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GPH 113 PHYSICAL GEOGRAPHY LABORATORY SEVERE WEATHER
Target Skills
Become familiar with public advisories issued by the National Weather Service
Track the development of a hurricane
Identify factors that influence the track a hurricane follows
TOOLS YOU’LL BE USING:
Colored pencils
Hurricane Katrina Supplementary Materials
KEY TERMS
Tropical disturbance
Hurricane
Spiral rain band
Tropical depression
Eye Storm surge
Tropical storm
Eyewall
Hurricane Katrina
EXERCISE 6, PART 1: HURRICANES
A hurricane is a well-organized system of severe thunderstorms rotating around a central eye. Several conditions are necessary in order for a storm to develop. First, ocean temperatures must exceed 26° C (about 80°F) in order to supply enough heat energy and moisture to fuel the storm. Second, an atmospheric disturbance must be present in order to create the initial low pressure and uplift of air. For Atlantic Hurricanes, these disturbances usually come from Africa and move east over the ocean. Third, the disturbance must develop between 5° and 30° latitude.
When all of these conditions are present, the disturbance may eventually develop into a hurricane. In the Northern Hemisphere, the Coriolis effect and the pressure gradient force cause winds to spiral in towards the low pressure in a counterclockwise manner. The thunderstorms begin to circulate into the low-pressure center. If the upper atmospheric winds are weak and do not sheer off the top of the storms, the system will continue to strengthen. During the initial phase of organization, when the circulation is still weak, the storm is known as a tropical disturbance. As the storm strengthens, the low pressure will deepen and there will be at least one closed-loop isobar around it. When isobars begin to close and winds reach 20 knots, the system becomes a tropical depression. As the low continues to deepen, more isobars will become closed and they will be very tightly spaced, indicating a steep pressure gradient. Wind speeds increase to 35 knots or greater, and the system becomes a tropical storm. At this stage, the storm is officially given a name. (For the list of storm names for the next few years, see Table 7.1.) The tropical storm becomes a hurricane when sustained wind speeds exceed 64 knots (64 mph).
INSERT DIAGRAM
Figure: Anatomy of a hurricane. Note the location of the eye, the eyewall, and the rain bands. Source: NOAA.
A fully developed hurricane has several important features. At the very center is the eye, a zone of clear skies and extreme calm caused by sinking air that is being sucked down into the extremely low central pressure at the surface. The lowest pressure of the storm is located within the eye. Although the central pressure is usually 915-935 mb, extremely strong hurricanes can have even lower pressures. (The hurricane that struck the Florida Keys in 1935 had a central pressure of 892 mb!) Immediately around the eye is the eyewall, the region with the strongest winds and most intense hurricanes. The rest of the storm consists of giant spiral rain bands – long lines of thunderstorms spiraling in towards the center of the storm. Once developed, a hurricane’s circulation tends to pull warm, moist air into the storm, where it rises, condenses, and releases latent heat into the storm. Therefore, a hurricane operates on a positive feedback cycle of growing stronger, pulling even more moisture and latent heat energy into the center, and therefore growing stronger. A hurricane will continue to strengthen as long as conditions remain favorable. Once it makes landfall or moves north into colder waters, however, it will eventually dissipate because it no longer has sufficient energy to continue operating.
While hurricane-force winds are incredibly intense and can be very destructive, one of the most damaging effects of the hurricane is often the storm surge. The storm surge is a large wall of water that is pushed up ahead of the storm as it travels over open water. When the hurricane approaches land, the storm surge can cause sea levels to instantly rise by up to 10 feet or more, causing widespread flooding in low-lying areas. If the storm surge strikes during high tide, the flooding can be even worse.
Hurricane strength is rated by the Saffir-Simpson scale. For a summary of the Saffir-Simpson categories, see Table 7.1.
INSERT TABLE
Table 7.1: Summary of the Saffir-Simpson Hurricane Scale
SAFFIR-SIMPSON CATEGORY
SUSTAINED WIND SPEEDS (knots)
STORM SURGE
(feet)
1
64-82
4-5
2
83-95
6-8
3
96-113
9-12
4
114-135
13-18
5
>135
>18
Table 7.2: Official list of hurricane names for upcoming years. List provided by the National Hurricane Center, via NOAA.
2023
2024
2025
2026
2027
2028
Arlene
Bret
Cindy
Don
Emily
Franklin
Gert
Harold
Idalia
Jose
Katia
Lee
Margot
Nigel
Ophelia
Philippe
Rina
Sean
Tammy
Vince
Whitney
Alberto
Beryl
Chris
Debby
Ernesto
Francine
Gordon
Helene
Isaac
Joyce
Kirk
Leslie
Milton
Nadine
Oscar
Patty
Rafael
Sara
Tony
Valerie
William
Andrea
Barry
Chantal
Dexter
Erin
Fernand
Gabrielle
Humberto
Imelda
Jerry
Karen
Lorenzo
Melissa
Nestor
Olga
Pablo
Rebekah
Sebastien
Tanya
Van
Wendy
Arthur
Bertha
Cristobal
Dolly
Edouard
Fay
Gonzalo
Hanna
Isaias
Josephine
Kyle
Leah
Marco
Nana
Omar
Paulette
Rene
Sally
Teddy
Vicky
Wilfred
Ana
Bill
Claudette
Danny
Elsa
Fred
Grace
Henri
Imani
Julian
Kate
Larry
Mindy
Nicholas
Odette
Peter
Rose
Sam
Teresa
Victor
Wanda
Alex
Bonnie
Colin
Danielle
Earl
Farrah
Gaston
Hermine
Idris
Julia
Karl
Lisa
Martin
Nicole
Owen
Paula
Richard
Shary
Tobias
Virginie
Walter
PRACTICE
On the picture of Hurricane Fran (1996) above, Draw lines to “cut” the hurricane into four quarters. Use the white arrow to help you to determine direction the hurricane is moving so that you know front/back and left/right; then use a green or blue pencil to draw arrows depicting the direction of wind flow.
