11/1/2011
Hurricane Basics
 Severe Tropical Cyclones
 Tropical cyclones with winds greater than 74 mph (120 km/h)
 Most destructive weather on earth
Hurricanes and Nor’Easters
Severe Thunderstorms and Tornadoes
Typhoons
Hurricanes
Hurricanes
Tropical
Cyclones
Hurricane Formation
 Formed over ocean water > 26o C
(80o F)
 Convection (Warm, moist,
rising air.) and Latent Heat
From Wikipedia
From NWS Jetstream
Hurricane Structure
Hurricane Formation
 Tropical Disturbance
 some T-storms
 Tropical Depression
 Organized T-storms (wind speed 0-38 mph)
 Tropical Storm
 Organized rotation (wind speed 39-73 mph)
 name given
 Eye Wall –strongest
T-storm and winds
 Eye – calm
descending air
 Feeder Bands /Rain
bands - storms
spiral in towards
center
From NWS Jetstream
 Storm Surge- dome
of water pushed by
the winds
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Hurricane Damage
Hurricane Rating Scale
 Storm Surge – may exceed 20 feet
 Winds – may exceed 150 mph
Saffir–Simpson Hurricane Scale
Category
 Waves – may exceed 50 feet
 Flooding – Rain intensity and speed of the storm may
cause flooding
 Tornadoes – occasional weak tornadoes may occur
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Hurricane Damage

Wind speed
Storm surge
mph
(km/h)
≥156
(≥250)
131–155
(210–249)
111–130
(178–209)
96–110
(154–177)
74–95
(119–153)
ft
(m)
>18
(>5.5)
13–18
(4.0–5.5)
9–12
(2.7–3.7)
6–8
(1.8–2.4)
4–5
(1.2–1.5)
“Winter Hurricane”
Category One Hurricane: Damaging winds are expected. Some damage to building structures could occur,. Some
damage is likely to poorly constructed signs. Loose outdoor items will become projectiles, causing additional damage.
Persons struck by windborne debris risk injury and possible death. Numerous large branches of healthy trees will
snap.. Many areas will experience power outages with some downed power poles.
Category Two Hurricane: Very strong winds will produce widespread damage. Some roofing material, door, and
window damage of buildings will occur. Considerable damage to mobile homes. A number of glass windows in high
rise buildings will be dislodged and become airborne. Numerous large branches will break. Many trees will be
uprooted or snapped. Extensive damage to power lines and poles will likely result in widespread power outages that
could last a few to several days
Category Three Hurricane: Dangerous winds will cause extensive damage. Some structural damage to houses and
buildings will occur with a minor amount of wall failures. Mobile homes and poorly constructed signs are destroyed.
Persons struck by windborne debris risk injury and possible death. Many trees will be snapped or uprooted and block
numerous roads. Near total power loss is expected with outages that could last from several days to weeks.
Category Four Hurricane: Extremely dangerous winds causing devastating damage are expected. Some wall failures
with some complete roof structure failures on houses will occur. All signs are blown down. Complete destruction of
mobile homes Extensive damage to doors and windows is likely. Numerous windows in high rise buildings will be
dislodged and become airborne. Most trees will be snapped or uprooted. Fallen trees could cut off residential areas for
days to weeks. Electricity will be unavailable for weeks after the hurricane passes.
Category Five Hurricane: Catastrophic damage is expected. Complete roof failure on many residences and industrial
buildings will occur. Some complete building failures with small buildings blown over or away are likely. All signs
blown down. Complete destruction of mobile homes). Severe and extensive window and door damage will occur.
Nearly all windows in high rise buildings will be dislodged and become airborne. Severe injury or death is likely for
persons struck by wind-blown debris. Nearly all trees will be snapped or uprooted and power poles downed. Fallen
trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months.
Nor’Easter Basics
Nor’Easter Formation
 Mid-Latitude
 Low pressure often forms over the Gulf
Cyclone forms off
the Carolina Coast
 Brings Snow, Rain
and Ice to Mid
Atlantic states
 Severe damage and
beach erosion to
the NC coast.
Stream
 Storm increases intensity with greater
temperature contrast in air masses
 Pressure drop of 24mB in 24 hrs – Bomb
cyclone
 Strongest winds from the NE give the storm
its name
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Storm with lightning, thunder, and strong winds
T-Storm Formation
 Ingredients:
 Moisture
 Unstable air
 Cold air over warm
allows more
rapid convection
Development of T-Storm
T-Storm Development
 Mature stage
 Cumulus stage
 Warm moist air
forms a large
cumulus cloud
 Violently rising air
forms
cumulonimbus
cloud

Heavy rain, wind, hail, and
lightening possible
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T-Storm Development
Types of T-storms
 Single cell
 Multi-cell
 Dissipating Stage
 Down drafts cool
and slow convection
 Multi-cell line (Squall line)
 Super cell
 A thunderstorm will be called SEVERE if:
 Hail > ¾”
 Winds in excess of 57 mph
 Tornado
Single Cell Storm
Squall Line
Multi Cell Storm
Super Cell Storm
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Smallest and most violent short lived
cyclones associated with thunderstorms
Tornado on Doppler RADAR
Tornado Formation
 Middle and high altitude wind shear
causes rotation (roll)
 Directional Shear
 Speed Shear
 Convection (Updrafts) causes roll to
become vertical
 Wind speeds may reach over 300 mph.
Tornado Rating Scale
Tornado Formation
 Fujita Scale
Tornado Alley
From Wikipedia
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