unit 2 the life of a cyclone - delmarlearning.com · 33 data detectives: tropical cyclones unit 2...

In this unit, you will learn • What are the stages in the life of a tropical cyclone?

• How, where, and why do tropical cyclones die?

• How do scientists track tropical storms?

• How do you determine the intensity of a tropical storm?

Unit 2

The Life of a Cyclone

Three satellite images superimposed on the same map show the development of Hurricane Georges on September 25, 1998.

NA

SA/G

SFC

33

Data Detectives: Tropical Cyclones Unit 2 – The Life of a Cyclone

34


Warm-up 2.1 Observing tropical cyclonesTh e 1900 Galveston Hurricane was the worst natural disaster in U.S. history. Why was this hurricane so destructive? Part of the answer is that, in 1900, forecasters had to make predictions based almost exclusively on direct observation. Th e skies, the tides, and the changing winds and air pressure were their only clues that a hurricane was coming. Unfortunately, by the time they realized it, the storm was nearly upon them. Th e forecasters had little time to warn the residents to evacuate or to seek shelter.

Today’s forecasters use sophisticated tools to locate and track tropical cyclones well before they make landfall. By modeling tropical cyclones on computers, researchers are improving their ability to accurately predict storm tracks. Using satellites and other modern tools, forecasters observe and measure cyclones remotely. To learn about both direct and indirect methods of observing tropical cyclones, point your Web browser to the following NOAA Web site:

hurricanes.noaa.gov/prepare/observation.htm

On the NOAA Web page, click on each of the technologies to learn more about it. Find out how each tool helps scientists monitor these dangerous storms. In Table 1, list the name of each technology and tell whether it provides a direct or an indirect method of observation.

Table 1 — Modern weather-observation technologies

Number Name of technology Direct or Indirect?

1

2

3

4

5

6

Who or what is NOAA?NOAA is the National Oceanic and Atmospheric Administration. It is a government agency that conducts research and gathers data about the oceans, atmosphere, space, and sun.

NOAA warns of dangerous weather, charts our seas and skies, guides our use and protection of ocean and coastal resources, and conducts research to improve our understanding and stewardship of the environment.

Part of the U.S. Department of Commerce, NOAA provides services through fi ve major organizations.

• National Weather Service

• National Ocean Service

• National Marine Fisheries Service

• National Environmental Satellite, Data and Information Service

• NOAA Research and special program units

Media Viewer web siteIf you have any view of the cyclones.apr project open in ArcView, you can click the Media Viewer button and choose Observing Tropical Cyclones from the media list to open this web page in your browser.

NO

AA

1 2 3

45 6

Observing tropical cyclones 35


1. What are the two main types of satellite imagery used in tropical cyclone forecasting?

2. Radiosondes and dropsondes help us understand the structure of tropical cyclones by measuring and transmitting information about which atmospheric properties?

3. What is the most direct method of measuring the wind speeds within a tropical cyclone?

4. Which two important properties of tropical cyclones are measured using radar?

36 Observing tropical cyclones


Tropical cyclones do not start out fully developed. Like people, who grow from infant to child to adult, tropical cyclones also experience stages of growth. Unfortunately, instead of “mellowing out,” hurricanes get nastier and generally more dangerous with age. Th is investigation explores the development of a single hurricane, Georges (pronounced zhorzh), that aff ected Atlantic and Caribbean coastal communities in September 1998.

Launch ArcView, locate and open the cyclones.apr fi le, and open the Tracking Georges view.

Th is view shows the path, or track, of Hurricane Georges as it swept across the Atlantic and the Caribbean. Hurricane Georges was the 19th most deadly tropical cyclone in history. Th e large number of casualties occurred as Georges passed directly over several Caribbean islands.

An eye in the skyTh is view’s table of contents contains seven themes, labeled by date and time, that show satellite images of Hurricane Georges at diff erent points along its journey across the Atlantic Ocean.

Begin with the 9/15 10:19 UT theme, then turn on each of the seven hurricane image themes in order by date.

