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AHS 104: Introduction to Atmospheric Science
Lab Exercise 11: Hurricanes
1. A radar image of Hurricane Michael (2018), which came ashore on the Florida panhandle, is shown below. The darker
red indicates heavy rain, while orange, yellow, blue, and green indicate successively lighter rain (black indicates no rain).
Match each letter to the corresponding hurricane feature:
Eye ______
eyewall ______ spiral rainband ________
2. A map of the track of Hurricane Michael (2018) is shown below. The location of the storm’s center is shown at
various times; landfall was about 18 Z (1 PM local time) on 10 October 2018, with a central pressure of 918 mb.
A map of the estimated storm surge levels (feet above ground level) along the Florida Gulf Coast is shown below. Circle
the area of highest storm surge. Given what you know about the flow around a hurricane, explain why the highest
storm surge was located where it was, given the path of the storm.
3. The position of Hurricane Nicholas, which struck Texas in September 2021, is shown at successive times. Connect the
hurricane center locations to draw the track of the storm as an arrow.
A hurricane’s path is highly dependent on upper-level wind patterns. Compare the path of Nicholas to the 500 mb wind
directions observed over the area at 12Z on 14 September.
The table below lists weather data for Galveston, TX (shown by the star on the storm track map). Note the wind direction
(“DIR” in the table) at Galveston changed from East (90 degrees) to South (180 degrees) to W (270 degrees) during the day.
Explain with a sketch why it did so.
Welcome to our 36th day of class:
13 April 2022





Today: start on the climate system
◆ Ref: Chap. 7, pp. 198-207
Assignments: Lab 11 (due tonight)
New lab: Lab 12 due Wednesday, 20 April
Today’s lecture quiz (due Friday by 11:59
PM)
Weather Notebook entry for today
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Components of the Climate System

Atmosphere:


Hydrosphere:


The air and everything in it (clouds, dust, etc.)
All the liquid water: oceans, lakes, rivers,
ground water
Cryosphere:

All the frozen water: continental ice sheets, sea
ice, alpine glaciers, snow cover
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Components of the Climate System

Lithosphere:


The upper part of the earth’s crust: continents,
ocean basins, soil, etc.
Biosphere:
All plant and animal life
◆ Vegetation determines albedo
◆ Plants and animals can influence the
atmosphere’s composition!
◆ This includes humans!!

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Earth’s Climate System


The components react to changes on different time
scales
◆ Atmosphere = fastest responding
◆ Hydrosphere = slower
◆ Cryosphere = still slower
◆ Lithosphere = Really slow response to change!
◆ Biosphere = somewhere in between atmosphere
and hydrosphere
The components all interact with one another
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How’s your geography knowledge?
1. Name a place that is a desert
◼ 2. Name a place with a rain forest

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World’s deserts: Mostly within a belt
centered on 30°N or 30°S latitude
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This is not where Australia really is
– The2022
mapmaker put it here to save
Spring
6
space…
World’s rain forests: Mostly in a
belt surrounding the Equator
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Question:
Why are these things where they are?
Answer:
The atmospheric circulation created them!
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• The general circulation


We begin by examining the average global
circulation in the atmosphere
This is sometimes called the “general circulation”
◆ It is the average response of the atmosphere to
the differential heating of the earth by solar and
terrestrial radiation
◆ It is modulated by the heat capacity of the
underlying surface (land versus ocean)
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What causes the average global
circulation?

The uneven heating over the surface of the
globe.

Recall the earth’s radiation balance:
◆ Tropics receive a net radiation surplus.
◆ Middle and high latitudes have a radiation
deficit.

General circulation moves warm air
poleward and cold air equatorward to
maintain the heat balance.
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Historical background

The need to understand the
general circulation was an
important issue in the 16th
century with increased
exploration and shipping
between Europe and Asia and
the New World using the “trade
winds”
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Trade winds


East-to-west winds at
tropical latitudes
These trade winds are
very steady (and
predictable). They made
transoceanic travel (and
trade) possible before the
Machine Age ushered in
by the Industrial
Revolution
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What about the trade winds?



