Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

Unit 1 Recipe for a Cyclone

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?

C F S /G SA NA

Earth’s complex atmospheric circulation and energy balance produce the conditions that form — or prevent the formation of — tropical cyclones.

1 Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

2 Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

Warm-up 1.1 The Great Hurricane of 1900 Hurricane vital statistics On September 8, 1900 the greatest natural disaster to strike the United • Maximum wind speed: States (in terms of human casualties) occurred at Galveston, Texas. 194 km/hr (120 mph). In the early evening hours of September 8, a hurricane made landfall, This is the estimated speed since bringing with it a 5-m (17-ft ) storm surge that inundated most of the instruments blew away after Galveston Island and the city of Galveston. By the next day, much of recording a sustained wind speed of 135 km/hr (84 mph) and gusts of 165 the city was destroyed, at least 8,000 people were killed, and many km/hr (102 mph). thousands more were made homeless. • Storm surge: Th e account beginning on page 5 is an eyewitness report of the storm 5 – 6 m (15 – 20 ft). and its aft ermath written by Isaac M. Cline, the senior employee and • Galveston’s highest point: 2.7 m (8.7 ft) above sea level. section director at the Galveston offi ce of the USDA Weather Bureau • Local tide at time of storm in 1900. Ironically, the citizens of Galveston had proposed to build a surge: high protective seawall in 1893. Th e project was dismissed, based partly on • Lowest observed air Cline’s assertions that no hurricane could pose so serious a threat, and pressure at the Galveston that the money would be better spent on other projects. Cline’s pregnant weather offi ce: wife was one of the thousands that perished in the storm. 964 mb (28.48 in Hg). Note: Standard atmospheric pressure = 29.92 in Hg or 1013.25 mb. Hurricane behavior and hazards mb stands for millibars, a metric unit of pressure. Hurricanes, also known as tropical cyclones, unleash massive amounts of Hg is the chemical symbol for energy over wide areas, and are capable of tremendous destruction. Aft er mercury, the silvery liquid metal used reading the story of the 1900 Galveston Hurricane, list and describe all in barometers. in Hg stands for inches of the hurricane-related hazards mentioned in the story. Feel free to add of mercury, a standard U.S. unit of atmospheric pressure. other hazards from your previous knowledge or experience with tropical • Lowest observed air cyclones. Be prepared to discuss your list with your classmates. pressure at sea: 1. 931 mb (27.49 in Hg). • Estimated Intensity: Category 4 hurricane. 2. • Population of Galveston: 37,789 (1900 Census). • Fatalities: 3. Estimated at 6000 – 8000 in Galveston, plus 2000 in surrounding area. Some place 4. the fi gure as high as 12,000. • Number of homes destroyed: 5. Over 3600 homes in Galveston (estimate). 6. • Total damage: $30 million (estimate, equivalent to $53 billion in 7. 2005 U.S. dollars).

The Great Hurricane of 1900 3 Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

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

Eyewitness account by Dr. Isaac M. Cline Special Report on the Galveston Hurricane

