moves from the center
of the cloud toward the ground When the stepped leader nears the ground, a branched channel of positively charged particles, called
the return stroke, rushes upward to meet it The return stroke
surges from the ground to the cloud, illuminating the connecting channel with about 100 million volts of electricity That illumination
is the brightest part of lightning
Thunder A lightning bolt heats the surrounding air to about 30,000°C That is about five times hotter than the surface of the Sun
The thunder you hear is the sound produced as this superheated air rapidly expands and contracts Because sound waves travel more slowly than light waves, you might see lightning before you hear thunder, even though they are generated at the same time
Lightning variations There are several names given to ning effects Sheet lightning is reflected by clouds, while heat light-ning is sheet lightning near the horizon Spider lightning can crawl across the sky for up to 150 km The most bizarre is ball lightning which is a hovering ball about the size of a pumpkin that disap-pears in a fizzle or a bang Blue jets and red sprites originate in clouds and rise rapidly toward the stratosphere as cones or bursts
light-■ Figure 13.5 When a stepped leader
nears an object on the ground, a powerful
surge of electricity from the ground moves
upward to the cloud and lightning is produced
Sequence Make an outline sequencing
the steps of lightning formation
Trang 8Self-Check Quiz glencoe.com Section 1 • Thunderstorms 349
Thunderstorm and lightning safety Each year in the
United States, lightning causes about 7500 forest fires, which result
in the loss of thousands of square kilometers of forest In addition,
lightning strikes in the United States cause a yearly average of 300
injuries and 93 deaths to humans Figure 13.6 indicates how
destructive a lightning strike might be
Avoid putting yourself in danger of being struck by lightning If
you are outdoors and feel your hair stand on end, squat low on the
balls of your feet Duck your head and make yourself the smallest
target possible Small sheds, isolated trees, and convertible
automo-biles are hazardous as shelters Using electrical appliances and
tele-phones during a lightning storm can lead to electric shock Stay out
of boats and away from water during a thunderstorm
■ Figure 13.6 Five times hotter than the surface of the Sun, a lightning bolt can be spectacular But when an object such as this pine tree is struck, it can be explosive.
Section Summary
◗◗ The cumulus stage, the mature stage,
and the dissipation stage comprise
the life cycle of a thunderstorm.
◗
◗ Clouds form as water is condensed
and latent heat is released.
◗
◗ Thunderstorms can be produced
either within air masses or along
fronts.
◗
◗ From formation to dissipation, all
thunderstorms go through the same
stages.
◗
◗ Lightning is a natural result of
thun-derstorm formation.
Understand Main Ideas
1 MAIN Idea List the conditions needed for a thunderstorm’s cumulus stage.
2 Explain how a thunderstorm is formed along a front.
3 Differentiate between a sea-breeze thunderstorm and a mountain thunderstorm.
4 Identify what causes a thunderstorm to dissipate.
5 Compare and contrast how a cold front and a warm front can create
thunderstorms.
6 Describe two different types of lightning.
Think Critically
7 Infer which stage of thunderstorm formation causes lightning.
8 Determine the conditions in thunderstorm formation that creates lightning.
Earth Science
9 Write a setting for a movie using a storm as part of the opening scene.
(l)G Grob/zefa/CORBIS, (r)Mark A Schneider/Visuals Unlimited
Trang 9Wall cloud
Wind shear Anvil cloud
350 Chapter 13 • The Nature of Storms
Objectives
◗ Explain why some thunderstorms
are more severe than others.
◗ Recognize the dangers of severe
weather, including lightning, hail,
and high winds.
◗ Describe how tornadoes form.
Review Vocabulary
air mass: large body of air that takes
on the characteristics of the area over
Real-World Reading Link Sliding down a park slide might seem mild and safe compared to a roller coaster’s wild and chaotic ride Similarly, while a gentle rain is appreciated by many, the same weather processes can create thunder- storms on a massive atmospheric scale resulting in disaster.
Weather Cells
All thunderstorms are not created equal Some die out within utes, while others flash and thunder throughout the night What makes one thunderstorm more severe than another? The increas-ing instability of the air intensifies the strength of a storm’s updrafts and downdrafts, which makes the storm severe
min-Supercells Severe thunderstorms can produce some of the most violent weather conditions on Earth They can develop into
self-sustaining, extremely powerful storms called supercells
Supercells are characterized by intense, rotating updrafts taking
10 to 20 minutes to reach the top of the cloud These furious storms can last for several hours and can have updrafts as strong
as 240 km/h It is not uncommon for a supercell to spawn lived tornadoes Figure 13.7 shows an illustration of a supercell
long-Notice the anvil-shaped cumulonimbus clouds associated with severe storms The tops of the supercells are chopped off by wind shear Of the estimated 100,000 thunderstorms that occur each year in the United States, only about 10 percent are considered to
be severe, and fewer still reach classic supercell proportions
■ Figure 13.7 An anvil-shaped
cumulo-nimbus cloud is characteristic of many severe
thunderstorms The most severe thunderstorms
are called supercells.