For simplicity, let’s first
consider a water-covered
Earth
Climatological mean
surface winds are depicted
Why do the winds blow
from these directions on
average?
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Average circulation on a water-covered planet



Heating at the equator
leads to rising motion.
Cooling at the poles leads
to sinking motion
The Coriolis Force
“breaks up” the circulation
into three “cells”
◆ In the NH, flow is
deflected to the right
by the Coriolis force
◆ In the SH, flow is
deflected to the left by
the Coriolis force
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Hadley cell: mean rising motion at Equator,
mean sinking motion in the subtropics
Equatorward subtropical flow is deflected by the Coriolis force to produce NE trade
winds in the Northern Hemisphere and SE trade winds in the SH
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Return to our two questions
Why are there rain forests near the equator?
◆ Net rising air motion (due to hundreds of
thunderstorms), leads to frequent heavy
rains there.
◼ Why are the major deserts in the subtropics?
◆ Net sinking motions there lead to
warming air temperatures, clear skies,
stable atmosphere and no rain!!

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Note where the
sinking and
rising motions
are!
See them right
now from Univ
of WI SSEC
Longer period
from NASA
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Likewise, poleward middle-latitude mean flow is deflected by
the Coriolis force to produce the “prevailing westerlies”
These westerlies are NOT very steady (or predictable) since they occur
in the same zone where middle-latitude cyclones dominate the weather
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Summary: Global Atmospheric Circulation
Cold and dry
Fronts and cyclones
Wet with clashing air
masses
Hot and dry
T-storms
Warm and wet
Hot and dry
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Add in the continents and what
happens?
Remember heat capacity?
◼ Land is warmer than oceans in summer
◆ Leads to warm, low pressure at surface
over continents (vice versa over oceans)
◼ Land is colder than oceans in winter
◆ Leads to cold, high pressure at surface
over continents (vice versa over oceans)

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Key to next slides
Red arrows depict the trade winds
Blue arrows depict the westerlies
Magenta arrows depict “monsoon” winds in
southern Asia
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Monsoons

What is a monsoon?
◆ Arabic word meaning “season”
◆ Note the reversal of the winds (the
magenta arrows) across southern Asia on
the following slides
◆ People often say a heavy rain is a
“monsoon”. That is incorrect!!
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Northern Hemisphere Winter
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Northern Hemisphere Summer
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Major monsoon in South Asia


Dry, NE monsoon in NH winter
◆ Monsoon wind brings dry cP air to the region
◆ These winds are like typical trade winds
Wet, SW monsoon in NH summer
◆ Winds bring mT air and generous seasonal
rains
◆ The reversal is due to the heating of the Tibetan
plateau in summer and the resulting “thermal
low” over the region
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Monsoons

In Asia:
◆ This is the main seasonal change across
southern Asia where ~3 billion people live
◆ The monsoon is critical to the success of south
Asian agriculture!
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Monsoons

There is a weaker monsoon in the SW US
◆ Peak rains in Arizona and New Mexico are in
the late summer (it doesn’t rain much, but
thunderstorms are frequent)
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A
30 deg N
B
D
Equator
C
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Review
Questions
Match the following
features of the global
circulation to the lettered
points on the satellite:
1. Intertropical
convergence zone
2. Polar front zone
3. Subtropical high
4. Place with trade
winds
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A
B
D
C
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Answers
1. Intertropical
convergence zone C
• Thunderstorms
near equator
2. Polar front zone A
• Cyclone (commashaped system)
3. Subtropical high B
• clear (dark) since
air is sinking
there
4. Trade winds D
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What’s next?

What we have just talked about is the climate of
the atmosphere in terms of the average surface
winds.

It represents only one component of the earth’s
climate system.

