Year 13 Hazards

Tropical Cyclones

Introduction• Tropical cyclones are amongst the most powerful and

destructive meteorological systems on earth.• Globally, 80 to 100 develop over tropical oceans each

year. • Many of these make land and can cause considerable

damage to property and loss of life as a result of high winds and heavy rain

• Tropical cyclone = general term for an area of: intense low pressure, high winds, torrential rain, thunder, lightning and swirling cloud which covers the tropics

Classification:= according to windspeed:

•Tropical Disturbances = low strength winds posing few problems

•Tropical Depressions = more developed features with windspeeds up to 63km/h

•Tropical Storms = windspeeds between 64-118km/h

•Hurricanes = Winds above 119km/h

Greenwich MeridianInternational Date Line

Typhoon

Willy-willy

Cyclone

Hurricane

Conditions:•Only a few tropical disturbances develop into full blown hurricanes. •If they do – a number of conditions must be met:

•High air humidity (moisture levels)•Sea surface temperatures must be at least 27°C

Where do Tropical Cyclones Occur?•Common on western sides of oceans where high pressure cells are less strongly developed

SST’s too low here

Tornado Fact:

“Tornadoes feed on an incessant supply of latent heat released from condensation in ascending moist air. The mechanical power generated is of the order of 20 million megawatts and the energy generated in one day is equivalent to about 20 years' supply of electricity for Hong Kong!”

Causes:

•Warm water evaporates from the oceans

•Air above the sea is then warm and humid

•Water vapour rises and cools - releasing latent heat on condensation

•This latent heat accentuates the uplift and perpetuates atmospheric instability

•Coriolis Force promotes the rotating effect of the parcel of air

•They track under the influence of prevailing winds

•Their power supply is cut as they hit land (or a cooler sea) because the energy supply from the warm sea has been cut off•They tend to blow themselves out as they progress inland

•Tornadoes = more localized events – think of it as a very very low pressure on a small scale

•Air uplifts very rapidly in a powerful vortex

•When they develop over water (eg Miami) they generate waterspouts

•It is still not entirely clear how they are formed although recent storm chasers suggest that it is to do with the development of “Super Cells” – atmospheric systems containing large cumulonimbus clouds

Tornado over water – a waterspout

•Tornadoes tend to form where cold dry air overlies a warm moist layer •and when local heating of the ground surface, or a vigorous cold front, triggers a vortex of rotating air.•Rotating windspeed = 150-200km/hr•They are only 50-100m wide and 2-5km in length•They travel at about 30-60km/hr•They “touch down” for a maximum of 20mins•Common in the southern States – eg Texas, Oklahoma and Wisconsin especially in spring and early summer when warm moist air moves up from the Gulf of Mexico and meets cold dry descending from the eastern side of the Rockies

Saffir-Simpson Scale: Hurricane intensity is categorized in several ways. One of the better known is the Saffir-Simpson Hurricane Scale.

-1-5 rating based on the hurricane's present intensity

-used to give an estimate of the potential property damage and flooding expected along the coast from a hurricane landfall

-wind speed is the determining factor in the scale

-storm surge values are highly dependent on the slope of the continental shelf in the landfall region

HURRICANE CATEGORIES

Category 1 (Minimal) -winds 64-82 knots (74-95

mph) -storm surge 4 to 5 ft above

normal -no real damage to building

structures -low lying coastal areas

flooded -minor damage to piers. (DOLLY 1996)

HURRICANE CATEGORIES

Category 2 Moderate) -winds 83-95 knots (96-110 mph) -storm surge 6 to 8 ft above

normal -minor damage to structures,

poorly constructed buildings major damage

-coastal and low lying escape routes flooded over

-considerable pier damage. (BOB 1991)  

HURRICANE CATEGORIES

Category 3 (Extensive) -winds 96-113 knots (111-130

mph) -storm surge 9 to 12 ft above

normal -major damage to structures,

poorly constructed buildings destroyed

-serious flooding along the coast, and extensive flooding may extend inland 8 miles.

(HUGO 1992)  

HURRICANE CATEGORIES

Category 4 (Extreme) -winds 114-135 knots (131-155

mph) -storm surge 13 to 18 ft above

normal -extensive roofing and window

damage, complete destruction of mobile homes,

-areas above 10 ft flooded inland up to 6 miles, major erosion of beaches, and massive evacuation of coastal areas

(ANDREW 1992)

HURRICANE CATEGORIES

Category 5 (Catastrophic) -winds above 135 knots (156 mph) -storm surge greater than 18 ft

above normal -complete failure of roof

structures and very severe window and door damage, some complete buildings fail, major damage to structures lower than 15 ft above sea level

-massive evacuations of residential units within 10 miles of the coast. (CAMILLE 1969)

TROPICAL CYCLONE CONDITIONS OF READINESS

CONDITION V - Destructive force winds (35 kts or as specified *) are possible within 96 hours. Also set when secured from destructive weather conditions and when it is hurricane season.

-Atlantic Ocean, 1 June – 30 November -Eastern Pacific, 15 May – 30 November -Western Pacific, year round

CONDITION IV - Destructive force winds are possible within 72 hours.

CONDITION IVA - Destructive force winds are possible within 72 hours. (Cuba and Puerto Rico maintain this condition throughout the season)

* DESTRUCTIVE FORCE WINDS ARE DELINEATED BY LOCAL 3140 INSTRUCTION

CONDITION III - Destructive force winds are possible within 48 hours.

CONDITION II - Destructive force winds are anticipated within 24 hours.

CONDITION I - Destructive force winds are anticipated within 12 hours.

CONDITION IA - Destructive force winds are imminent. This condition is also used to sortie aircraft.

CONDITIONS OF READINESS

They form in the atmosphere over warm ocean areas with at least 26°C water temperature (mainly in latitudes 5° to 20° north or south) although their exact trigger-mechanism is not fully understood. If conditions are right, an ordinary tropical depression, or 'low' can develop into a tropical cyclone. In the southern hemisphere, in a 'low', the winds spiral in a clockwise direction towards its centre, where they rise and spill over in an outward flow at high altitude. Summer heat beating on the warm ocean evaporates water, creating a deep layer of moist air. The uplift of this moist air in the centre of a 'low' cools it, causing the intense rain characteristic of tropical cyclones. Higher in the upper levels the rising air spirals outward, removing air faster than it flows in, resulting in a fall in barometric pressure.

THE BIRTH AND ANATOMY OF A CYCLONE

Hurricane GilbertMEDC