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In this unit, you will explore answers to these questions • What
caused the worst natural disaster ever to occur on American
soil?
• Where, when, and how do tropical cyclones form?
• What source of energy powers tropical cyclones?
• What forces drive tropical cyclones?
Unit 1
Recipe for a Cyclone
NASA
/GSF
C
Earth’s complex atmospheric circulation and energy balance
produce the conditions that form — or prevent the formation of —
tropical cyclones.
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Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
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Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
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Warm-up 1.1 The Great Hurricane of 1900On September 8, 1900 the
greatest natural disaster to strike the United States (in terms of
human casualties) occurred at Galveston, Texas. In the early
evening hours of September 8, a hurricane made landfall, bringing
with it a 5-m (17-ft ) storm surge that inundated most of Galveston
Island and the city of Galveston. By the next day, much of the city
was destroyed, at least 8,000 people were killed, and many
thousands more were made homeless.Th e account beginning on page 5
is an eyewitness report of the storm and its aft ermath written by
Isaac M. Cline, the senior employee and section director at the
Galveston offi ce of the USDA Weather Bureau in 1900. Ironically,
the citizens of Galveston had proposed to build a protective
seawall in 1893. Th e project was dismissed, based partly on
Cline’s assertions that no hurricane could pose so serious a
threat, and that the money would be better spent on other projects.
Cline’s pregnant wife was one of the thousands that perished in the
storm.
Hurricane behavior and hazardsHurricanes, also known as tropical
cyclones, unleash massive amounts of energy over wide areas, and
are capable of tremendous destruction. Aft er reading the story of
the 1900 Galveston Hurricane, list and describe all of the
hurricane-related hazards mentioned in the story. Feel free to add
other hazards from your previous knowledge or experience with
tropical cyclones. Be prepared to discuss your list with your
classmates. 1.
2.
3.
4.
5.
6.
7.
Hurricane vital statistics • Maximum wind speed:
194 km/hr (120 mph). This is the estimated speed since
the instruments blew away after recording a sustained wind speed
of 135 km/hr (84 mph) and gusts of 165 km/hr (102 mph).
• Storm surge:5 – 6 m (15 – 20 ft).
• Galveston’s highest point:2.7 m (8.7 ft) above sea level.
• Local tide at time of storm surge: high
• Lowest observed air pressure at the Galveston weather offi
ce:
964 mb (28.48 in Hg). Note: Standard atmospheric pressure
= 29.92 in Hg or 1013.25 mb. mb stands for millibars, a metric
unit
of pressure. Hg is the chemical symbol for
mercury, the silvery liquid metal used in barometers. in Hg
stands for inches of mercury, a standard U.S. unit of atmospheric
pressure.
• Lowest observed air pressure at sea:
931 mb (27.49 in Hg). • Estimated Intensity:
Category 4 hurricane. • Population of Galveston:
37,789 (1900 Census). • Fatalities: Estimated at 6000 – 8000
in Galveston, plus 2000 in surrounding area. Some place the fi
gure as high as 12,000.
• Number of homes destroyed:
Over 3600 homes in Galveston (estimate).
• Total damage: $30 million (estimate,
equivalent to $53 billion in 2005 U.S. dollars).
The Great Hurricane of 1900 3
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
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Questions 1. Which of the hazards you listed was responsible for
the greatest
damage and loss of life?
2. Why didn’t the people of Galveston evacuate the city before
the hurricane struck?
3. What, if anything, do you think the city of Galveston could
have done to prevent the high death toll and property damage caused
by the 1900 hurricane?
4. If this hurricane struck Galveston Island today, would it
have the same destructive eff ect? Explain.
5. With today’s technology, do you think a tropical cyclone
could cause a disaster of this magnitude anywhere around the world?
Explain.
4 The Great Hurricane of 1900
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
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Isaac M. Cline, meteorologist and chief of the U.S. Department
of Agriculture Weather Service bureau in Galveston
September 8, 1900Th e hurricane which visited Galveston Island
on Saturday, September , , was no doubt one of the most important
meteorological events in the world’s history. Th e ruin which it
wrought beggars description, and conservative estimates place the
loss of life at the appalling fi gure, ,. A brief description of
Galveston Island will not be out of place as introductory to the
details of this disaster. It is a sand island about thirty miles in
length and one and one-half to three miles in width. Th e course of
the island is southwest to northeast, parallel with the southeast
coast of the State. Th e City of Galveston is located on the east
end of the island. To the northeast of Galveston is Bolivar
Peninsula, a sand spit about twenty miles in length and varying in
width from one-fourth of a mile to about three miles. Inside of
Galveston Island and Bolivar Peninsula is Galveston bay, a shallow
body of water with an area of nearly fi ve hundred square miles. Th
e length of the bay along shore is about fi fty miles and its
greatest distance from the Gulf coast is about twenty-fi ve miles.
Th e greater portion of the bay lies due north of Galveston. Th at
portion of the bay which separates the island west of Galveston
from the mainland is very narrow, being only about two miles in
width in places, and discharges into the Gulf of Mexico through San
Louis Pass. Th e main bay discharges into the Gulf between the
jetties; the south one being built out from the northeast end of
Galveston Island and the north one from the most southerly point of
Bolivar Peninsula. Th e channel between the jetties is twenty-seven
to thirty feet in depth at diff erent stages of the tide. Th ere
are channels in the harbor with a depth of thirty to thirty-fi ve
feet, and there is an area of nearly two thousand acres with an
anchorage depth of eighteen feet or more. Th e mainland for several
miles back of the bay is very low, in fact
much of it is lower than Galveston Island, and it is so
frequently overfl owed by high tide that large areas present a
marshy appearance. Th ese are in brief the physical conditions of
the territory devastated by the hurricane. Th e usual signs which
herald the approach of hurricanes were not present in this case. Th
e brick-dust sky was not in evidence to the smallest degree. Th is
feature, which has been distinctly observed in other storms that
have occurred in this section, was carefully watched for, both on
the evening of the th and the morning of the th. Th ere were cirrus
clouds moving from the southeast during the forenoon of the th, but
by noon only altostratus from the northeast were observed. About
the middle of the afternoon the clouds were divided between cirrus,
altostratus, and cumulus, moving from the northeast. A heavy swell
from the southeast made its appearance in the Gulf of Mexico during
the afternoon of the th. Th e swell continued during the night
without diminishing, and the tide rose to an unusual height when it
is considered that the wind was
Special Report on the Galveston HurricaneEyewitness account by
Dr. Isaac M. Cline
From A
Weekend in Septem
ber, ©1980 John Edw
ard Weem
s, Used w
ith permission
Figure 1. Map of the Galveston Bay area, Texas.
The Great Hurricane of 1900 5
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
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from the north and northwest. About a.m. of the th Mr. J. L.