Gene & Karen Rhoden/Visuals Unlimited
Trang 10Section 2 • Severe Weather 351
Strong Winds
Recall that rain-cooled downdrafts descend to
Earth’s surface during a thunderstorm and spread
out as they reach the ground Sometimes, instead
of dispersing that downward energy over a large
area underneath the storm, the energy becomes
concentrated in a local area The resulting winds
are exceptionally strong, with speeds of more than
160 km/h Violent downdrafts that are
concen-trated in a local area are called downbursts
Based on the size of the area they affect,
down-bursts are classified as either macrodown-bursts or
microbursts Macrobursts can cause a path of
destruction up to 5 km wide They have wind
speeds of more than 200 km/h and can last up to
30 minutes Smaller in size, though deadlier in
force, microbursts affect areas of less than 3 km
but can have winds exceeding 250 km/h Despite
lasting fewer than 10 minutes on average, a
micro-burst is especially deadly because its small size
makes it extremely difficult to predict and detect
Figure 13.8shows a microburst
Hail
Each year in the United States, almost one billion
dollars in damage is caused by hail—precipitation
in the form of balls or lumps of ice Hail can do
tremendous damage to crops, vehicles, and
roof-tops, particularly in the central United States
where hail occurs most frequently Hail is most
common during the spring growing season
Figure 13.9 shows some conditions associated
with hail
Hail forms because of two characteristics
com-mon to thunderstorms First, water droplets enter
the parts of a cumulonimbus cloud where the
temperature is below freezing When these
super-cooled water droplets encounter ice pellets, the
water droplets freeze on contact and cause the ice
pellets to grow larger The second characteristic
that allows hail to form is an abundance of strong
updrafts and downdrafts existing side by side
within a cloud The growing ice pellets are caught
alternately in the updrafts and downdrafts, so
that they constantly encounter more supercooled
water droplets The ice pellets keep growing until
they are too heavy for even the strongest updrafts
to keep aloft, and they finally fall to Earth as hail
■ Figure 13.9 This hail storm in Sydney, Australia, caused slippery conditions for the traffic as well as damage to property.
■ Figure 13.8 A microburst, such as this one in Kansas, can
be as destructive as a tornado.
(t)Jim Reed/Photo Researchers, (b)David Gray/Reuters/CORBIS
Trang 11352 Chapter 13 • The Nature of Storms
VOCABULARY
A CADEMIC VOCABULARY
Phenomenon
an object or aspect known through
the senses rather than by thought or
intuition
Students observing the phenomenon
realized later that the powerful wind
was a microburst.
Tornadoes
In some parts of the world, the most feared form of severe weather is
the tornado A tornado is a violent, whirling column of air in contact
with the ground Before a tornado reaches the ground, it is called a funnel cloud Tornadoes are often associated with supercells—the most severe thunderstorms The air in a tornado is made visible by dust and debris drawn into the swirling column, sometimes called the vortex, or by the condensation of water vapor into a visible cloud
Reading Check Define the term tornado.
Development of tornadoes A tornado forms when wind speed and direction change suddenly with height, a phenomenon associated with wind shear Current thinking suggests that torna-does form when small pockets of cooler air are given a horizontal, rolling-pin type of rotation near Earth’s surface, as shown in
Figure 13.10. If this rotation occurs close enough to the storm’s updrafts, the twisting column of wind can be tilted from a horizontal to a vertical position As updrafts stretch the column the rotation is accelerated Air is removed from the center of the column, which in turn lowers the air pressure in the center The extreme pressure gradient between the center and the outer por-tion of the tornado produces the violent winds associated with tor-nadoes Although tornadoes rarely exceed 200 m in diameter and usually last only a few minutes, they can be extremely destructive
thunder-A tornado is classified according to its destructive force
■ Figure 13.10 Tornado formation is associated with changes in wind speed and direction
Infer what would cause the updrafts.
Interactive Figure To see an animation
of tornado formation, visit glencoe.com.