Th e hurricane’s clouds were not bright colors like those you see on your screen (Figure 1). Th ese false colors show the temperatures of the cloud tops recorded by the satellite’s infrared sensor. Grays and whites represent the warmest temperatures, whereas greens, yellows, and reds show the coldest (Figure 2). Temperature decreases with

Investigation 2.2 Tracking Hurricane Georges

Higher cloud tops = Cooler temperatures = Stronger convection

Lower cloudtops =

Warmer temperatures =

Weaker convection

Incr

ea

sin

g a

ltit

ud

e

De

cre

asi

ng

te

mp

era

ture

red

orange

yellow

green

blue

violet

white

gray

black

Figure 2. Relationship between color, cloud temperature, and height in false-color satellite images.

NO

AA

Figure 1. False-color image of Hurricane Georges. The highest (coldest) cloudtops shown in orange and yellow, indicate where upward convection is strongest.

To turn a theme on or off , click its checkbox in the Table of Contents.

Tracking Hurricane Georges 37


altitude, so colder temperatures indicate higher cloud tops. As the upward convection in a storm intensifi es, clouds rise higher into the atmosphere. Th us, the cloud temperatures tell us about the strength of the convection, which in turn indicates the energy of the hurricane.

1. On what date did Hurricane Georges make landfall on the U.S. mainland? How many days did it take Hurricane Georges to travel across the Atlantic Ocean before landfall?

2. Convert the number of days from question 1 into hours.

3. In what direction did Georges travel as it crossed the ocean?

Now you will measure the total distance Georges traveled and use it to fi nd the average rate of travel, or speed of the storm.

Using the Measure tool , click on the September 15 storm center. Drag the measuring tool through each of the fi ve storm centers. Double-click on the last storm center (September 27) to fi nish the measurement. Th e total distance Georges traveled (Length) is reported in the status bar.

4. How far (in km) did Georges travel?

5. Use this distance and the number of hours you calculated in question 2 to fi nd Georges’ average speed (in km/hr) as it crossed the Atlantic Ocean (speed = distance ÷ time).

Universal Time (UT)The times listed in the Hurricane Georges IR Images layer group are in 24-hour Universal Time format. Abbreviated UT, this is the time at 0° longitude, or the prime meridian.

The U.S. Atlantic Coast is 5 time zones west of the prime meridian.Subtract 5 hours from Universal Time to get local time for the Atlantic Coast.

Read distance here(your distance will be different).

38 Tracking Hurricane Georges


6. An average person walks at a speed of about 6.5 km/hr, and runs at about 10 km/hr. Compare these rates with the average speed you calculated for Hurricane Georges.

7. If a major tropical cyclone is spotted 500 km off shore, how much warning time might a coastal community have before the storm strikes? (time = distance ÷ speed)

On the path of a killerNext you will learn about the countries aff ected by Georges as it traveled across the Atlantic. Look at the Storm Track theme, which shows the path of the center of the hurricane as it crossed the Atlantic Ocean.

Turn the Georges images on and off as needed to better see the track.

Activate the Countries theme.

Using the Identify tool , click on a country or island and read the Name of the country in the Identify Results window.

If you need to identify U.S. states, activate the States theme before using the Identify tool .

8. Identify these landmarks associated with Hurricane Georges as it crossed the Atlantic Ocean:

a. Hurricane Georges formed south of this group of islands off the west coast of Africa: _______________________.

b. Georges passed directly over three large Caribbean island countries and one U.S. territory. Th ese are ________________, ______________, _____________, and _______________.

c. Th e fi rst U.S. state aff ected by Georges’ heavy rainfall and strong winds was _________________.

d. Th e U.S. state where the eye of Hurricane Georges fi rst made landfall was ____________________.

Hurricane formation Click the QuickLoad button , select the Formation and

Movement view, and click OK.

Turn on the Formation Points theme.

To activate a theme, click on its name in the Table of Contents.



Each point in this theme indicates where a storm system fi rst showed the typical features of a tropical cyclone in the Atlantic between 1950 – 2005. Notice that the points are not evenly distributed.

9. On Map 1, identify and label two areas where hurricane formation in the Atlantic appears to be concentrated.

Map 1 — North Atlantic Ocean hurricane-formation regions

Turn on the End Points theme.

Look for areas where hurricanes appear to lose strength and die.