Cline, Observer, called me and stated that the tide was well up in
the low parts of the city, and that we might be able to telegraph
important information to Washington. He having been on duty until
nearly midnight, was told to retire and I would look into
conditions. I drove to the Gulf, where I timed the swells, and then
proceeded to the offi ce and found that the barometer was only
one-tenth of an inch lower than it was at the p.m. observation of
the th. I then returned to the Gulf, made more detailed
observations of the tide and swells, and fi led the following
telegram addressed to the Central Offi ce in Washington: Unusually
heavy swells from the southeast,
intervals of one to fi ve minutes, overfl owing low places south
portion of city three to four blocks from beach. Such high water
with opposing winds never observed previously.
Broken stratus and stratocumulus clouds predominated during the
early forenoon of the th, with the blue sky visible here and there.
Showery weather commenced at : a.m., but dense clouds and heavy
rain were not in evidence until about noon, after which dense
clouds with rain prevailed.Th e wind during the forenoon of the th
was generally north, but oscillated, at intervals of from fi ve to
ten minutes, between northwest and northeast, and continued so up
to p.m. After p.m., the wind was mostly northeast, although as late
as : p.m. it would occasionally go back to the northwest for one or
two minutes at a time. Th e prevailing wind was from the northeast
until : p.m., when it shifted to the east, continuing from this
direction until about p.m. After p.m. the wind was from the
southeast, and after about p.m. the prevailing direction was from
the south or southwest. Th e directions after p.m. are from
personal observations. A storm velocity was not attained until
about p.m. after which the wind increased steadily and reached a
hurricane velocity about p.m. Th e greatest velocity for fi ve
minutes was miles per hour at : p.m. With two minutes at the rate
of miles per hour. Th e anemometer blew away at this time, and it
is estimated that prior to p.m. the wind attained a velocity of at
least miles per hour. For a short
time, about p.m., just before the wind shifted to the east,
there was a distinct lull, but when it came out from the east and
southeast it appeared to come with greater fury than before. After
shifting to the south at about p.m. the wind steadily diminished in
velocity, and at a.m. on the morning of the th was blowing at the
rate of miles per hour from the south. Th e barometer commenced
falling on the afternoon of the th and continued falling steadily
but slowly up to noon of the th, when it read . inches. Th e
barometer fell rapidly from noon until : p.m. of the th, when it
registered . inches, a fall of pressure of about one inch in eight
and one-half hours. After : p.m. the barometer rose at the same
rapid rate that had characterized the fall.On account of the rapid
fall in pressure, Mr. John D. Blagden, observer, took readings of
the mercurial barometer as a check on the barograph, and readings
are as follows:
Time Reading Time Reading : p.m. ..... . : p.m. ..... . : p.m.
..... . : p.m. ...... : p.m. ...... : p.m. ....... : p.m. .... . :
p.m. ...... : p.m. ..... . : p.m. ..... . : p.m. ....... : p.m.
..... .
Figure 2. The hurricane, born some 4000 miles away, was first
observed on August 30, east of Puerto Rico. Galvestonians became
aware of the storm on September 4, but seriously underestimated its
strength. The city was considered safe from such disasters, having
survived major storms in 1875 and 1886 with minimal damage and loss
of life.
Sept. 13
Sept. 11
Sept. 8
Sept. 1Aug. 27
Sept. 5
6 The Great Hurricane of 1900
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
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Th ese readings confi rm the low pressure shown by barograph and
indicate the great intensity of the hurricane. Mr. Blagden looked
after the instruments during the hurricane in a heroic and
commendable manner. He kept the wires of the self-registering
apparatus intact as long as it was possible for him to reach the
roof. Th e rain gauge blew away about p.m. and the thermometer
shelter soon followed. All the instruments in the thermometer
shelter were broken, except the thermograph which was found
damaged, but has been put in working order. Storm warnings were
timely and received a wide distribution not only in Galveston but
throughout the coast region. Warning messages were received from
the Central Offi ce at Washington on September , , , , and . Th e
high tide on the morning of the th, with storm warning fl ying,
made it necessary to keep one man constantly at the telephone
giving out information. Hundreds of people who could not reach us
by telephone came to the Weather Bureau offi ce seeking advice. I
went down on Strand street and advised some wholesale commission
merchants who had perishable goods on their fl oors to place them
feet above the fl oor. One gentleman has informed me that he
carried out my instructions, but the wind blew his goods down. Th e
public was warned, over the telephone and verbally, that the wind
would go by the east to the south and that the worst was yet to
come. People were advised to seek secure places for the night. As a
result thousands of people who lived
near the beach or in small houses moved their families into the
center of the city and were thus saved. Th ose who lived in large
strong buildings, a few blocks from the beach, one of whom was the
writer of this report, thought that they could weather the wind and
tide. Soon after p.m. conditions became so threatening that it was
deemed essential that a special report be sent at once to
Washington. Mr. J. L. Cline, Observer, took the instrumental
readings while I drove fi rst to the bay and then to the Gulf, and
fi nding that half the streets of the city were under water added
the following to the special observation at : p.m.: “Gulf rising,
water covers streets of about half of city.” Having been on duty
since a.m., after giving this message to the observer, I went home
to lunch. Mr. J. L. Cline went to the telegraph offi ces through
water from two to four feet deep, and found that the telegraph
wires had all gone down; he then returned to the offi ce, and by
inquiry learned that the long distance telephone had one wire still
working to Houston, over which he gave the message to the Western
Union telegraph offi ce at Houston to be forwarded to the Central
Offi ce at Washington. I reached home and found the water around my
residence waist deep. I at once went to work assisting people, who
were not securely located, into my residence, until forty or fi fty
persons were housed therein. About : p.m. Mr. J. L. Cline, who had
left Mr. Blagden at the offi ce to look after the instruments,
reached my residence, where he found the water neck deep. He
informed me that the barometer had fallen below . inches;
NO
AA
/National W
eather Service Collection
Figure 4. After the disaster, survivors returned to salvage what
they could from the debris, but little remained of their former
homes and businesses.
NO
AA
/National W
eather Service Collection
Figure 3. Many people went to upper floors and climbed onto
roofs to escape the rising water. Many wood-frame buildings were
knocked from their foundations and disintegrated to become part of
the sea of floating debris.