Isaac M. Cline, meteorologist and chief of the U.S. Department From

of Agriculture Weather Service bureau in Galveston A Weekend in September September 8, 1900 Th e hurricane which visited Galveston Island on Saturday, September , , was no doubt one of , ©1980 John Edward Weems, Used with permission 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 Figure 1. Map of the Galveston Bay area, Texas. in length and varying in width from one-fourth of a mile to about three miles. Inside of Galveston much of it is lower than Galveston Island, and Island and Bolivar Peninsula is Galveston bay, it is so frequently overfl owed by high tide that a shallow body of water with an area of nearly large areas present a marshy appearance. Th ese are fi ve hundred square miles. Th e length of the bay in brief the physical conditions of the territory along shore is about fi fty miles and its greatest devastated by the hurricane. distance from the Gulf coast is about twenty- Th e usual signs which herald the approach of fi ve miles. Th e greater portion of the bay lies due hurricanes were not present in this case. Th e north of Galveston. Th at portion of the bay which brick-dust sky was not in evidence to the smallest separates the island west of Galveston from the degree. Th is feature, which has been distinctly mainland is very narrow, being only about two observed in other storms that have occurred in miles in width in places, and discharges into the this section, was carefully watched for, both on Gulf of Mexico through San Louis Pass. Th e main the evening of the th and the morning of the bay discharges into the Gulf between the jetties; th. Th ere were cirrus clouds moving from the the south one being built out from the northeast southeast during the forenoon of the th, but by end of Galveston Island and the north one from noon only altostratus from the northeast were the most southerly point of Bolivar Peninsula. observed. About the middle of the afternoon the Th e channel between the jetties is twenty-seven clouds were divided between cirrus, altostratus, to thirty feet in depth at diff erent stages of the and cumulus, moving from the northeast. A heavy tide. Th ere are channels in the harbor with a depth swell from the southeast made its appearance in of thirty to thirty-fi ve feet, and there is an area the Gulf of Mexico during the afternoon of the of nearly two thousand acres with an anchorage th. Th e swell continued during the night without depth of eighteen feet or more. Th e mainland for diminishing, and the tide rose to an unusual several miles back of the bay is very low, in fact height when it is considered that the wind was

The Great Hurricane of 1900 5 Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

from the north and northwest. About  a.m. of the th Mr. J. L. Cline, Observer, called me and Sept. 13 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 Sept. 11 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 Sept. 8 than it was at the  p.m. observation of the th. Sept. 5 I then returned to the Gulf, made more detailed Sept. 1 observations of the tide and swells, and fi led Aug. 27 the following telegram addressed to the Central Offi ce in Washington: Unusually heavy swells from the southeast, Figure 2. The hurricane, born some 4000 miles away, was first observed on August 30, east of Puerto Rico. Galvestonians intervals of one to fi ve minutes, overfl owing became aware of the storm on September 4, but seriously low places south portion of city three to four underestimated its strength. The city was considered safe blocks from beach. Such high water with from such disasters, having survived major storms in 1875 and opposing winds never observed previously. 1886 with minimal damage and loss of life. Broken stratus and stratocumulus clouds time, about  p.m., just before the wind shifted predominated during the early forenoon of the th, to the east, there was a distinct lull, but when it with the blue sky visible here and there. Showery came out from the east and southeast it appeared weather commenced at : a.m., but dense clouds to come with greater fury than before. After and heavy rain were not in evidence until about shifting to the south at about  p.m. the wind noon, after which dense clouds with rain prevailed. steadily diminished in velocity, and at  a.m. on Th e wind during the forenoon of the th was the morning of the th was blowing at the rate of generally north, but oscillated, at intervals of  miles per hour from the south. from fi ve to ten minutes, between northwest and Th e barometer commenced falling on the afternoon northeast, and continued so up to  p.m. After  of the th and continued falling steadily but slowly p.m., the wind was mostly northeast, although as up to noon of the th, when it read . inches. late as : p.m. it would occasionally go back to Th e barometer fell rapidly from noon until : the northwest for one or two minutes at a time. p.m. of the th, when it registered . inches, a Th e prevailing wind was from the northeast until fall of pressure of about one inch in eight and one- : p.m., when it shifted to the east, continuing half hours. After : p.m. the barometer rose at from this direction until about  p.m. After  the same rapid rate that had characterized the fall. p.m. the wind was from the southeast, and after about  p.m. the prevailing direction was from On account of the rapid fall in pressure, Mr. the south or southwest. Th e directions after  p.m. John D. Blagden, observer, took readings of the are from personal observations. A storm velocity mercurial barometer as a check on the barograph, was not attained until about  p.m. after which the and readings are as follows: wind increased steadily and reached a hurricane Time Reading Time Reading velocity about  p.m. Th e greatest velocity for fi ve : p.m...... . : p.m...... . minutes was  miles per hour at : p.m. With : p.m...... . : p.m...... . two minutes at the rate of  miles per hour. : p.m...... . : p.m...... . Th e anemometer blew away at this time, and it is : p.m. .... . : p.m...... . estimated that prior to  p.m. the wind attained a : p.m...... . : p.m...... . velocity of at least  miles per hour. For a short : p.m...... . : p.m...... .