A change in wind direction and speed
creates a horizontal rotation in the lower
Trang 12Section 2 • Severe Weather 353
Tornado classification Tornadoes vary greatly in size
and intensity The Fujita tornado intensity scale, which ranks
tornadoes according to their path of destruction, wind speed,
and duration, is used to classify tornadoes The Fujita scale was
named for Japanese tornado researcher Dr Theodore Fujita The
scale ranges from F0, which is characterized by winds of up to
118 km/h, to the incredibly violent F5, which can pack winds of
more than 500 km/h Most tornadoes do not exceed the F1
cate-gory In fact, only about 1 percent reach F4 or F5 Those that do,
however, can lift entire buildings from their foundations and toss
automobiles and trucks around like toys The Fujita scale is
shown in Table 13.1.
Tornado distribution While tornadoes can occur at any
time and at any place, there are some times and locations where
they are more likely to form Most tornadoes — especially violent
ones — form in the spring during the late afternoon and evening,
when the temperature contrasts between polar air and tropical
air are the greatest Large temperature contrasts occur most
fre-quently in the central United States, where cold con tinental polar
air collides with maritime tropical air moving northward from
the Gulf of Mexico These large temperature contrasts often spark
the development of supercells, which are each capable of
produc-ing several strong tornadoes More than 700 tornadoes touch
down each year in the United States Many of these occur in a
region called “Tornado Alley,” which extends from northern
Texas through Oklahoma, Kansas, and Missouri
Strong (F2 and F3)
19 percent of all tornadoes Path: 24 km +
Duration: 20 min + Wind speed: 181–332 km/h
Violent (F4 and F5)
1 percent of all tornadoes Path: 80 km +
Duration: 1 h + Wind speed: 333–512+ km/h
Photo of
tornado
Interactive Table To explore more about the Fujita tornado intensity scale, visit glencoe.com.
(l)H Baker/Weatherstock, (c)Keith Brewster/Weatherstock
Trang 13Self-Check Quiz glencoe.com
354 Chapter 13 • The Nature of Storms
Tornado safety In the United States, an average of 80 deaths and
1500 injuries result from tornadoes each year In an ongoing effort to reduce tornado-related fatalities, the National Weather Service issues tor nado watches and warnings before a tornado strikes These adviso-ries are broadcast on local radio stations when tornadoes are indicated
on weather radar or spotted in the region During a severe storm, the presence of dark, greenish skies, a towering wall of clouds, large hailstones, and a loud, roaring noise similar to that of a freight train are signs of an approaching or developing tornado
thunder-The National Weather Service stresses that despite advanced tracking systems, some tornadoes develop very quickly In these cases, advance warnings might not be possible However, the threat
of tornado-related injury can be substantially decreased when ple seek shelter, such as the one shown in Figure 13.11, at the first sign of threatening skies
Section Summary
◗◗ Intense rotating updrafts are
associ-ated with supercells.
◗
◗ Downbursts are strong winds that
result in damage associated with
thunderstorms.
◗
◗ Hail is precipitation in the form of
balls or lumps of ice that accompany
severe storms.
◗
◗ The worst storm damage comes from
a vortex of high winds that moves
along the ground as a tornado.
Understand Main Ideas
1 MAIN Idea Identify the characteristics of a severe storm.
2 Describe two characteristics of thunderstorms that lead to hail formation.
3 Explain how some hail can become baseball sized.
4 Compare and contrast a macroburst and a microburst.
5 Identify the steps that change wind shear into a tornado.
6 Identify the conditions that lead to high winds, hail, and lightning.
9 Design a pamphlet about tornado safety.
■ Figure 13.11 In some areas, tornado shelters are common
If you are caught in a tornado, take shelter in the southwest
corner of a basement, a small downstairs room or closet, or
a tornado shelter like this one.
Peter Guttman/CORBIS
Trang 14Section 3 • Tropical Storms 355
Objectives
◗ Identify the conditions required for
tropical cyclones to form.
◗ Describe the life cycle of a tropical
cyclone.
◗ Recognize the dangers of
hurricanes.
