ddtc ag hs unit 1 final student - Access Training MaterialsTitle...

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

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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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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


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


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.



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



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;

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Figure 4. After the disaster, survivors returned to salvage what they could from the debris, but little remained of their former homes and businesses.

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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.



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.

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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

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Figure 6. Many survivors took refuge in a handful of large stone buildings such as churches and hospitals. Here, survivors inspect the devastation.



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 , .



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


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


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.



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.”



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



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.

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SA/G

SFC



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


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


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.



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



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



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



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



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.



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).



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


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


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


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


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


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


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