6 The Great Hurricane of 1900 Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

Th ese readings confi rm the low pressure shown by NOAA/National Weather Collection Service 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 Figure 4. After the disaster, survivors returned to salvage working order. Storm warnings were timely and what they could from the debris, but little remained of their received a wide distribution not only in Galveston former homes and businesses. but throughout the coast region. Warning near the beach or in small houses moved their messages were received from the Central Offi ce families into the center of the city and were thus at Washington on September , , , , and . saved. Th ose who lived in large strong buildings, Th e high tide on the morning of the th, with a few blocks from the beach, one of whom was storm warning fl ying, made it necessary to keep the writer of this report, thought that they could one man constantly at the telephone giving out weather the wind and tide. Soon after  p.m. information. Hundreds of people who could conditions became so threatening that it was not reach us by telephone came to the Weather deemed essential that a special report be sent at Bureau offi ce seeking advice. I went down once to Washington. Mr. J. L. Cline, Observer, on Strand street and advised some wholesale took the instrumental readings while I drove commission merchants who had perishable goods fi rst to the bay and then to the Gulf, and fi nding on their fl oors to place them  feet above the fl oor. that half the streets of the city were under water One gentleman has informed me that he carried added the following to the special observation out my instructions, but the wind blew his goods at : p.m.: “Gulf rising, water covers streets of down. Th e public was warned, over the telephone about half of city.” Having been on duty since  and verbally, that the wind would go by the east a.m., after giving this message to the observer, to the south and that the worst was yet to come. I went home to lunch. Mr. J. L. Cline went to People were advised to seek secure places for the the telegraph offi ces through water from two night. As a result thousands of people who lived to four feet deep, and found that the telegraph NOAA/National Weather Collection Service 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 Figure 3. Many people went to upper floors and climbed onto roofs to escape the rising water. Many wood-frame after the instruments, reached my residence, where buildings were knocked from their foundations and he found the water neck deep. He informed me disintegrated to become part of the sea of floating debris. that the barometer had fallen below . inches;

The Great Hurricane of 1900 7 Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

that no further messages could be gotten off on myself clinging to my youngest child, who had account of all wires being down, and that he gone down with myself and wife. Mr. J. L. Cline had advised everyone he could see to go to the joined me fi ve minutes later with my other two center of the city; also, that he thought we had children, and with them and a woman and child better make an attempt in that direction. At this we picked up from the raging waters, we drifted time, however, the roofs of houses and timbers for three hours, landing  yards from where were fl ying through the streets as though they we started. Th ere were two hours that we did not were paper, and it appeared suicidal to attempt a see a house nor any person, and from the swell journey through the fl ying timbers. Many people we inferred that we were drifting to sea, which, were killed by fl ying timbers about this time while in view of the northeast wind then blowing, was endeavoring to escape to town. more than probable. During the last hour that we were drifting, which was with southeast and south Th e water rose at a steady rate from  p.m. until winds, the wreckage on which we were fl oating about : p.m., when there was a sudden rise of knocked several residences to pieces. When about four feet in as many seconds. I was standing we landed about : p.m., by climbing over at my front door, which was partly open, watching fl oating debris to a residence on Twenty-eighth the water, which was fl owing with great rapidity street and Avenue P, the water had fallen about from east to west. Th e water at this time was  feet. It continued falling, and on the following about eight inches deep in my residence, and the morning the Gulf was nearly normal. While we sudden rise of  feet brought it above my waist were drifting we had to protect ourselves from before I could change my position. Th e water had the fl ying timbers by holding planks between us now reached a stage  feet above the ground at and the wind, and with this protection we were Rosenberg Avenue (Twenty-fi fth street) and Q frequently knocked great distances. Many persons street, where my residence stood. Th e ground was were killed on top of the drifting debris by . feet elevation, which made the tide . feet. fl ying timbers after they had escaped from their Th e tide rose the next hour, between : and : wrecked homes. In order to keep on the top of p.m., nearly fi ve feet additional, making a total the fl oating masses of wrecked buildings one had tide in that locality of about twenty feet. Th ese to be constantly on the lookout and continually observations were carefully taken and represent to climbing from drift to drift. Hundreds of people within a few tenths of a foot the true conditions. had similar experiences. Other personal observations in my vicinity confi rm these estimates. Th e tide, however, on the bay or Sunday, September , , revealed one of the north side of the city did not obtain a height of most horrible sights that ever a civilized people more than  feet. It is possible that there was  looked upon. About three thousand homes, feet of backwater on the Gulf side as a result of NOAA/National Collection Weather Service 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 Figure 5. The dead were carried by wagons to be loaded drowned and became unconscious, but recovered onto barges for burial at sea. Many bodies later washed ashore, requiring them to be buried again. though being crushed by timbers and found