Next, we’ll consider the other components of the
climate system.
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Welcome to our 35th day of class:
11 April 2022


Chapter 15: atmospheric
optics
Assignments
◆ Today’s lecture quiz is due
11:59 PM Wednesday
◆ Lab 11 due Wednesday
◆ Weather Notebook entry
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“Ingredients” needed
for optical displays
light
ice crystals
Water
droplets
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Basic Optical Principles

1) Reflection: light bouncing off a surface
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2) Refraction: bending of light as it passes
from one medium to another

Shorter wavelengths (violet and blue) bent
more than longer wavelengths (orange and
red) when entering B
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Example: prism
beam of
white light
entering
prism
Violet light is bent
more than red
resulting
spectrum
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Raindrops refract and reflect
sunlight, producing the rainbow
Note:
Sun is
behind
observer!
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red at top, violet at bottom
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of rainbow
6
Violet is bent most; this produces a “circle of light”
with violet at the center, red on the outside.
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Rainbows were first explained
by Rene Descartes in 1637
“Cogito, ergo sum”
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Angle between Sun, drops, and
observer’s shadow = 42º for red light
42°
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Can see full circle from airplane
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Q. Why can’t you see a rainbow at noon?
A. With high Sun, rainbow is “below the ground”
example of rainbow with high Sun
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Two reflections give a
secondary rainbow
secondary
primary
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Formation of secondary bow
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Perspective view:
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A rain shower isn’t the only source
of rainbows
That feels
fabulous!
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Sun dogs: caused by refraction
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Sun dogs are spots of light 22º
on either side of Sun
Plate
crystal
think of glints on
fresh snow
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Why “sun dog”?17
Halo: “ring around the Sun”
copyright Alan and Kimberly Klein
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Halo is also caused by refraction, but
in column ice crystals
cross section
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Perspective view:
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Can also be seen around Moon
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Thought Questions
1.
2.
3.
Might you see a rainbow at night? (That is, is it
theoretically possible to have a rainbow (colored or
not) at night?) If so, what might cause it?
The most vivid sun dogs occur on very cold winter
days. Why?
Could you see a sun dog on a hot summer day?
Explain.
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1. Answer: yes! “moonbow”
Q: Would you see
the colors, like
in this photo?
A: No.
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2. Lots of tiny plate crystals are in the
air on very cold days.
“diamond dust”
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3. Sure, if it’s formed by a cirrus
cloud.

Recall cirrus clouds look wispy, because
they are made of ice crystals.
cool sun dog video
sun dog
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Sun
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Sun pillar



Vertical shaft of light
extending upward or
downward from Sun
Typically seen during
sunrise or sunset
Forms when sunlight
reflects off the
surfaces of falling ice
crystals
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Pillars caused by streetlights
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Ice crystals
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Corona: caused by diffraction
corona
Moon
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Another basic principle of optics

3) Diffraction: light bending as it passes
through an opening
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Corona

If thin cloud is present, diffraction of
moonlight spreads the light
cloud droplet
Light waves
from Moon
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Glory: Like the corona, diffraction is involved,
but the light is back-scattered toward observer
Shadow of
aircraft
glory
video 1
video 2
cloud bank below
aircraft
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Another ghostly image of the glory
(“Specter of the Brocken”): taken
from mountaintop
person’s
shadow
In China, this phenomenon is called Buddha’s light (佛光), and
was often taken to show the observer’s personal enlightenment
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Heiligenschein (German for “holy light”)
Bright area
around head of
observer’s
shadow on
dewy grass


Name comes from technique
artists used to indicate holiness
light is reflected
directly back to
observer
no colored rings
as with corona
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The Green Flash
air acts as a prism,
splitting sunlight
into colors
◼ green “sets” last

video
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Recall why the sky is blue

Shorter wavelengths (blue) scatter more than
longer ones (red)
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Why are sunsets red, then?


