METEOROLOGY
GEL-1370
Chapter Eleven
Hurricanes
What we are going to learn?
We are going to learn the answers to the following questions
– 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
• 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
• Most of tropics are marked by seasonal differences in
amount of precipitation, as temperature remains warm
all year long
• Greatest cloudiness & precipitation occur during highsun 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.
• 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
• 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
• 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
• 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
• 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
• 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
• Hurricane
Middle latitude storms
• Energy from warm water & horizontal temp gradients
latent heat of condensation
Hurricane weakens with height
Contains an eye where air is
sinking
Strongest near the surface
Intensifies with height
Characterized by centers of
rising air
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 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.
• 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
Rank Hurricane
Year
Cen. P
Catego. Death
•1 .
Florida (Keys)
1935
892 mb 5
408
2
Camille
1969
909
5
256
3
Andrew
1992
922
4
53
4
Florida/South TX
1919
927
4
>600
5
FL-Lake
Okeechobee
1928
929
4
1836
6
Donna
1960
930
4
50
7
Texas (Galveston)
1900
931
4
>6000
8
Louisiana (Grand
Isle)
1909
931
4
350
9
Louisiana (New
Orleans)
1915
931
4
275
10
Carla
1961
931
4
46
11
Hugo
1989
934
4
49
12
Florida (Miami)
1926
935
4
243
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
Scale # Central Winds
• .
Pressure Knots
(mb)
64-82
Storm
Surge
(m)
Damage
~1.5
Damage mainly to trees, shrubbers
and unanchored mobile homes
1
>980
2
965-979 83-95
~2.02.5
Some trees blown down; some
damage to to exposed mobile homes;
some damage to roofs
3
945-964 96-113
~2.54.0
Large trees blown down; mobile
homes destroyed; some structural
damage
4
920-944 114-135 ~4.05.5
All signs blown down; extensive
damage to roofs, windows and doors;
destruction of mobile homes
5
< 920
Most serious damage to buildings; <
4.5 m above sea level within 500 m
shore
>135
>5.5
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
• 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, 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