Review Vocabulary
Coriolis effect: caused by Earth’s
rotation, moving particles, such as air,
are deflected to the right north of the
equator, and to the left, south of the
Overview of Tropical Cyclones
During summer and fall, the tropics experience conditions ideal for the formation of large, rotating, low-pressure tropical storms
called tropical cyclones In different parts of the world, the largest
of these storms are known as hurricanes, typhoons, and cyclones
Cyclone location Favorable conditions for cyclone formation exist in all tropical oceans except the South Atlantic Ocean and the Pacific Ocean off the west coast of South America The water in these areas is somewhat cooler and these areas contain regions of nearly permanently stable air As a consequence, tropical cyclones do not normally occur in these areas They do occur in the large
expanse of warm waters in the western Pacific Ocean where they are known as typhoons To people living near the Indian Ocean, they are known as cyclones In the North Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico, and along the western coast of Mexico, the strongest of these storms are called hurricanes Figure 13.12 shows where cyclones generally form
30ºN
■ Figure 13.12 Tropical cyclones are common in all of Earth’s tropical oceans except in the relatively cool waters of both the South Pacific and South Atlantic Oceans.
Interactive Figure To see an animation
of tropical cyclones, visit glencoe.com.
Trang 15356 Chapter 13 • The Nature of Storms
Cyclone formation Tropical cyclones require two basic conditions to form: an abundant supply of warm ocean water and some sort of mechanism to lift warm air and keep it rising
Tropical cyclones thrive on the tremendous amount of energy in warm, tropical oceans As water evaporates from the ocean sur-face, latent heat is stored in water vapor This latent heat is later released when the air rises and the water vapor condenses
The air usually rises because of some sort of existing weather disturbance moving across the tropics Many disturbances origi-nate along the equator Others are the result of weak, low-pressure systems called tropical waves Tropical disturbances are common during the summer and early fall Regardless of their origin, only
a small percentage of tropical disturbances develop into cyclones
There are three stages in the development of a full tropical cyclone
Reading Check Infer what is produced when water vapor condenses.
Formative stage The first indications of a building tropical cyclone is a moving tropical disturbance Less-dense, moist air is lifted, triggering rainfall and air circulation As these disturbances produce more precipitation, more latent heat is released In addi-tion, the rising air creates an area of low pressure at the ocean sur-face As more warm, dense air moves toward the low-pressure center to replace the air that has risen, the Coriolis effect causes the moving air to turn counterclockwise in the northern hemi-sphere This produces the cyclonic (counterclockwise) rotation of a tropical cyclone, as shown in Figure 13.13. When a disturbance over a tropical ocean acquires a cyclonic circulation around a cen-ter of low pressure, it has reached the developmental stage and is known as a tropical depression, as illustrated in Figure 13.14
Mature stage As the moving air approaches the center of the growing storm, it rises, rotates, and increases in speed as more energy is released through condensation In the process, air pres-sure in the center of the system continues to decrease As long as warm air is fed into the system at the surface and removed in the upper atmosphere, the storm will continue to build and the winds
of rotation will increase as the air pressure drops
When wind speeds around the low-pressure center of a tropical depression exceed 65 km/h, the system is called a tropical storm If air pressure continues to fall and winds around the center reach at least 120 km/h, the storm is officially classified as a cyclone Once winds reach these speeds, another phenomenon occurs — the devel-
opment of a calm center of the storm called the eye, shown in Figure 13.14. The eye of the cyclone is a span of 30 to 60 km of calm weather and blue sky The strongest winds in a hurricane are
usually concentrated in the eyewall—a tall band of strong winds
and dense clouds that surrounds the eye The eyewall is visible because of the clouds that form there and mark the outward edge
of the eye
■ Figure 13.13 The characteristic
rotating nature of cyclonic storms is
evident in this tropical depression that
formed over the Atlantic Ocean.
VOCABULARY
S CIENCE USAGE V C OMMON USAGE
Depression
Science usage: a pressing down or
low-ering, the low spot on a curved line
Common usage: a state of feeling sad
NASA/Photo Researchers
Trang 161 Warm air absorbs
moisture from the
ocean.
2 Water vapor
is lifted into the atmosphere.
3 As the water
vapor rises, the cooler upper air condenses it into liquid droplets.
4 Condensation
releases latent heat into the atmosphere, making the air less dense.
5 As the lighter air
rises, moist air from the ocean takes its place, creating a wind current.
Eye Eyewall
Rainbands
Section 3 • Tropical Storms 357
Visualizing Cyclone Formation
Moving air starts to spin as a result of the Coriolis effect.
Tropical Depression The
first indications of a building storm are a tropical depression with good circulation, thunder- storms, and sustained winds of 37−62 km/h.
Tropical Storm As winds
increase to speeds of 63–117 km/h, strong thunderstorms develop and become well defined They are now tropical storms.
Cyclone With sustained winds of 118 km/h, an
intense tropical weather system with well-defined circulation becomes a cyclone, also called a typhoon
or hurricane
To explore more about cyclone mation, visit glencoe.com.