Sun low in sky; rays pass through much more air
than midday sun.
Having crossed so much air, most of blue & green
light is scattered out, leaving red, yellow, & orange
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Mirages

Examples: “wet road”,
desert oasis

Light bends because of
temperature differences
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Superior Mirage
image of ship, but
upside down!
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Welcome to our 33rd day of class:
6 April 2022


Finish Chapter 11: Hurricanes
Assignments:
◆ Lecture quiz for today
◆ Lab 11 due Wed., 13 Apr
◆ Weather Notebook entry
◆ Keep reading Chap. 11
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Hurricane Threats to Land

1) Storm surge: the wind drags along ocean surface
and causes water to pile up
◆ Add the waves and possibly high tide
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Tacloban City Airport in Philippines after Typhoon Haiyan (2013);
Haiyan had 23-foot storm surge
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Hurricane Michael (Florida, 2018)
Before
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After
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Front right part of storm is most
dangerous
Water is “piled
up” against the
coast, increasing
storm surge
Storm’s forward
motion adds to
the wind speed
and storm surge
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Question: If a hurricane was moving westward and about to hit
Florida, the worst storm surge would be in:
A) Miami
B) Naples
C) Orlando
D) West Palm Beach
(think about the
structure/winds in the storm)
Path of hurricane’s eye
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Cyclone Bhola (1970): deadliest
tropical cyclone in recorded
history
◆ 225,000 people died
(estimates range much higher)
◆ Storm track shown in red
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Note the very low
elevation where Bhola
made landfall
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Why was Cyclone Bhola so deadly?



Very low land elevations meant the storm surge
(measured as high as 34’) could move far inland
◆ Landfall occurred at high tide
Structures and housing not well-built
Very high population density
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Hurricane Threats to Land
2) High winds
◼ Strongest in eyewall
◼ Tornado-type damage,
but over wider area!
◼ Example: Category 5
Hurricane Maria (2017)

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Hurricane Maria





September 2017
Best known for destroying
Puerto Rico electrical grid
Destroyed all
infrastructure on island of
Dominica
97 people killed (54 in
Puerto Rico)
Losses of $90 billion,
mostly in Puerto Rico
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Saffir-Simpson scale of
hurricane strength
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Hurricane Threats to Land
3) Flooding rains (often 10-20 inches if storm
moves slowly or encounters mountains)
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12
Hurricane Harvey (2017)




Lingered over Texas
Catastrophic flooding in
Houston and other parts of
south Texas (25-60 inches
of rain)
Estimated $125 billion
91 confirmed deaths
Flooded vehicles
Before and after
Simonton, TX
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For really high rainfall, add in
mountains!


Typhoon Morakot: 91.6 inches in 48 hours
Cyclone Gamede: 13 FEET in 3 days (16’ in 4 days)
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Hurricane Mitch (1998): greatest devastation was in
Honduras, where multiple feet of rain fell
Led to major landslides and caused over 11,000 deaths
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Lesson: we shouldn’t focus only on
Category 4 or 5 hurricanes

Large, wet, slow Category 1 storms like Hurricane
Harvey (or even tropical storms) that result in
torrential rainfall and flooding can be just as
destructive and deadly.
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National Hurricane Center forecasts

NHC issues
forecasts of



track
intensity
There is
uncertainty
in both!
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Forecasted track:
eye location shown
by the black line
“Forecast cone”
shows likely error,
not how wide
storm will be
Also, storm is
larger than the thin
black line
Line says nothing
about how large or
strong a storm is!
18

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Track can be anywhere
inside cone, in fact there’s
a one-in-three chance the
eye will pass outside the
cone!
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Evacuation issues

It takes a lot of time to move
large populations out of harm’s
way



Beachfront property on barrier
islands is hard to evacuate
Florida Keys and New Orleans can
take 2-3 days to evacuate
We could start the evacuation
process earlier, but forecasts are
more uncertain then!
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Emergency managers are in a
difficult situation
Delaying evacuation puts huge numbers at risk
◼ Calling for evacuation too early is expensive in
money and time
◆ Business and wages are lost
◆ “Cry wolf” syndrome if wrong

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Hurricane track forecasts have
significantly improved….
By 2017, error was
down to only 75
miles for 72 hours
out….
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However, 75 miles in each
direction is still a lot of
coastline!
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We have computer models: why is
prediction still difficult?
Each color is a
different track
forecast
Different computer
forecast models give
different solutions
Which one, if any, is
right?
This is the path
the actual
hurricane took!
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