Hurricanes

• Sustained winds at least 74 mph

• Circulation (c-clkws in N Hem; clkws in S Hem)

synonyms

• Hurricane : north Atlantic, eastern North Pacific

• Typhoon: western North Pacific

• Baguio : Philippines

• Cyclone : India, Australia

Typhoon Angela

Progression of tropical storms leads to hurricane:

1. tropical disturbances

disorganized groups of thunderstorms; some spin: 90% fizzle out

2. tropical depressions : wind speeds 23 mph

3. tropical storm : wind speeds 39 mph (named)

4. hurricane : wind speed 74 mph

Begin as “easterly waves”

Most Atlantic hurricanes begin in Africa

Monsoonal LowAzores Bermuda HighPacific High

equator

ITCZ

Africa

ocean

SE Trades

NE Trades

Turned SW by Coriolis deflection

• At this point, storm is a “tropical disturbance”

• If storm moves off African continent towards Atlantic, becomes “easterly wave” (carried west by equatorial easterlies)

• Of approx. 100 each year, 6 become hurricanes

Requirements:

• Warm ocean surface– (>78°F)– Warm to depth of 200 ft

– Vast– Basin or current

• Surface convergence

• Coriolis deflection– Do not form at equator (5 – 20 °N,S)

Hurricane source regions (red arrows) :

Anatomy of a hurricane• Spiral bands of

thunderstorms organized around low pressure center

• Surface winds converge towards central LOW– Warm, moist air

• Air aloft diverges around a central HIGH

anatomy of a hurricane• eyewall

– Tallest clouds – Most rain

• 100 in / day– Highest wind speed

• Wind speeds strongest here due to conservation of angular momentum

• eye– air is sinking– Clear skies– Warmer temperatures– 15 miles diameter; 1

hour

• Source of fuel: latent heat of condensation

Process of hurricane development:

• Hurricane starts as tropical storm, clustered thunderstorms spinning around a central low:– Heavy rain– High winds– Release of latent heat (condensation)

• Increases temperature of cluster• Central pressure drops

• The lower the Low, the higher the winds speeds towards the center

• Bringing warm flow of warm, moist air• More latent heat is released• Pressure continues to drop

Edge of storm:Outflow sinks and warms

• Converging moist air and resulting release of latent heat of condensation fuels the process

• Rises, condenses in eyewall

• Coriolis deflection makes it spin– Cclkws surface; clkws aloft (N. Hem)

Central low pressure

Average: 950 mb

Typhoon Tip (1979): 870 mb

Hurricane Gilbert (1988): 888 mb

Hurricane Katrina: 907 mb

IR shows height of clouds in eyewall

SH_ _ ! We’re in the middle of a ………..

…hurricane???

At top of storm:

Ice crystals spiral out of storm

• Create a blanket of cirrostratus cloud cover– Obscures view from above of spiral bands of

clouds– On satellite images, hurricanes look like they

have uniform thickness and density but they are really strongly banded

Andrew

Katrina

Hurricane Destruction:

• High Winds (exceed 74 mph)

• High rainfall (can be as great as several meters per day)

• Storm surge

• May contain clusters of short-lived tornadoes

flooding

Storm surge

• Sea surface rises and high waves push onshore

• 2 processes:• Piling up of water as heavy winds drag surface

water forward to land, causing sea surface to rise• Low pressure in storm center causes water to rise

– For every 1 mb drop in pressure, water level rises 1 cm

• Storm surge usually increases sea surface by 3 – 6 feet, but can be extreme

Hurricane Camille: storm surge 25 feet

1900 Galveston Hurricane: storm surge 14.5 ft.

8000 drowned (book: Isaac’s Storm)

1970 Bay of Bengal / Bangladesh :

storm surge 40 ft.

300,000 – 500,000 fatalities

Zones of high storm surge and wind speed:

• Right-hand side of storm (relative to direction it is moving) in front (“forward right flank”)

– Additive effect of wind speed and storm speed

Right-forward

flank

Hurricane Andrew

Andrew’s destructionAugust 1992

– $ 26.5 billion

• 164 mph peak gust• 26 deaths• 17’ storm surge• 1 tornado• 922 mb central pressure• Began as tropical wave off west coast Africa,

August 14, became Tropical Storm Andrew on Aug 17, became hurricane on Aug. 22, Category 4 Aug 23, blasted Florida Aug. 24

Floyd’s destructivenessSeptember 1999

• 155 mph sustained winds; gusts 185 mph

• 921 mb

• One of the largest peacetime evacuations in US

• 52 deaths

• rain : 2-3 inches / hour

• $1 billion

Gilbert

Gilbert

Saffir-Simpson Scale

CATEGORY DAMAGE WINDS STORM SURGE

1 minimal 74 -95mph 4 -5 ft.

2 moderate 96 -110 6 -8 ft.

3 extensive 111-130 9 -12 ft

4 extreme 131 -155 13 -18 ft

5 catastrophic > 155 > 18 ft

Naming hurricanes

• WWII named by Air Corps and Navy meteorologists after wives and girlfriends

• Gradually added men’s names

• North Pacific basin after 2000, given Asian names, not necessarily personal names (flowers, birds, etc)

• Hurricane watch: landfall > 24 hours

• Hurricane warning: landfall < 24 hours

• ArleneBretCindyDennisEmilyFranklinGertHarveyIreneTD #10JoséKatrinaLeeMariaNateOpheliaPhilippeRitaTD #19StanTammySTD #22VinceWilmaAlphaBetaGammaDelta