The Great Hurricane of 1900 7
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that no further messages could be gotten off on account of all
wires being down, and that he had advised everyone he could see to
go to the center of the city; also, that he thought we had better
make an attempt in that direction. At this time, however, the roofs
of houses and timbers were fl ying through the streets as though
they were paper, and it appeared suicidal to attempt a journey
through the fl ying timbers. Many people were killed by fl ying
timbers about this time while endeavoring to escape to town. Th e
water rose at a steady rate from p.m. until about : p.m., when
there was a sudden rise of about four feet in as many seconds. I
was standing at my front door, which was partly open, watching the
water, which was fl owing with great rapidity from east to west. Th
e water at this time was about eight inches deep in my residence,
and the sudden rise of feet brought it above my waist before I
could change my position. Th e water had now reached a stage feet
above the ground at Rosenberg Avenue (Twenty-fi fth street) and Q
street, where my residence stood. Th e ground was . feet elevation,
which made the tide . feet. Th e tide rose the next hour, between :
and : p.m., nearly fi ve feet additional, making a total tide in
that locality of about twenty feet. Th ese observations were
carefully taken and represent to within a few tenths of a foot the
true conditions. Other personal observations in my vicinity confi
rm these estimates. Th e tide, however, on the bay or north side of
the city did not obtain a height of more than feet. It is possible
that there was feet of backwater on the Gulf side as a result of
debris accumulating four to six blocks inland. Th e debris is piled
eight to fi fteen feet in height.By p.m. a number of houses had
drifted up and lodged to the east and southeast of my residence,
and these with the force of the waves acted as a battering ram
against which it was impossible for any building to stand for any
length of time, and at : p.m. my residence went down with about fi
fty persons who had sought it for safety, and all but eighteen were
hurled into eternity. Among the lost was my wife, who never rose
above the water after the wreck of the building. I was nearly
drowned and became unconscious, but recovered though being crushed
by timbers and found
myself clinging to my youngest child, who had gone down with
myself and wife. Mr. J. L. Cline joined me fi ve minutes later with
my other two children, and with them and a woman and child we
picked up from the raging waters, we drifted for three hours,
landing yards from where we started. Th ere were two hours that we
did not see a house nor any person, and from the swell we inferred
that we were drifting to sea, which, in view of the northeast wind
then blowing, was more than probable. During the last hour that we
were drifting, which was with southeast and south winds, the
wreckage on which we were fl oating knocked several residences to
pieces. When we landed about : p.m., by climbing over fl oating
debris to a residence on Twenty-eighth street and Avenue P, the
water had fallen about feet. It continued falling, and on the
following morning the Gulf was nearly normal. While we were
drifting we had to protect ourselves from the fl ying timbers by
holding planks between us and the wind, and with this protection we
were frequently knocked great distances. Many persons were killed
on top of the drifting debris by fl ying timbers after they had
escaped from their wrecked homes. In order to keep on the top of
the fl oating masses of wrecked buildings one had to be constantly
on the lookout and continually climbing from drift to drift.
Hundreds of people had similar experiences. Sunday, September , ,
revealed one of the most horrible sights that ever a civilized
people looked upon. About three thousand homes,
Figure 5. The dead were carried by wagons to be loaded onto
barges for burial at sea. Many bodies later washed ashore,
requiring them to be buried again.
NO
AA
/National W
eather Service Collection
8 The Great Hurricane of 1900
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
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nearly half the residence portion of Galveston, had been
completely swept out of existence, and probably more than six
thousand persons had passed from life to death during that dreadful
night. Th e correct number of those who perished will probably
never be known, for many entire families are missing. Where ,
people lived on the th not a house remained on the th, and who
occupied the houses may, in many instances, never be known. On
account of the pleasant Gulf breezes many strangers were residing
temporarily near the beach, and the number of these that were lost
can not yet be estimated. I enclose a chart, fi g. [not included],
which shows, by shading, the area of total destruction. Two charts
of this area have been drawn independently; one by Mr. A. G.
Youens, inspector for the local board of underwriters, and the
other by myself and Mr. J. L. Cline. Th e two charts agree in
nearly all particulars, and it is believed that the chart enclosed
represents the true conditions as nearly as it is possible to show
them. Th at portion of the city west of Forty-fi fth street was
sparsely settled, but there were several splendid residences in the
southern part of it. Many truck farmers and dairy men resided on
the west end of the island, and it is estimated that half of these
were lost, as but very few residences remain standing down the
island. For two blocks, inside the shaded area, the damage amounts
to at least fi fty per cent of the property. Th ere is not a house
in Galveston that escaped injury, and there are houses totally
wrecked in all parts of the city. All goods and supplies not
over
eight feet above fl oor were badly injured, and much was totally
lost. Th e damage to buildings, personal, and other property in
Galveston County is estimated at above thirty million dollars. Th e
insurance inspector for Galveston states that there were ,
residences located prior to the hurricane in the area of total
destruction, and he estimates , houses totally destroyed in other
portions of the city, making a total of , houses totally destroyed.
Th e value of these buildings alone is estimated at $,,. Th e grain
elevators which were full of grain suff ered the smallest damage.
Ships have resumed loading and work is being rushed day and night.
Th e railroad bridges across the bay were washed away, but one of
these has been repaired and direct rail communication with the
outside world was established within eleven days after the
disaster. Repairs and extensions of wharves are now being pushed
forward with great rapidity. Notwithstanding the fact that the
streets are not yet clean and dead bodies are being discovered
daily among the drifted debris, the people appear to have confi
dence in the place and are determined to rebuild and reestablish
themselves here. Galveston being one of the richest cities of its
size in the United States, there is no question but that business
will soon regain its normal condition and the city will grow and
prosper as she did before the disaster. Cotton is now coming in by
rail from diff erent parts of the State and by barge from Houston.
Th e wheels of commerce are already moving in a manner which gives
assurance for the future. Improvements will be made stronger and
more judiciously; for the past twenty-fi ve years they have been
made with the hurricane of in mind, but no one ever dreamed that
the water would reach the height observed in the present case. Th e
railroad bridges are to be built ten feet higher than they were
before. Th e engineer of the Southern Pacifi c Company has informed
me that they will construct their wharves so that they will
withstand even such a hurricane as the one we have just
experienced. I believe that a sea wall, which would have broken the
swells, would have saved much loss of both life and property. I
base this view upon observations
NO
AA
/National W
eather Service Collection
Figure 6. Many survivors took refuge in a handful of large stone
buildings such as churches and hospitals. Here, survivors inspect
the devastation.
The Great Hurricane of 1900 9
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which I have made in the extreme northeastern portion of the
city, which is practically protected by the south jetty; this part
of the city did not suff er more than half the damage that other
similarly located districts, without protection, sustained. From
the offi cers of the U. S. Engineer tug Anna, I learn that the wind
at the mouth of the Brazos River went from north to southwest by
way of west. Th is shows that the center of the hurricane was near
Galveston, probably not more than miles to the westward. Th e
following towns have suff ered great damage, both in the loss of
life and property: Texas City, Dickinson, Lamarque [La Marque],
Hitchcock, Arcadia, Alvin, Manvel, Brazoria, Columbia, and Wharton.
Other towns further inland have suff ered, but not so seriously. Th
e exact damage at these places can not be ascertained.