10. Identify and label two areas on Map 1 where hurricanes tend to weaken and die out.

Turn on the Hurricane Tracks theme.

Th e tangled lines are the paths traveled by Atlantic hurricanes from 1950 to 2005. Each path was defi ned by recording the location of a storm’s center at regular intervals and then connecting the dots. Before the existence of weather satellites, storm locations were discovered by aircraft and by reports from ships at sea. Next, you will determine the number of hurricanes that have crossed the Atlantic over the past 50 years.

How many hurricanes?Every hurricane is a unique event with its own “personality.” Th is may be one reason why it seems fi tting to give hurricanes human names. Still, most hurricanes go through similar life cycles. Next, you will determine how many hurricanes occurred between 1950 and 2005.

Activate the Hurricane Tracks theme.

Click the Open Th eme Table button .

Read the total number of hurricanes from the status bar above the table (as shown below):

Do not be fooledRemember that each hurricane track represents the path traveled by a large and dangerous storm. Each storm aff ected an area far wider than the thin lines you see.

Read total here.Your answer will be different.



Prevailing winds — surface winds that generally fl ow in the same direction for long time periods. In general, winds over the U.S. mainland blow from the southwest, and are called the prevailing westerlies.

11. How many Atlantic-basin hurricanes occurred between 1950 – 2005?

12. What is the average number of hurricanes that occur each year in the Atlantic Ocean basin? (total number of hurricanes ÷ years)

Close the theme table.

Turn off the Formation Points and End Points themes.

Global wind patterns and hurricane tracksNext you will look at wind patterns and determine their infl uence on hurricane movement in the Atlantic Ocean and the Gulf of Mexico.

Turn on the Wind Patterns (km/hr) theme.

Use the Zoom In to examine the wind data in the North

Atlantic Ocean.

Th e arrows indicate the direction of prevailing winds over the Atlantic Ocean.

13. In what direction do the prevailing winds over the Atlantic Ocean appear to be moving near the equator?

14. In what direction do the prevailing winds over the Atlantic Ocean appear to be moving near 40° N latitude?

15. Do the prevailing winds over the Atlantic appear to be rotating? If so, are they rotating clockwise or counterclockwise?

Turn off the Wind Patterns (km/hr) theme.

Activate the Hurricane Tracks theme.

Using the Select Feature tool , click on one of the hurricane tracks. One or more hurricane paths will be highlighted.

Repeat this several times at random locations and try to identify any general patterns in hurricane movement.

Reading coordinatesThe cursor’s longitude and latitude are displayed on the right end of the tool bar.

longitude

latitude



16. Draw the general pattern(s) of hurricane movement on Map 2. Be sure to include arrows to indicate the direction of movement.

Map 2 — Atlantic hurricane-movement patterns

Choose View Full Extent to view the entire map.

Turn on the Global Tropical Cyclone Movement theme.

17. Describe the relationship between the paths of Atlantic tropical cyclones and global wind patterns.

Quit ArcView and do not save changes.



Classifying tropical cyclonesAn average tropical cyclone lasts about 12 days. If conditions are favorable, it may progress through four stages of development.

1 – Tropical disturbanceA tropical cyclone begins as a low-pressure area with warm, moist air rising from the ocean surface. As the moisture condenses, clouds and precipitation form. If the system sustains itself for 24 hours, it is called a tropical disturbance. Winds in a tropical disturbance blow at up to 36 km/hr (22 mph), but the system has very little rotation.

2 – Tropical depressionAs convection intensifi es and the surface pressure decreases further, the tropical disturbance begins to rotate. When the winds reach speeds over 36 km/hr (22 mph), the system is classifi ed as a tropical depression.

3 – Tropical stormIf a tropical depression intensifi es, with wind speeds increasing to 63 km/hr (39 mph) or higher, it is called a tropical storm, and is assigned a name. Th e eye of the storm may become visible, and the storm begins to resemble a tropical cyclone in many ways.

4 – Tropical cycloneWhen wind speeds exceed 117 km/hr (73 mph), the storm is classifi ed as a tropical cyclone or hurricane. Distinct bands of thunderstorms rotate around the eye of the storm. Th e eye is an area of calm surrounded by the eye wall, where the winds reach their maximum speed.