8 The Great Hurricane of 1900 Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

NOAA/National Collection Weather Service 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 Figure 6. Many survivors took refuge in a handful of large suff ered the smallest damage. Ships have stone buildings such as churches and hospitals. Here, resumed loading and work is being rushed day survivors inspect the devastation. and night. Th e railroad bridges across the bay nearly half the residence portion of Galveston, were washed away, but one of these has been had been completely swept out of existence, and repaired and direct rail communication with probably more than six thousand persons had the outside world was established within eleven passed from life to death during that dreadful days after the disaster. Repairs and extensions night. Th e correct number of those who perished of wharves are now being pushed forward with will probably never be known, for many entire great rapidity. Notwithstanding the fact that families are missing. Where , people lived the streets are not yet clean and dead bodies are on the th not a house remained on the th, and being discovered daily among the drifted debris, who occupied the houses may, in many instances, the people appear to have confi dence in the place never be known. On account of the pleasant Gulf and are determined to rebuild and reestablish breezes many strangers were residing temporarily themselves here. Galveston being one of the near the beach, and the number of these that were richest cities of its size in the United States, there lost can not yet be estimated. I enclose a chart, is no question but that business will soon regain fi g.  [not included], which shows, by shading, its normal condition and the city will grow and the area of total destruction. Two charts of this prosper as she did before the disaster. Cotton is area have been drawn independently; one by now coming in by rail from diff erent parts of the Mr. A. G. Youens, inspector for the local board State and by barge from Houston. Th e wheels of of underwriters, and the other by myself and commerce are already moving in a manner which Mr. J. L. Cline. Th e two charts agree in nearly gives assurance for the future. Improvements all particulars, and it is believed that the chart will be made stronger and more judiciously; for enclosed represents the true conditions as nearly the past twenty-fi ve years they have been made as it is possible to show them. Th at portion of the with the hurricane of  in mind, but no one city west of Forty-fi fth street was sparsely settled, ever dreamed that the water would reach the but there were several splendid residences in the height observed in the present case. Th e railroad southern part of it. Many truck farmers and dairy bridges are to be built ten feet higher than men resided on the west end of the island, and it they were before. Th e engineer of the Southern is estimated that half of these were lost, as but very Pacifi c Company has informed me that they will few residences remain standing down the island. construct their wharves so that they will withstand For two blocks, inside the shaded area, the damage even such a hurricane as the one we have just amounts to at least fi fty per cent of the property. experienced. Th ere is not a house in Galveston that escaped I believe that a sea wall, which would have broken injury, and there are houses totally wrecked in all the swells, would have saved much loss of both life parts of the city. All goods and supplies not over and property. I base this view upon observations

The Great Hurricane of 1900 9 Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

which I have made in the extreme northeastern A list of those lost in Galveston, whose names portion of the city, which is practically protected have been ascertained up to the present time, by the south jetty; this part of the city did not contains , names. [Th is was later revised to as suff er more than half the damage that other many as ,.] similarly located districts, without protection, sustained. UNITED STATES DEPARTMENT OF From the offi cers of the U. S. Engineer tug Anna, AGRICULTURE WEATHER BUREAU I learn that the wind at the mouth of the Brazos OFFICE, 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  GALVESTON, TEX., September , . 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.