A list of those lost in Galveston, whose names have been
ascertained up to the present time, contains , names. [Th is was
later revised to as many as ,.]
UNITED STATES DEPARTMENT OF AGRICULTURE WEATHER BUREAU
OFFICE,
GALVESTON, TEX., September , .
10 The Great Hurricane of 1900
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
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Investigation 1.2A tropical cyclone is one of the most powerful
and destructive natural events on Earth. If you could convert the
energy released by a tropical cyclone in a single day into
electricity, it would power the entire United States for six
months! Called hurricanes, cyclones, or typhoons depending on where
they occur, these massive storms kill thousands of people and cause
billions of dollars in damage each year.
A global viewIn this activity, you will investigate where and
when tropical cyclones form. Th is will help you understand the
conditions that create and sustain these huge storms.
Launch ArcMap, then locate and open the ddtc_unit_1.mxd fi
le.
Refer to the tear-out Quick Reference Sheet located in the
Introduction to this module for GIS defi nitions and instructions
on how to perform tasks. In the Table of Contents, right-click the
Global Patterns data
frame and choose Activate. Expand the Global Patterns data
frame.
Th e green dots show the starting point of each of the 4814
tropical cyclones recorded between 1950 and 2005. Of course, real
tropical cyclones are not just dots — they average 560 km (350 mi)
in diameter. To grasp the size of these storms, you will look at a
series of weather satellite images of Hurricane Andrew. Andrew was
one of the costliest Atlantic hurricanes in history, causing over
$26 billion in damage.
Click the Media Viewer button .Choose Hurricane Andrew Movie
from the media list.
Th e Hurricane Andrew movie is an animated sequence of weather
satellite images that covers a six-day period from August 22 – 27,
1992. It shows the motion and development of the storm as it moved
toward the Florida coast. At the beginning of the movie, you should
also see Hurricane Lester making landfall on the west coast of
Mexico. A day or two later, Hurricane Andrew crosses Florida before
slamming into Louisiana. 1. In which direction do Andrew and Lester
spin — clockwise or
counterclockwise?
2. What happens to both storms when they cross over land?
Detecting cyclone patterns
Costliest Atlantic Hurricanes
Rank Name Location Costbillions
1Katrina
2005LA/MS 80.0
2Andrew
1992FL/LA 26.5
3Charley
2004FL 15.0
4Wilma2005
FL 14.4
5Ivan2004
AL/FL 14.2
Source: NOAANote: Damages not adjusted for infl ation.Damages
for Katrina and Wilma are estimated.
Detecting cyclone patterns 11
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
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Close the Media Viewer window. Look at the distribution of
hurricanes on the map. Using the
latitude lines on the map, estimate the northern and southern
boundaries of the region where tropical cyclones form.
3. Most tropical cyclones form between about ________ °N
latitude and ________ °S latitude.
Click the QuickLoad button . Select Spatial Bookmarks, choose
Eastern Hemisphere Equator,
and click OK.
Look closely at the area near the equator. Th ere appears to be
a narrow, cyclone-free zone centered on the equator.
Use the Measure tool to measure the distance from the equator to
the edge of the cyclone-free zone as shown in sidebar at left . Th
e distance (Total) is given in the status bar. (Note: Your Total
will be diff erent than the one shown below.)
Click the QuickLoad button . Select Spatial Bookmarks, choose
Western Hemisphere Equator,
and click OK. Use the Measure tool to repeat your measurements
of the
cyclone-free zone in the Western Hemisphere.
4. Th e cyclone-free zone extends approximately ________ km from
the equator.
For now, it is enough to know that tropical cyclones do not form
in this band. Later, you will learn why they do not form there.
Click the Full Extent button to view the entire map.
5. Do tropical cyclones form over land, over oceans, or
both?
In some areas, the tropical cyclone formation points show a
symmetrical pattern above and below the equator. In other areas,
this symmetrical pattern is absent. 6. On Map 1 on the following
page, circle the areas where you would
expect, based on symmetry, that tropical cyclones should form,
yet they do not.
Symmetry in natureSymmetrical (mirror image) patterns are common
in nature. For example, animal faces are symmetrical about a
vertical line, or axis of symmetry.
Using the Measure tool
Equator (0°)
To measure the distance from the equator to the edge of the
cyclone-free zone, click the crosshair cursor on the equator, move
the crosshair to the edge of the zone, and read the distance in the
status bar. Double-click to stop measuring.
12 Detecting cyclone patterns
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
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Map 1 — Areas where tropical cyclones should form, but do
not
7. Explain why you think it would make sense to expect tropical
cyclones to form in these areas.
Tropical cyclone basinsTropical cyclones are not distributed
evenly around the globe. Instead, they occur in large clusters. Th
ese clusters help defi ne tropical cyclone basins.
Turn off the Cyclones layer. Turn on the Tropical Cyclone Basins
layer.
Th is layer shows seven major regions, called basins, where
tropical cyclones form. Each basin is identifi ed by the ocean in
which it occurs and the land areas aff ected by its storms. A
storm’s name (hurricane, cyclone, or typhoon) also depends on the
basin in which it forms.
To learn more about these basins, click the Identify tool . In
the Identify Results window, select the Tropical Cyclone
Basins layer from the drop-down list of layers. Next, click
within each of the tropical cyclone basins to answer the
question below.
8. In Table 1 on the following page, record the hemisphere,
direction of rotation, average number of cyclones per year, and
storm type for each basin.
Detecting cyclone patterns 13
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
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Table 1 — Tropical cyclone basin characteristics
Basin name Hemisphere
N/S
Direction of rotation
CW/CCW
Cyclones per year
Average
Storm type
Atlantic
Australia / SE Indian
Australia / SW Pacifi c
N Indian
NE Pacifi c
NW Pacifi c
SW Indian
Close the Identify Results window.
Using the information from Table 1, answer the following
questions. 9. What are all tropical cyclones in the Southern
Hemisphere called?
10. What would you call a tropical cyclone that strikes China or
the Philippines — a cyclone, a typhoon, or a hurricane?
11. In which direction do storms in each hemisphere rotate?
Northern = ____________________
Southern = ____________________
When do tropical cyclones occur?For people living on the
Atlantic and Gulf Coasts, a typical year has fi ve, not four
seasons. Th e fi ft h season is hurricane season. With it comes the
fear that it might just be the year of the “BIG ONE.” Is tropical
cyclone season the same everywhere on Earth? To fi nd out, you will
change the legend to show the time of year during which each storm
occurred. Click the QuickLoad button . Select Layers, choose
Cyclones, and click OK. Locate the new Cyclones layer in the Table
of Contents (Figure 1).