Reading 2.3

Figure 2. Tropical cyclone family portrait. This unique satellite image taken on August 31, 1996 shows systems at each of the four stages described. Hurricane Edouard, Tropical Storm Fran, Tropical Storm Gustav (soon to be downgraded to a tropical depression), and Tropical Disturbance #8 (which eventually developed into Hurricane Hortense).

Naval R

ese

arch Lab

orato

ry, Mo

nte

rey, C

A

Edouardtropical cyclone

Frantropical storm Gustav

tropical depression Hortensetropical disturbance

Eye — a region of relatively calm, clear air that forms as rising air from the thunderstorms converges and sinks in the center of the storm.

Eye wall — ring of tall thunderstorms surrounding the eye, where the strongest winds and rain are found.

Rain bands — long bands of thunderstorms spiraling inward toward the eye wall .

Centraleye

NA

SA/G

SFC

Eyewall

Rain bands

Figure 1. Tropical cyclone structure.

Classifying tropical cyclones 43


1. If a tropical storm is downgraded to a tropical depression, what does this mean about the wind speeds?

2. Fewer than 10 percent of tropical disturbances evolve into tropical cyclones. Based on what you have learned about tropical cyclones, why do you think this percentage is so low?

Intensity scalesForecasters responsible for monitoring tropical cyclones have developed scales to describe a storm’s potential for destruction. Th e U.S. uses the Saffi r-Simpson hurricane intensity scale, shown below. Th is scale ranks hurricanes in fi ve categories from 1, the weakest, to 5, the strongest.

Table 1 — Saffi r-Simpson Hurricane Scale

Saffi r-Simpson Category

Maximum sustainedwind speed

Min. surface

pressure

Storm surge

mph km/hr knots mb ft m

Tropical depression

< 39 < 63 < 34 --- --- ---

Tropical storm

39 – 73 63 – 118 34 – 63 --- --- ---

H1 74 – 95 119 – 153 64 – 82 > 980 3 – 5 1.2 – 1.5

H2 96 – 110 154 – 177 83 – 95 979 – 965 6 – 8 1.8 – 2.4

H3 111 – 130 178 – 209 96 – 113 964 – 945 9 – 12 2.7 – 3.6

H4 131 – 155 210 – 250 114 – 135 944 – 920 13 – 18 3.9 – 5.5

H5 > 155 > 250 > 135 < 920 > 18 > 5.5

Why rank hurricanes?Th e categories of the Saffi r-Simpson and similar scales used around the world estimate the potential for fl ooding and other damage from tropical cyclones. Wind speed is related to the surface pressure in the center of the hurricane. Th e maximum wind speed and air pressure refl ect the storm’s energy and destructive potential. Category 3 – 5 hurricanes are considered to be major, and are capable of causing tremendous damage.

Knot — a measure of speed, used in air and water travel, equal to

• 1 nautical mile per hour

• 1.852 km/hr

• 1.15 mph

The word knot comes from an early device used to measure a ship’s speed. The device consisted of a rope with regularly spaced knots tied to a weighted wooden fl oat called a log. The pilot threw the log overboard, and counted the number of knots that ran out while a sand glass emptied. This gave the speed of the ship in nautical miles per hour.

Mb — stands for millibar, a metric unit of atmospheric pressure. One mb is 1/1000 of a bar, and is equal to 100 Pa (pascals). Standard atmospheric pressure at sea level is 1013.25 mb (29.92 in Hg).

What is the “H” in H1?In the North Atlantic, tropical cyclones are called hurricanes, so “H” stands for Hurricane. Here, it means the same thing as category.

44 Classifying tropical cyclones


Measuring wind speedTh e most meaningful way to measure wind speed is to calculate the average surface wind speed over a period of time. It is then reported as the maximum sustained surface wind speed.

Th e main diff erence between intensity scales used by the U.S. and other countries is the length of time over which the wind speed is averaged. Th e World Meteorology Organization recommends using a 10-minute average, whereas U.S. forecasters continue to use a 1-minute average.