10 The Great Hurricane of 1900 Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

Investigation 1.2 Detecting cyclone patterns A 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 view In 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. Costliest Atlantic Hurricanes Refer to the tear-out Quick Reference Sheet located in the Introduction to Rank Name Location Cost this module for GIS defi nitions and instructions on how to perform tasks. billions In the Table of Contents, right-click the Global Patterns data Katrina 1 LA/MS 80.0 frame and choose Activate. 2005 Andrew Expand the Global Patterns data frame. 2 FL/LA 26.5 1992 Th e green dots show the starting point of each of the 4814 tropical Charley 3 FL 15.0 cyclones recorded between 1950 and 2005. Of course, real tropical 2004 cyclones are not just dots — they average 560 km (350 mi) in diameter. Wilma 4 FL 14.4 To grasp the size of these storms, you will look at a series of weather 2005 satellite images of Hurricane Andrew. Andrew was one of the costliest Ivan 5 AL/FL 14.2 Atlantic hurricanes in history, causing over $26 billion in damage. 2004 Source: NOAA Click the Media Viewer button . Note: Damages not adjusted for infl ation. Damages for Katrina and Wilma are estimated. 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 11 Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

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. Using the Measure tool 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.)

Equator (0°) Click the QuickLoad button . Select Spatial Bookmarks, choose Western Hemisphere Equator, To measure the distance from and click OK. the equator to the edge of the cyclone-free zone, click the Use the Measure tool to repeat your measurements of the crosshair cursor on the equator, cyclone-free zone in the Western Hemisphere. move the crosshair to the edge of the zone, and read the distance 4. Th e cyclone-free zone extends approximately ______km from in the status bar. Double-click to the equator. stop measuring. 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?

Symmetry in nature In some areas, the tropical cyclone formation points show a symmetrical Symmetrical (mirror image) pattern above and below the equator. In other areas, this symmetrical patterns are common in nature. pattern is absent. For example, animal faces are symmetrical about a vertical line, 6. On Map 1 on the following page, circle the areas where you would or axis of symmetry. expect, based on symmetry, that tropical cyclones should form, yet they do not.

12 Detecting cyclone patterns Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

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 basins Tropical 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 Cyclone

What’s in a name? Table 1 — Tropical cyclone basin characteristics Cyclone comes from the Greek word kuklos, meaning circle. Basin name Hemisphere Direction of Cyclones Storm Indeed, tropical cyclones not only rotation per year type spin, but they usually move along N/S CW/CCW Average curved paths. Atlantic Hurricane comes Australia / SE from Hurakán, Indian the Mayan god of the skies and lightning. Hurakán Australia / SW literally translates as “one-legged.” Pacifi c Typhoon comes from the N Indian Japanese word taifuu. The NE Pacifi c characters translate literally as “pedestal wind.” 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 Figure 1. Location of newly- loaded Cyclones layer in the Table represents. of Contents.

14 Detecting cyclone patterns Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

Earth’s Seasons are 12. For each tropical cyclone basin, record the hemisphere and dates • Caused by the tilt of Earth’s of greatest tropical cyclone activity in Table 2. Use the information axis. at left to convert each range of dates to a season. • Opposite in the Northern and Southern Hemispheres. Table 2 — Table of tropical cyclone activity Dates Hemisphere Basin name Hemisphere Dates of greatest tropical Season N/S cyclone activity See sidebar NS Atlantic Dec 21 Winter Summer – Mar 20 Australia / SE Indian Mar 20 Spring Fall – Jun 21 Australia / SW Pacifi c Jun 21 Summer Winter – Sep 22 N Indian Sep 22 NE Pacifi c Fall Spring – Dec 21 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?

NASA/GSFC The ITCZ Th e ITCZ, or Inter-Tropical Convergence Zone, is also called Earth’s ITCZ 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 Figure 2. The ITCZ appears as a thunderstorms form as the heated air rises and cools. Th e ITCZ is also band of clouds near the equator. 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?

Detecting cyclone patterns 15 Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

Rare, but not impossible 15. On Map 2, label areas where the ITCZ never crosses the equator. Since the beginning of the satellite Map 2 — Areas near the equator where the ITCZ is absent 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. NASA/GSFC

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.

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.

Quit ArcMap and do not save changes.