Which quadrant(s) of the storm (front/left, front/right, back/left, or back/right) has wind directions that match the direction of the hurricane’s movement (shown by large white arrow)?
Which quadrant would you expect to have the weakest winds? Why?
Which quadrant would be the most likely to produce very severe weather, such as tornadoes? Why?
In which quadrant would the storm surge be the greatest? Why?
EXERCISE 7, PART 2: HURRICANE KATRINA
THE 2005 HURRICANE SEASON
The 2005 Atlantic hurricane season has been one of the most prolific ever recorded. By early October, only one name remained on the list, with several weeks of hurricane season yet to go. There are many factors that contribute to the strength of a hurricane season, such as how much rainfall has recently occurred in Eastern Africa, the strength of the subtropical high, and current El Niño conditions. Despite popular belief, according to NOAA, research has shown that it is unlikely that global warming has had any significant impacts on hurricane activity.
Hurricane Stan struck Guatemala on October 4, 2005 and triggered massive flooding and mudslides that wiped out entire villages. Search and rescue officials were forced to stop looking because they didn’t know where to dig under the mud, and entire cities were declared official cemeteries. In the United States, the most prolific hurricane of the season was Hurricane Katrina, which made landfall three times (once in Florida, once on the peninsula tip of Louisiana, and once on mainland Louisiana/Mississippi) before it finally dissipated! The remainder of this section of the lab exercise consists of a case study of Hurricane Katrina.
HURRICANE KATRINA
Read through the series of public advisories and discussions issued by the National Hurricane Center from August 23 through August 29, 2005. For each day (from the advisories), mark Katrina’s location on the map above with a small hurricane symbol () and label the date (and time if necessary) for each position.
In green, circle the position where Katrina became a named tropical storm.
In blue, circle the position where Katrina became a hurricane.
In red, circle the position where Katrina became a category 5 hurricane.
On the following plot, chart the central pressure of the storm on each given day and time. When you have plotted all the points, connect them. Central pressure can be found by scanning the advisories
Central Pressure (mb)
1010mb
1000mb
990mb
980mb
970mb
960mb
950mb
940mb
920mb
910mb
900mb
8/23
5pm
8/24 11am
8/25
8am
8/25
5pm
8/26
9am
8/27
10am
8/28
10am
8/29
2pm
Date (month/day) and Time (hours)
According to your plot, what happened to the central pressure of the storm between August 25 and August 27? Why did the central pressure of Hurricane Katrina behave in this way?
What happened to the central pressure of Hurricane Katrina between August 26 and August 28? Why did the central pressure of the hurricane behave in this way?
Look at the satellite imagery of Hurricane Katrina (in supplementary information). On which day can you first see an organized storm system with an eye?
9. Look at the satellite image of Hurricane Katrina as it was about to make landfall on the Gulf Coast.
Given what you have learned about wind speeds in various quadrants of the hurricane, for which state(s) would you forecast the most severe weather and tornado outbreaks? Why?
Where (which state(s)) would you guess that the greatest storm surge would occur? Why?
Browse through the public advisories for this date. Do your forecasts match those that were issued by NOAA? How deep was the storm surge?
Look at the surface analysis map of weather conditions in the United States on August 29, 2005. Returning to your map above, label any centers of high pressure with a blue ‘H’ and any centers of low pressure with a red ‘L’.
11. A. Based on what you know about the pressure gradient force, do winds blow into or out of a high pressure? Given this, would a hurricane move towards a center of high pressure, or be forced to move around it?
12. B. Based on what you know about the pressure gradient force, do winds blow into or out of a low pressure? Given this, would a hurricane move towards a center of low pressure, or be forced to move around it?
Find the five-day cone forecast for Hurricane Katrina after it made landfall along the Gulf. Trace the inner portion of this cone onto your map above. What explains the path that Katrina was expected to (and did) follow?
The city of New Orleans, LA, was originally the city targeted by the eye of Hurricane Katrina, although a last minute turn to the northwest spared the city the brunt of the storm surge and the worst of the powerful storms in the eye-wall. The city initially thought they had escaped the worst-case scenario: their below-sea level city being flooded by a powerful storm surge. However, within a short time, several levees holding the waters of Lake Pontchartrain broke, allowing the lake to drain into the city of New Orleans. The vast majority of the city was soon under water.
Read through some of the news articles describing the landfall and aftermath of Hurricane Katrina in New Orleans. In all likelihood, New Orleans will continue to sink as the weight of sediment from the Mississippi River deposited at the river’s mouth causes the Earth’s crust to compress under the city. Discuss what happened in New Orleans after Hurricane Katrina – what was life like for those who remained behind? Should we spend tax dollars rescuing people who ignore evacuation orders? Should we rebuild New Orleans? At what point do you think people should surrender a piece of land to Mother Nature and choose to build somewhere else? Defend your arguments.
ESSAY:
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