METEOROLOGY
GEL-1370
Chapter Eleven
Hurricanes
What we are going to learn?
We are going to learn the answers to the following questions
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What are hurricanes?
How do they form?
Why they strike the east coast of the US than the west coast of the US?
Conditions necessary for the hurricane development
Why are skies are clear in a hurricane’s eye?
What is the fuel that derives the hurricane?
What factors tend to weaken hurricane?
Why hurricanes are more likely to strike New Jersey than Oregon?
Why do hurricanes move westward over tropical waters?
Anatomy of a hurricane
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Hurricane: An intense storm of tropical origin, with sustained winds >64 knots (74 mi/hr)
Other Names: Typhoon: Western North Pacific
Beguio: In Phillipines
Cyclone: In India and Australia
International agreement: Tropical cyclone
Tropics: region between 23.5° north – 23.5° south; noon sun is always high in the sky, and so
diurnal and seasonal changes in temperature are small; daily heating of the surface & high
humidity favor the development of cumulus clouds & afternoon thunderstorms
A tropical wave as shown by the bending of streamlines; the waves move westward, bringing fair
weather on its western side & showers on the eastern side
Tropical weather
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Most of tropics are marked by seasonal differences in amount of precipitation, as temperature
remains warm all year long
• Greatest cloudiness & precipitation occur during high-sun period (when the ITCZ moves into
the region)
• Winds blow from the east, northeast or southeast – the trade winds
• Streamlines: depict wind flow direction; useful to see where surface air converges and
diverges
• Tropical wave: A wavelike disturbance in the tropical easterlies; have wavelengths ~2,500 km
& travel from east to west at speeds 10-20 knots
Hurricane Elena over the Gulf of Mexico, about 130 km southwest of Apalachicola, FL (1985);
winds flow counterclockwise about its center; central press. 955 mb
Hurricane Anatomy
• Average diameter of a hurricane ~500 km
• Elena’s eye is ~ 40 km wide; within the eye, the winds are light and clouds are mainly broken
• Surface winds increase in speed as they blow counter-clockwise and inward toward the center
• Eye wall: A wall of dense thunderstorms that surrounds the eye of a hurricane; within the eye wall,
heaviest precipitation & strongest winds are found; extend up to ~15 km above sea level
• Observations when we cross a hurricane (W to E):
– When we approach, sky is overcast with cirrostratus clouds;
– Pressure drops slowly at first & then more rapidly as we move closer to the center
– When we approach the eye, winds blow from the north and northwest with increasing speed; heavy
rain showers
Hurricane anatomy – contd.
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Inside the eye: air temperature rises, winds slacken, rainfall ceases & sky brightens; lowest
pressure reading (~50 mb lower than just outside the storm)
East side of the eye wall: heavy rain, strong southerly winds
As we move away from the eye wall, pressure rises, winds diminish, rain decreases, &
eventually sky clears
Why no thunderstorm inside the eye: In the eyewall, severe thunderstorms lead to release of
latent heat which in turns warms the eye --- leading to higher pressure which initiates a
downward air motion (sinking air) --- sinking air warms by compression and absence of
thunderstorms (so much heat is added to the air that surface air temp remains uniform in the
hurricane)
A model vertical view of air motions, clouds & precipitation in a typical hurricane
Hurricane formation & dissipation
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Hurricanes form over tropical waters (5-20 °) where winds are light, surface water temp is
warm (>26.5°C) over a vast area, high humidity; warm surface water must extend >200 m in
the water; 2/3 hurricanes in 10-20 °
These conditions prevail over tropical and subtropical North Atlantic and North Pacific oceans
during summer and early fall (June-November)
The surface winds must converge for a mass of unorganized thunderstorms to develop into a
hurricane; surface winds along ITCZ; when a wave forms along the ITCZ, convection
becomes organized leading to the formation of hurricanes
Hurricane formation and dissipation
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Hurricanes do not where the surface winds are strong
During El Nino event, strong winds aloft typically occurs over the tropical Atlantic --- fewer
Atlantic hurricanes than normal
During El Nino event, warmer water of El Nino in the northern tropical Pacific favors the
development of hurricanes in that region
Energy for a hurricane come from the direct transfer of sensible het from the warm water into
the atmosphere and from the transfer of latent heat from the ocean surface; if thunderstorm
start to organize along the ITCZ or along a tropical wave, and if the trade wind inversion is
weak, conditions will lead to the formation of a hurricane
Hurricane formation & dissipation - contd
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Tropical disturbance: A thunderstorm with a slight wind circulation