16 Detecting cyclone patterns Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

Reading 1.3 Understanding tropical cyclones Solar-powered storms In 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 seasons Th 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). 23.5°

Summer Winter

June 21 December 21

Winter Summer

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.) 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 Tropics Imaginary 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).

Understanding tropical cyclones 17 Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

North Pole Surface heating is greatest near 90° N the equator, where the sun’s 66.5° N Arctic Circle rays strike Earth’s surface S more directly, and pass U 23.5° N Tropic of Cancer through less atmosphere. N The 0° Equator L Surface heating is lowest Tropics near the poles, where the I 23.5° S TTropic of Capricorn sun’s rays pass through more G atmosphere and spread over H 66.5° S Antarctic Circle T a greater area, due to Earth’s 90° S curved surface. South Pole Figure 2. Variation in solar heating with latitude. Outside the tropics, the sun never passes directly overhead. Areas north 0° 45° N 60° N or south of the tropics receive more solar radiation during their summer, Equator New York City Moscow when their hemisphere is tilted toward the sun. 1 m2 1.4 m2 2 m2 Energy and latitude Figure 3. Due to the curvature of Earth’s surface, the amount of light In addition to the seasons, the tropics are warm because of the shape that falls on 1 square meter at of Earth. As latitude increases toward the poles, the sun’s rays strike the the equator spreads out to cover ground at lower angles, spreading the same amount of energy over a about 2 square meters at 60° N greater area, as shown in Figure 3. latitude. 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.

Solid water Liquid water Water vapor

Melting Evaporation

Freezing Condensation

Latent heat energy absorbed

Latent heat energy released

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. Storing energy To understand how solar radiation aff ects the formation of tropical cyclones, you need to know a few things about water and heat energy. Latent heat — heat energy Water can exist in three states — solid, liquid, or vapor (Figure 4). When released or absorbed during a water changes state, it absorbs or releases more heat (called latent heat) change of phase. than do most other substances.

18 Understanding tropical cyclones Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

Heat capacity — the amount Water can absorb more energy than most other substances because it of energy required to raise the has a high heat capacity. Raising the temperature of water requires a lot temperature of one unit of mass of energy. Th erefore, it is very diffi cult to change the temperature of the of a substance by 1 °C. 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?

Understanding tropical cyclones 19 Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

Putting a spin on tropical cyclones Two 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. NASA/GSFC

North

Figure 5. Hurricane Floyd approaches the Florida coast in September 1999. The Coriolis effect Each 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.

45˚ N

Equator

4 5˚ N E quator Figure 6. As Earth rotates, points on the surface move more slowly as latitude increases from the equator toward the poles. 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

20 Understanding tropical cyclones Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

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 Earth’s rotation curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

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 latitude As 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 cyclones Due 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).

90° N 60° N Polar easterlies Prevailing westerlies 30° N Figure 8. Global air circulation Trade winds patterns. The ITCZ forms where ITCZ the trade winds from the two hemispheres meet. Trade winds 30 °S Prevailing westerlies 60° S Polar easterlies 90° S

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.

July Equator January

Figure 9. Average position of the ITCZ in January and July. 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 Tropical depression — weather and their rotation and uplift (increasing elevation from the surface to system with a maximum sustained the top of the system) increase. If the winds in the rotating system reach surface wind speed of 38 mph (62 km/hr) or less. certain speeds, the system is upgraded to a tropical depression or a tropical cyclone. Driving storms In 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 L (Figure 10a). In the Southern Hemisphere, the winds veer to their left , spiraling inward in a clockwise direction (Figure 10b). Th is spiral motion, a in opposite directions in each hemisphere, produces the characteristic shape of tropical cyclones. Low-pressure storm systems are embedded in air that moves according L 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 b 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 Figure 10. Low-pressure systems in the Northern Hemisphere (a) eff ect: rotate in a counterclockwise direc- • In the Northern Hemisphere, storms rotate in a counterclockwise tion. Low-pressure systems in the Southern Hemisphere (b) rotate in direction, but follow clockwise paths. a clockwise direction. • In the Southern Hemisphere, storms rotate in a clockwise direction, but follow counterclockwise paths.