Th e cyclones are now classifi ed based on the time of year in
which each one formed. Note the predominant color of the tropical
cyclone formation
points in each basin. Th e legend shows the dates that each
color represents.Figure 1. Location of newly-loaded Cyclones layer
in the Table
of Contents.
What’s in a name?Cyclone comes from the Greek word kuklos,
meaning circle. Indeed, tropical cyclones not only spin, but they
usually move along curved paths.
Hurricane comes from Hurakán, the Mayan god of the skies and
lightning. Hurakán literally translates as “one-legged.”
Typhoon comes from the Japanese word taifuu. The characters
translate literally as “pedestal wind.”
14 Detecting cyclone patterns
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
12. For each tropical cyclone basin, record the hemisphere and
dates of greatest tropical cyclone activity in Table 2. Use the
information at left to convert each range of dates to a season.
Table 2 — Table of tropical cyclone activity
Basin name HemisphereN/S
Dates of greatest tropical cyclone activity
SeasonSee sidebar
Atlantic
Australia / SE Indian
Australia / SW Pacifi c
N Indian
NE Pacifi c
NW Pacifi c
SW Indian
13. According to the table, regardless of the hemisphere, during
which season(s) of the year do most tropical cyclones occur?
The ITCZTh e ITCZ, or Inter-Tropical Convergence Zone, is also
called Earth’s thermal equator. Th ere, the heating of Earth’s
surface is highest, due to the tilt of Earth’s axis. Viewed by
satellite, the ITCZ appears as persistent bands of clouds
encircling Earth (Figure 2). Th ere, showers and thunderstorms form
as the heated air rises and cools. Th e ITCZ is also important for
its role in creating tropical cyclones, as low-pressure systems
move away from the ITCZ and gradually evolve into tropical
depressions. Turn off the Tropical Cyclones Basins and
newly-loaded
Cyclones layers. Turn on the ITCZ (Jun – Aug) and ITCZ (Dec –
Feb) layers.
Notice how the ITCZ changes position during the year. 14. How
does the location of the ITCZ appear to be related to Earth’s
seasons?
Earth’s Seasons are • Caused by the tilt of Earth’s
axis. • Opposite in the Northern and
Southern Hemispheres.
Dates Hemisphere
N S
Dec 21 – Mar 20 Winter Summer
Mar 20 – Jun 21 Spring Fall
Jun 21 – Sep 22 Summer Winter
Sep 22 – Dec 21 Fall Spring
NA
SA/G
SFC
Figure 2. The ITCZ appears as a band of clouds near the
equator.
ITCZ
Detecting cyclone patterns 15
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
15. On Map 2, label areas where the ITCZ never crosses the
equator.Map 2 — Areas near the equator where the ITCZ is absent
16. Does the location of the ITCZ appear to be related to the
formation of tropical cyclones? Use examples from the Northern and
Southern Hemisphere to support your position.
Quit ArcMap and do not save changes.
Rare, but not impossibleSince the beginning of the satellite age
(the early 1960s), no tropical cyclone had ever been observed in
the South Atlantic Ocean. Amazingly, on March 28, 2004 a Category 1
hurricane struck the coast of Brazil, causing moderate damage and
loss of life (Figure 3). Hurricanes are so rare in the South
Atlantic that no warning system existed to alert coastal residents,
and the storm was never named.
Figure 3. Cyclone approaches the coast of Brazil on March 26,
2004. This low-pressure system formed outside the ITCZ, and had
much in common with extratropical (outside the tropics)
cyclones.
NA
SA/G
SFC
16 Detecting cyclone patterns
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
Solar-powered stormsIn the previous activity, you learned that
most tropical cyclones form during the summer and early fall. Th is
is because tropical cyclones are powered by solar energy, and
summer is when Earth receives the most energy from the sun. Summer
does not occur at the same time of year everywhere and neither do
tropical cyclones. In the Northern Hemisphere, tropical cyclone
season is from June through September. In the Southern Hemisphere
it is from December through March.
The reason for seasonsTh e seasons are opposite in each
hemisphere for the same reason that the seasons themselves exist;
Earth’s axis is tilted 23.5° from “vertical” as it orbits the sun
(Figure 1).
Around June 21 of each year, the north pole tilts toward the
sun. Th is day marks the fi rst day of summer in the Northern
Hemisphere. On the same day, the south pole is pointing most
directly away from the sun, marking the fi rst day of winter in the
Southern Hemisphere. Around December 21, the opposite tilt of the
poles marks the fi rst day of Northern Hemisphere winter and
Southern Hemisphere summer.
The TropicsImaginary lines 23.5° north and south of the equator
mark where the sun passes overhead on the fi rst day of summer in
each hemisphere. Th ese latitudes are called the Tropic of Cancer
and the Tropic of Capricorn, respectively. Th e area between these
latitudes, called the tropics, receives the most direct sunlight
throughout the year (Figure 2 on the following page).
Summer
Winter
June 21
Winter
Summer
23.5°
December 21
Figure 1. Earth’s axis is tilted 23.5° from a line perpendicular
to its orbital plane. When viewed from above, Earth’s orbit around
the sun is nearly circular. (Note: Earth and sun are not drawn to
scale.)
Reading 1.3 Understanding tropical cyclones
Understanding tropical cyclones 17
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
Outside the tropics, the sun never passes directly overhead.
Areas north or south of the tropics receive more solar radiation
during their summer, when their hemisphere is tilted toward the
sun.
Energy and latitudeIn addition to the seasons, the tropics are
warm because of the shape of Earth. As latitude increases toward
the poles, the sun’s rays strike the ground at lower angles,
spreading the same amount of energy over a greater area, as shown
in Figure 3.Th is explains the temperature diff erences at diff
erent latitudes. Th e same amount of sunlight that heats up one
square meter at the equator is spread over 1.4 square meters at 45°
latitude, 2 square meters at 60° latitude, and over 11 square
meters near the poles at 85° latitude. Closer to the poles, each
square meter receives less solar energy.
Storing energyTo understand how solar radiation aff ects the
formation of tropical cyclones, you need to know a few things about
water and heat energy. Water can exist in three states — solid,
liquid, or vapor (Figure 4). When water changes state, it absorbs
or releases more heat (called latent heat) than do most other
substances.
Figure 4. As ice melts or water evaporates, they absorb latent
heat; and as water vapor condenses or water freezes, they release
latent heat. The transition between liquid water and water vapor
involves about seven times as much heat energy as the transition
between liquid water and ice.
Solid water
Latent heat energy released
EvaporationMelting
Water vaporLiquid water
Latent heat energy absorbed
CondensationFreezing
1 m2 1.4 m2 2 m2
0° Equator
45° N New York City
60° N Moscow
Figure 3. Due to the curvature of Earth’s surface, the amount of
light that falls on 1 square meter at the equator spreads out to
cover about 2 square meters at 60° N latitude.