Storm surgeStorm surge can be the most destructive aspect of a tropical storm (Figure 3). Storm surge is a mound of water piled up on the leading edge of a tropical cyclone by strong winds blowing across the ocean surface. As the surge washes ashore, it temporarily raises sea level over wide areas of the shoreline.

Storm intensity and damageTh e Saffi r-Simpson Hurricane Scale provides an estimate of the damage potential of hurricanes based on wind speeds and storm-surge heights.

Use the Saffi r-Simpson chart to answer the following questions.

3. As a storm grows from a tropical depression to a Category 5 hurricane, how do the surface pressure and the wind speed change?

4. Storm surge adds to the normal tides for a section of coastline. How would the time of day aff ect the severity of the storm surge striking coastal areas?

5. How high would sea level be if an H3 storm made landfall at high tide and the normal tides range ±2 m from mean sea level?

Figure 3. For Atlantic storms, the surge is usually located in the right front quadrant, where the storm’s rotation and its forward motion add to produce the strongest winds. This satellite photograph shows Hurricane Andrew approaching the Louisiana coast on August 25, 1992.

Path

NA

SA/G

SFC

Strongest winds

Highestsurge

Classifying tropical cyclones 45


6. How high would sea level be if the same H3 storm made landfall at low tide?

Use Figure 4 to answer the following questions.

7. In general, what happens to the damage caused by hurricanes as their intensity increases?

8. About how many times more destructive, in terms of dollar damage, is an average H5 hurricane than an average H1 hurricane?

Logically, property damage should increase with hurricane intensity. However, this chart shows that the most destructive H1 hurricanes caused more damage than the most destructive H2 and H3 hurricanes.

9. Describe a situation where a H1 hurricane could strike land and cause more damage than more intense storms.

0

5

20

15

10

25

30

35

40

H1 H2 H5H4H3

Storm category (n = number of storms)

Da

ma

ge

in b

illio

ns

of

do

llars

n = 3 n = 4 n = 15 n = 5 n = 4

$ 7.1

$ 3.5$ 5.7

$ 7.0

$ 36.2

Figure 4. Bar graph depicting the average monetary damage inflicted by the top 31 most damaging Atlantic hurricanes of varying strength that made landfall between 1950 and 2005.

46 Classifying tropical cyclones


Monitoring cyclone growthInvestigation 2.4In an earlier investigation, you examined the path of Hurricane Georges across the Atlantic Ocean. In this investigation, you will use measurements of the wind speed to classify the intensity of the hurricane at each point along its path.

Launch ArcView, locate and open the cyclones.apr fi le, and open the Tracking Georges view.

Turn on the 9/15 10:19 UT theme. Th e other Georges satellite image themes should be turned off .

Turn on the Storm Centers theme.

Activate the Storm Centers theme.

Using the Identify tool , click the Georges 9/15 storm center. Th e Identify Results window provides information about the conditions at the center of the developing cyclone.

1. Record the pressure in millibars (mb) and wind speed in km/hr for September 15th in Table 1.

Table 1 — Development of Hurricane Georges

Hurricane Georges image date

Characteristic How to fi nd 9/15 9/17 9/18 9/20 9/23 9/27

Pressure (mb) use Identify tool

Wind speed (km/hr) use Identify tool

Category Saffi r-Simpson Scale

Rotation look at images

2. Use the pressure and wind speed to fi nd the Saffi r-Simpson category (Table 1 on page 44) on this date and enter it in Table 1 above.

Early in the development of a tropical cyclone, the storm begins to rotate. Look carefully at the image for evidence of rain bands, a central eye, or any other visual clue that the storm has begun to spin (Figure 1).

3. Write either N (none), F (faint), or S (strong) for the storm’s rotation in Table 1.

Repeat this process by turning on the other Georges satellite image themes and complete the table for the remaining dates.

Close the Identify Results window when you are fi nished.

Answer the questions on the following page using the table and your observations.

NA

SA/G

SFC

Figure 1. Hurricane Andrew approaches the Louisiana coast on August 25, 1992.

pathstrongest

winds

highestsurge

To turn a theme on or off , click its checkbox in the Table of Contents.

To activate a theme, click on its name in the Table of Contents.