22 Understanding tropical cyclones Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

Steering Atlantic hurricanes Th 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 Area shown resembles the tracks of many of the Atlantic hurricanes you examined. below 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.

H H H H

July 15, 1978 July 15, 1983 July 15, 1988 July 15, 1993 Figure 11. Location of the Bermuda High on July 15 in various years. 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?

Understanding tropical cyclones 23 Data Detectives: Tropical Cyclones Unit 1 – Recipe for a Cyclone

Developing the cyclone Heat, 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.

E

D

C

B

A

Figure 12. Process of intensification in the formation of tropical storms. Required ingredients Th 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.

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 shear Even 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). Low wind shear High wind shear

Surface Surface Figure 13. Vertical wind shear disrupts the formation of tropical cyclones by cutting off the uplift of warm, moist air from the surface. 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. Troposphere — the lowest layer To form or maintain a tropical cyclone, the vertical wind shear between of Earth’s atmosphere. It begins the surface and the upper troposphere must be less than 37 km/hr (23 at Earth’s surface and extends mph). upward 8 – 14.5 km (5 – 9 mi). 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?

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

Investigation 1.4 Powering tropical cyclones 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 cyclones So 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 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 seasons To show how sea-surface temperatures change throughout the year, a Earth’s Seasons series of SST maps have been assembled into a movie. Click the Media Viewer button . Dates Hemisphere Choose SST Movie from the media list and view the movie several NS times. Dec 21 Winter Summer – Mar 20 Th e colors represent sea-surface temperature averaged over a 9-year period. Mar 20 Spring Fall – Jun 21 5. How does the shift in sea-surface temperatures correspond to the Jun 21 seasons in the Northern and Southern Hemispheres? Summer Winter – Sep 22 Sep 22 Fall Spring – Dec 21

Close the Media Viewer window. Turn off the February SST layer.

Searching for a minimum cyclone formation temperature Is 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.

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 at 25 ˚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

1100 1000 900 800 700 600 500 400 300 200

Number of tropical cyclones Number of tropical 100 0 25 26 27 28 29 30 Temperature (°C) 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?

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?

Hint for question 12 12. Logically, the warmer the water, the more tropical cyclones should occur. Why do you think the number of tropical cyclones on the Look at one of the sea-surface temperature layers. What would graph actually decreases for temperatures above 29 °C? a graph of the sea-surface temperature versus area look like?

What do scientists say? 13. If global warming is a real phenomenon, and ocean temperatures For a detailed though somewhat increase worldwide, how do you think this could affect the technical discussion of the eff ects frequency, latitude range, and intensity of tropical cyclones. Justify of global warming on tropical your answer. 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 Quit ArcMap and do not save changes. 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 1.5 Solving the cyclone puzzle Earlier, 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 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 Tropical cyclone checklist shear should be low — less than 37 km/hr (23 mph) from In this unit, you have examined specifi c conditions or “ingredients” that the surface to the upper must be present for tropical cyclones to form and grow. Th e four key troposphere. (Reading 1.3) ingredients are described at left . Based on everything you have read and • A stable energy source — observed in this unit, see if you can fi gure out why tropical cyclones warm ocean water (≥ 26.7 ˚C) extending to a depth of almost never form in these two areas (marked by question marks in the about 50 meters or more. As above map). surface water evaporates, it 1. Complete this tropical cyclone formation checklist for the South intensifi es the convection within the system. (Reading Atlantic and Southeastern Pacifi c Oceans. 1.3 and Investigation 1.4) Don’t Yes No know a. Do weak, tropical, low-pressure A process of elimination systems form there? One way to solve a problem is to b. Does the ocean surface there reach use a process of elimination to temperatures of 27 °C or warmer? fi nd the solution. By identifying the ingredients that are not the c. Do the regions have low vertical cause of the problem, you may be wind shear? able to narrow the choices of the correct cause down to just one or d. Are parts of the region more than two possible “culprits.” If you can’t eliminate a cause based on the 500 km away from the equator? evidence, you must consider it as e. Does the ITCZ cross the region at a possible factor. some time of year?

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