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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 64118km/h
•Hurricanes = Winds above 119km/h
International Date Line
Greenwich Meridian
Typhoon
Hurricane
Cyclone
Willy-willy
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 (131155 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
CONDITIONS OF READINESS
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.
THE BIRTH AND ANATOMY OF A CYCLONE
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
Hurricane Gilbert
MEDC