Latent heat — heat energy released or absorbed during a change
of phase.
Figure 2. Variation in solar heating with latitude.
Surface heating is greatest near the equator, where the sun’s
rays strike Earth’s surface more directly, and pass through less
atmosphere.
Surface heating is lowest near the poles, where the sun’s rays
pass through more atmosphere and spread over a greater area, due to
Earth’s curved surface.
Arctic Circle
Tropic of Cancer
Equator
Tropic of Capricorn
Antarctic Circle
South Pole
SUNLIGHT
23.5° S
66.5° S
0°
23.5° N
66.5° N
T
The Tropics
North Pole
90° S
90° N
18 Understanding tropical cyclones
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
Water can absorb more energy than most other substances because
it has a high heat capacity. Raising the temperature of water
requires a lot of energy. Th erefore, it is very diffi cult to
change the temperature of the ocean even a small amount. Th is
resistance to change is called thermal inertia. Even small changes
to the energy content of the ocean can have major eff ects on
global climate. In contrast, the heat capacity and thermal inertia
of the atmosphere are much lower, making it easier to change the
temperature of the atmosphere.At the equator, where solar energy is
highest, about 75 percent of Earth’s surface is covered by water.
Water’s higher heat capacity allows the ocean to absorb and retain
more solar energy than the land or atmosphere. In fact, the upper
few hundred meters of ocean store approximately 30 times more heat
than the entire atmosphere. Without some type of circulation in the
ocean and atmosphere, the equator would be 14 °C (25 °F) warmer on
average, and the north pole would be 25 °C (45 °F) colder.
Fortunately, temperature imbalances drive atmospheric and oceanic
circulation, which redistribute energy more evenly over Earth’s
surface. 1. Today, 75 percent of the area at the equator is covered
with water.
How might global temperatures change if 75 percent of the area
near the equator were covered with land? Explain.
2. Where are the Tropics, and why do cyclones form there?
3. During which months does summer occur in the Southern
Hemisphere?
4. Why is there a temperature diff erence between the equator
and the poles?
Heat capacity — the amount of energy required to raise the
temperature of one unit of mass of a substance by 1 °C.
Understanding tropical cyclones 19
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
Putting a spin on tropical cyclonesTwo characteristic features
of tropical cyclones are their spiral shape and the curved path
they follow (Figure 5). Both are controlled by a phenomenon called
the Coriolis eff ect.
The Coriolis effectEach day, Earth makes one full rotation on
its axis. To complete this trip, a point at the equator must travel
more than 40,000 km in 24 hours — a speed of about 1,670 km/hr
(1,035 mph). At higher latitudes, the distance required to complete
a rotation decreases (Figure 6). Th us, the speed at which the
surface is moving also decreases. For example, a point at 45° N
travels only about 28,000 km per day, or about 1170 km/hr (725
mph). At the poles themselves, the speed of the surface is
essentially zero.
Air near the surface travels at about the same speed as the
ground below it. When the sun heats air near the equator, it rises
and begins moving toward the pole. As it moves poleward, the speed
of the surface below decreases. Th e air moves faster than the
surface, and appears to curve or defl ect in the direction of
Earth’s rotation, the east. Th e air appears to veer to its right
in the Northern Hemisphere or to its left in the Southern
Hemisphere.At the poles air cools, sinks, and spreads out toward
the equator. Because the air has no rotational speed, it lags
behind the ground moving eastward beneath it and appears to defl
ect toward the west. Again, the
Equator
45˚ N
45˚ NEquator
Figure 6. As Earth rotates, points on the surface move more
slowly as latitude increases from the equator toward the poles.
North
Figure 5. Hurricane Floyd approaches the Florida coast in
September 1999.
NA
SA/G
SFC
20 Understanding tropical cyclones
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
air appears to veer to its right in the Northern Hemisphere and
to its left in the Southern Hemisphere (Figure 7). For more complex
reasons, air moving due east or west follows the same pattern, defl
ecting to the right in the Northern Hemisphere and to the left in
the Southern Hemisphere.
Coriolis effect and latitudeAs latitude increases north or south
on Earth’s surface, the rotational speed of the surface changes. Th
e rate of change is small near the equator and increases toward the
poles. Th erefore, the strength of the Coriolis eff ect at the
equator is zero, and increases with latitude. Within about 5
degrees of the equator, the eff ect is so weak that there is not
enough rotation to generate or sustain tropical cyclones. Tropical
cyclones do not cross the equator into the opposite hemisphere,
because they cannot maintain their rotation without the Coriolis
eff ect, nor can they change the direction of their rotation.
The ITCZ: birthplace of tropical cyclonesDue to Earth’s
spherical shape, as well as to the distribution of land and water,
the surface is heated unevenly. Where the surface is heated more,
it warms the air above it. Th e warmed air expands and rises,
forming a low-pressure region at the surface. Where the surface is
cooler, the air above it cools, compresses, and sinks, forming a
high-pressure region.At Earth’s surface, air moves from
high-pressure regions toward low-pressure regions in order to
equalize these pressure diff erences. Th is moving air forms three
global wind belts in each hemisphere — the trade winds, prevailing
westerlies, and polar easterlies (Figure 8).
Earth’s rotation
Figure 7. Imagine yourself in a hot-air balloon traveling along
with the air, following the paths shown by the arrows. As you look
ahead in the direction of travel, your balloon would seem to curve
to the right in the Northern Hemisphere and to the left in the
Southern Hemisphere.
Figure 8. Global air circulation patterns. The ITCZ forms where
the trade winds from the two hemispheres meet.
90° N
90° S
Trade winds
Prevailing westerlies
Polar easterlies
30° N
30 °S
60° N
60° S
ITCZ
Prevailing westerlies
Polar easterlies
Trade winds
Understanding tropical cyclones 21
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
Near the equator, the trade winds from both hemispheres meet. As
they converge, warm, moist air is forced upward and condenses to
form a band of clouds and heavy precipitation around the globe. Th
is band of low-pressure air is called the Intertropical Convergence
Zone, or ITCZ.Th e ITCZ moves with the seasons, following the
region of greatest solar heating. Th e average summer position of
the ITCZ in each hemisphere is shown in Figure 9. Note that over
the Atlantic and eastern Pacifi c Oceans, the ITCZ remains north of
the equator throughout the year.
A tropical cyclone begins as a weak, disorganized low-pressure
system along the ITCZ called a tropical disturbance. Under the
right conditions, these systems are pushed by surface winds toward
the poles and their rotation and uplift (increasing elevation from
the surface to the top of the system) increase. If the winds in the
rotating system reach certain speeds, the system is upgraded to a
tropical depression or a tropical cyclone.