Tropical depression (TD): When Tropical disturbance becomes TD when winds speeds reach
20-34 knots & several closed isobars appear about its center in a surface weather map
Tropical Storm: When TD wind speed reaches 35-64 knots & when the isobars are packed
together
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Hurricane: When tropical storm speed exceeds 64 knots
Stages: Tropical disturbance crossing over Panama; tropical disturbance speed 25 knots;
tropical storm speed < 40 knots; hurricane >110 knots
Visible satellite image of 4 tropical systems, each in a different stage of its life style
Hurricane movement
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Hurricanes form in tropical oceans (except in the S.Atlantic & eastern South Pacific: water
temp is too cold); hurricanes formed over the N. Pacific & N. Atlantic are steered by easterly
winds and move west or northwestward
Actual path of a hurricane is determined by the structure of the storm & the storm’s interaction
with the environment
Off coast of Mexico over the N. Pacific spawns ~8/yr
Tropical North Atlantic ~ 6/yr
Hawaiian Islands (20-23°N) is in direct path of eastern Pacific hurricanes & tropical storms
Regions where tropical storms form, their names and typical paths
Some erratic paths taken by hurricanes
Hurricane and middle latitude storms
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Hurricane
Middle latitude storms
Energy from warm water & horizontal temp gradients
latent heat of condensation
Hurricane weakens with height Intensifies with height
Contains an eye where air is
Characterized by centers of
sinking
rising air
Strongest near the surface
Strongest winds found aloft
in the jet stream
Around the hurricanes, isobars are more circular, pressure gradient is much steeper & winds are
stronger
Destruction & warning
• A hurricane moving northward over the Atlantic will normally survive for a longer time than will the
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one from the same latitude over the eastern Pacific – surface water of Atlantic is warmer than the
Pacific
When a hurricane is approaching from the east, its highest winds are usually on its north side (winds
that push the storm along add to the winds on the north & subtract from the winds on the south)
When wind blows over water, the water beneath is set in motion; bending of water with depth (Ekman
spiral) causes a net transport of water (Ekman transport)
High winds of hurricanes generate large waves (10-15 m in height); waves move outward in the form of
swells – storm’s energy is carried; effects of storm is felt days before the hurricane arrives
Destruction and warning – contd.
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Region of low pressure allows the ocean level to rise (~50 cm); 1mb drop ~ 1 cm rise
Storm surge is produced by a combination of high winds, high waters and the net transport of
water toward the coast – inundates low-lying areas and turns beachfront homes into piles of
splinters
Hurricane watch: issued ~24-48 hrs before the storm arrives
Hurricane warning: When storm appears to strike an area within 24 hrs, warning is issued;
issued for a large coastal area ~550 km (5 degrees) in length
After landfall, hurricanes become tropical storms with peak winds < 50 knots
12 Most intense hurricanes from 1900-1999
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A composite IR images of Hurricane Georges (September 18-28, 1998); its trek across
Caribbean and northward into US
Hurricane Gloria on Sept. 27, 1985; moving northward @25 knots, winds of 100 knots on its
right & 50 knots on left side; 945 mb at the central
When a storm surge moves in at high tide it can inundate and destroy a wide swath of coastal
lowlands
Color-enhanced IR satellite image of Hurricane Hugo near Charleston, SC
Saffir-Simpson Hurricane damage-potential scale
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Beach homes at Folly Beach, South Carolina before hurricane Hugo
Beach homes at Folly Beach, South Carolina after Hurricane Hugo
IR satellite image of Hurricane Andrew moving across South Florida on the morning of August
24, 1992; central pressure 932 mb and sustained winds of 126 knots
Number of hurricanes (3,4,5 are major) in each category that made landfall along the coastal US
from 1900-1999; all hurricanes struck the Gulf or Atlantic coasts
Naming Hurricanes
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A name is assigned when a storm reaches tropical storm strength
Beginning 1953 (-1977), the National Weather Service began using female names
Beginning 1978-1979, alternately male and female names were assigned
Chapter Summary
• What are streamlines
• Hurricane-eye, eyewall, where the pressure is higher & lower; vertical structure, rotation of hurricanes,
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source of energy, conditions for the formation of hurricanes
Commonness between middle latitude cyclones & hurricanes
Various stages of hurricane, areas where more hurricanes
Why hurricanes don’t form in certain regions?
Saffir-Simpson scale
How hurricanes can be modified?
Damage by hurricanes
Other names of hurricanes