What is the “H” in H1?In the North Atlantic, tropical cyclones are called hurricanes, so “H” stands for Hurricane. Here, it means the same thing as category.

Monitoring cyclone growth 47


4. When did Georges fi rst reach hurricane status? How did its appearance change from the previous image?

5. When was Georges at peak intensity?

6. What was Georges’ maximum wind speed at its peak intensity?

Use the Measure tool to measure the diameter of Hurricane Georges at its peak, when its wind speeds were the highest (see Sidebar). Read the diameter in the status bar.

7. How wide (in km) was Georges at its peak intensity?

8. How did Georges’ structure change aft er it passed over Haiti and the Dominican Republic?

9. Look at the 9/29 image of Georges. How did the storm’s appearance change? What caused this change?

Using the Measure tool

Position the measuring tool cursor on one side of the hurricane, then click and drag across to the opposite side of the storm. Read the distance in the status bar. Double-click to stop measuring.

Read distance here(your distance will be different).

48 Monitoring cyclone growth


Selecting areasTo select an area, click its border using the Pointer tool . “Handles” appear when the area is selected. Below, Area 3 is selected.

handle

Catastrophic hurricanesTh e fi nal part of this exercise looks at the life cycles of the most powerful hurricanes, using the 80 Category 4 and 5 hurricanes that have occurred between 1950 and 2005. Th ese storms had wind speeds greater than 210 km/hr (130 mph) and were capable of catastrophic damage. As hurricanes develop and move across sea and land, their energy, wind speed, and potential for damage can change. In this section, you will examine some of these changes and identify where hurricane intensity appears to increase or decrease.

Click the QuickLoad button , select the Hurricane Intensity view, and click OK.

Th is view shows the storm tracks for all of the Category 4 and 5 hurricanes between 1950 and 2005. Changes in the width and color of the tracks indicate changes in the wind speed as the storms progressed from Tropical Storm to a Category 4 or 5.

Now you will examine the relationship of wind speed to storm location in more detail. Th e hurricanes in each of the fi ve outlined areas have unique characteristics.

Activate the Catastrophic Hurricanes (km/hr) theme.

Select the outline of Area 1 (see Sidebar for Area numbers) by clicking on its boundary line using the Pointer tool . Handles (small boxes) appear around the boundary to indicate that it has been selected.

Click the Select Features Using Graphics button . Hurricane tracks that fall within the outline will be highlighted yellow.

Click the Statistics button .

Calculate statistics for the selected features of the Catastrophic Hurricanes (km/hr) theme, using the Winds Speed (km/hr) fi eld.

Select the Basic output option and click OK.

Area locator

4 5

213



Th e average wind speed for Category 4 and 5 hurricanes in Area 1 (Coast of Africa to Mid-Atlantic) between 1950 – 2005 is reported in the Statistics window as the Mean.

10. In Table 2, record the average wind speed for the Area 1, and whether the area is over land or over water. Round to the nearest whole number.

Area Average wind speed km/hr

Over land or Over water?

1 Coast of Africa to Mid-Atlantic

Mid-Atlantic to U.S. East Coast

3 Caribbean and Gulf Coast

4 Eastern U.S.

5 North Atlantic

Close the Statistics window.

Repeat the selection and statistics procedures for the other four areas to complete Table 2.

Click the Clear Selected Features button to deselect the highlighted hurricane tracks.

Turn on the August SST (C) theme.

Th e August SST (C) theme shows the average sea-surface temperature, in degrees Celsius, for the month of August. Use the sea-surface temperature map and Table 2 to answer the following questions about Atlantic hurricanes:

11. Describe the location(s) and, if available, the sea-surface temperature where the wind speeds are the highest.

12. Describe the location(s) and, if available, the sea-surface temperature where the wind speeds appear to diminish.

Area locator

4 5

213



13. Briefl y describe how wind speed (and thus storm energy) changes

a. when hurricanes move over warm, tropical water.

b. when hurricanes move onto land.

c. when hurricanes move over colder water.

d. when hurricanes move over the Gulf of Mexico.

Quit ArcView and do not save changes.



unit 2 the life of a cyclone - delmarlearning.com · 33 data detectives: tropical cyclones unit 2...

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