Driving stormsIn low-pressure systems, air fl ows inward toward
the center of the system. In the Northern Hemisphere, the Coriolis
eff ect defl ects the air to the right, causing it to spiral inward
in a counterclockwise direction (Figure 10a). In the Southern
Hemisphere, the winds veer to their left , spiraling inward in a
clockwise direction (Figure 10b). Th is spiral motion, in opposite
directions in each hemisphere, produces the characteristic shape of
tropical cyclones.Low-pressure storm systems are embedded in air
that moves according to diff erences in air pressure and the
Coriolis eff ect. As global winds defl ect these systems toward the
poles, they may be carried into the prevailing westerlies, which
push them eastward, as shown by the white dashed line in Figure 8
on the previous page. As a result of the Coriolis eff ect: • In the
Northern Hemisphere, storms rotate in a counterclockwise
direction, but follow clockwise paths. • In the Southern
Hemisphere, storms rotate in a clockwise
direction, but follow counterclockwise paths.
L
L
a
bFigure 10. Low-pressure systems in the Northern Hemisphere (a)
rotate in a counterclockwise direc-tion. Low-pressure systems in
the Southern Hemisphere (b) rotate in a clockwise direction.
Tropical depression — weather system with a maximum sustained
surface wind speed of 38 mph (62 km/hr) or less.
EquatorJanuary
July
Figure 9. Average position of the ITCZ in January and July.
L
22 Understanding tropical cyclones
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
Steering Atlantic hurricanesTh e Bermuda High is a
semi-permanent, high-pressure region in the North Atlantic Ocean.
Air moving outward from this high-pressure center is defl ected to
its right creating a clockwise, or anticyclonic circulation.Th e
images in Figure 11 show the position of the Bermuda High on the
same day in diff erent years. Th e circulation around the High
closely resembles the tracks of many of the Atlantic hurricanes you
examined. If the Bermuda High shrinks or shift s eastward,
hurricanes stay away from the U.S. coast. Conversely, if the
Bermuda High gets larger or shift s westward, hurricanes are more
likely to make landfall on Gulf or Atlantic shores.
5. As surface winds blow toward the equator in the Southern
Hemisphere, which way are they defl ected by the Coriolis eff
ect?
6. In the Northern Hemisphere, does a tropical cyclone generally
follow a clockwise path or a counterclockwise path?
7. Why don’t tropical cyclones form very near the equator?
8. If the Coriolis eff ect is strongest near the poles, why
don’t tropical cyclones form in these areas?
HH
July 15, 1988
H
July 15, 1983 July 15, 1993
H
July 15, 1978Figure 11. Location of the Bermuda High on July 15
in various years.
Area shown below
Understanding tropical cyclones 23
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
Developing the cycloneHeat, or thermal energy, is a critical
factor in forming tropical cyclones. Figure 12 illustrates how heat
energy from warm ocean water and warm, moist air masses powers
these immense storms. Th e letters refer to the letters in Figure
12.
Required ingredientsTh e process of forming and sustaining a
tropical cyclone requires special conditions: A. Warm ocean waters
extending to a depth of at least 50 m (150 ft )
and located at least 500 km from the equator.
B. Converging winds caused by a weak tropical low-pressure
system.
C. Warm, moist air that is unstable, meaning that it tends to
rise into the atmosphere.
As a result: D. Air cools as it rises, eventually reaching a
temperature called
the dew point, where water vapor condenses into droplets. Th is
process releases heat, called the latent heat of condensation.
E. Th e released heat warms the surrounding air, creating
stronger updraft s that draw in more warm, moist air at the
surface.
E
D
C
B
A
Figure 12. Process of intensification in the formation of
tropical storms.
24 Understanding tropical cyclones
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
Th is cycle continues until a cyclone develops or something
disrupts the process.
Vertical wind shearEven with all of these ingredients present,
some conditions can keep tropical cyclones from forming or can cut
off their energy source aft er they have formed. One such
atmospheric condition is called vertical wind shear (Figure
13).
Vertical wind shear is a signifi cant change in wind speed
and/or direction with increasing altitude. High vertical wind shear
disrupts strong convection by spreading the latent heat released by
the condensing water vapor over a wider area.To form or maintain a
tropical cyclone, the vertical wind shear between the surface and
the upper troposphere must be less than 37 km/hr (23 mph). 9. If
surface winds are blowing eastward at 15 km/hr and winds in
the upper troposphere are blowing westward at 30 km/hr, will
there be enough vertical wind shear to prevent a tropical cyclone
from forming?
SurfaceSurface
Low wind shear High wind shear
Figure 13. Vertical wind shear disrupts the formation of
tropical cyclones by cutting off the uplift of warm, moist air from
the surface.
Troposphere — the lowest layer of Earth’s atmosphere. It begins
at Earth’s surface and extends upward 8 – 14.5 km (5 – 9 mi).
Understanding tropical cyclones 25
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
26 Understanding tropical cyclones
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
Energy from the sun drives Earth’s weather. Solar radiation
travels through space and Earth’s atmosphere and is absorbed by
Earth’s surface. As the surface heats up, it warms the air above
the surface. Our weather is complex for two reasons: Earth’s
surface does not heat up evenly, and the planet is spinning.
Energy for tropical cyclonesSo far, you have seen that tropical
cyclones form only within a limited range of latitudes, only over
oceans, and only during the summer and early fall in the Northern
and Southern Hemispheres. Th is seasonal pattern appears to be
related to the warming of Earth’s surface by the sun. Over what
range of ocean-surface temperatures do tropical cyclones form? In
this part of the investigation, you will try to answer this
question.
Launch ArcMap, then locate and open the ddtc_unit_1.mxd fi
le.
Refer to the tear-out Quick Reference Sheet located in the
Introduction to this module for GIS defi nitions and instructions
on how to perform tasks.
In the Table of Contents, right-click the Powering Tropical
Cyclones data frame and choose Activate.
Expand the Powering Tropical Cyclones data frame. Turn on the
August SST layer.
Th is layer shows the average sea-surface temperature (SST) in
degrees Celsius (°C) for the month of August, the warmest summer
month in the Northern Hemisphere. Working with temperatures in
Celsius instead of Fahrenheit can be confusing at fi rst. To get a
better feeling for the temperatures involved, you will convert two
temperatures from Fahrenheit to Celsius. 1. Use the formula °C =
(5/9) × (°F – 32) to convert 70 °F and 80 °F
to degrees Celsius (°C). Round to the nearest degree.
70 °F = ______ °C 80 °F = ______ °C
Use the legend for the August SST layer to answer the following
questions about the sea-surface temperature. 2. What color
represents the warmest water? What is the
temperature of the warmest water?
Powering tropical cyclonesInvestigation 1.4
Powering tropical cyclones 27
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
3. In August, water with a temperature of 27 – 28 °C is found as
far north as ______° N and as far south as _______° S latitude.
Turn off the August SST layer and turn on the February SST
layer.
4. In February, water with a temperature of 27 – 28 °C is found
as far north as ______° N and as far south as _______° S
latitude.
Sea-surface temperature and the seasonsTo show how sea-surface
temperatures change throughout the year, a series of SST maps have
been assembled into a movie.
Click the Media Viewer button . Choose SST Movie from the media
list and view the movie several
times.
Th e colors represent sea-surface temperature averaged over a
9-year period. 5. How does the shift in sea-surface temperatures
correspond to the
seasons in the Northern and Southern Hemispheres?
Close the Media Viewer window.Turn off the February SST
layer.
Searching for a minimum cyclone formation temperatureIs there a
minimum temperature needed for tropical cyclones to form?
Turn on the Tropical Cyclones (Jun – Sep) layer.
Th is layer shows the starting location of every tropical
cyclone that formed during June, July, or August in the years 1950
– 2005. Th e points are classifi ed according to the average
sea-surface temperature during that season, in degrees Celsius.
Next, you will count the number of tropical cyclones that formed at
each temperature, and look for any signifi cant patterns in the
data.
Click the Summarize button . In the Summarize window, select the
Tropical Cyclones
(Jun – Sep) as the feature layer. Select Temp (C) as the fi eld
to summarize in the drop-down
menu. Click OK.
Earth’s Seasons
Dates Hemisphere
N S
Dec 21 – Mar 20 Winter Summer
Mar 20 – Jun 21 Spring Fall
Jun 21 – Sep 22 Summer Winter
Sep 22 – Dec 21 Fall Spring
28 Powering tropical cyclones
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
Th e resulting summary table shows the frequency, or number of
tropical cyclones (Count_TEMP_C) that formed over water at each
average temperature. 6. Record the frequency of each temperature in
Table 1. (Temperatures
below 25 °C and above 30 °C are uncommon, so ignore them.)Table
1 — Tropical-cyclone formation and sea-surface temperature
Season Number of tropical cyclones forming at25 ˚C 26 ˚C 27 ˚C
28 ˚C 29 ˚C 30 ˚C
Jun – Sep
Dec – Mar
Close the summary table. Turn off the Tropical Cyclones (Jun –
Sep) layer. Turn on and select the Tropical Cyclones (Dec – Mar)
layer. Repeat the Summarize operation using the Tropical
Cyclones
(Dec – Mar) layer and the Temp (C) fi eld. 7. Record the summary
table results for December through February
in Table 1. (Th e value 9999 represents “no data,” so ignore
it.) Close the summary table.
8. Plot the number of tropical cyclones versus temperature (°C)
for Jun – Sep and for Dec – Mar on Graph 1. Use a solid line for
Jun – Sep and a dashed line for Dec – Mar.
Graph 1 — Number of cyclones vs average sea-surface
temperature
9. According to Graph 1, between which two temperatures does the
number of tropical cyclones in the Northern (Jun – Sep) and
Southern (Dec – Mar) Hemispheres begin to increase signifi
cantly?
Temperature (°C)
Num
ber
of t
rop
ical
cyc
lon
es
25 26 27 28 29 30
1100
1000
900
800
700
600
500
400
300
200
100
0
Powering tropical cyclones 29
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
10. Over what sea-surface temperature do tropical cyclones form
most frequently?
Draw a vertical line on the graph at 26.7 °C. Th is is the
temperature that experts say is needed for a tropical cyclone to
form. 11. How well do the data you graphed agree with the
experts?
12. Logically, the warmer the water, the more tropical cyclones
should occur. Why do you think the number of tropical cyclones on
the graph actually decreases for temperatures above 29 °C?
13. If global warming is a real phenomenon, and ocean
temperatures increase worldwide, how do you think this could affect
the frequency, latitude range, and intensity of tropical cyclones.
Justify your answer.
Quit ArcMap and do not save changes.
Hint for question 12Look at one of the sea-surface temperature
layers. What would a graph of the sea-surface temperature versus
area look like?
What do scientists say?For a detailed though somewhat technical
discussion of the eff ects of global warming on tropical cyclone
formation, click the Media Viewer button and choose the NOAA
Hurricane FAQ Web site.
If this does not work, enter the following address in your Web
browser:
www.aoml.noaa.gov/hrd/tcfaq/tcfaqG.html#G3
Another Web site that addresses a pattern of increasing
hurricane intensity over the past three decades can be found at
www.nsf.gov/news/news_summ .jsp?cntn_id=104325
30 Powering tropical cyclones
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
Wrap-up 1.5Earlier, you observed a mysterious situation.
Tropical cyclones form in the lower latitudes of most of the
world’s oceans, yet are virtually absent from the South Atlantic
and Southeastern Pacifi c Oceans.
Tropical cyclone checklistIn this unit, you have examined
specifi c conditions or “ingredients” that must be present for
tropical cyclones to form and grow. Th e four key ingredients are
described at left . Based on everything you have read and observed
in this unit, see if you can fi gure out why tropical cyclones
almost never form in these two areas (marked by question marks in
the above map). 1. Complete this tropical cyclone formation
checklist for the South
Atlantic and Southeastern Pacifi c Oceans. Don’t
Yes No know a. Do weak, tropical, low-pressure
systems form there?
b. Does the ocean surface there reachtemperatures of 27 °C or
warmer?
c. Do the regions have low verticalwind shear?
d. Are parts of the region more than500 km away from the
equator?
e. Does the ITCZ cross the region atsome time of year?
Solving the cyclone puzzle
??Tropical cyclone ingredients • A weak low-pressure system
forms along the ITCZ. (Reading 1.3)
• The system must form ≥ 500 km from the equator, where the
Coriolis eff ect is strong enough to cause the system to rotate.
(Investigation 1.2 and Reading 1.3)
• To maintain strong convection, vertical wind shear should be
low — less than 37 km/hr (23 mph) from the surface to the upper
troposphere. (Reading 1.3)
• A stable energy source — warm ocean water (≥ 26.7 ˚C)
extending to a depth of about 50 meters or more. As surface water
evaporates, it intensifi es the convection within the system.
(Reading 1.3 and Investigation 1.4)
A process of eliminationOne way to solve a problem is to use a
process of elimination to fi nd the solution. By identifying the
ingredients that are not the cause of the problem, you may be able
to narrow the choices of the correct cause down to just one or two
possible “culprits.” If you can’t eliminate a cause based on the
evidence, you must consider it as a possible factor.
Solving the cyclone puzzle 31
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone
-
2. Which ingredient(s) do you think is (are) most likely missing
in these cyclone-free areas?
3. What would you do to fi nd out if your answer to this puzzle
is right? What data would you like to collect and add to your
map?
32 Solving the cyclone puzzle
Data Detectives: Tropical Cyclones Unit 1 – Recipe for a
Cyclone