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HURRICANE KATRINA 2005
CLOUD SEEDING IN NZ 1950-1970
9.3 HAZARDS FROM
Global distribution of areas most at risk from large scale
tropical disturbances (cyclones, hurricanes, typhoons) and
small scale atmospheric disturbances (tornadoes).
Processes causing the formation and development of cyclones,
hurricanes, typhoons and tornadoes.
Hazards from large scale atmospheric disturbances: storm
surges, coastal flooding, intense rainfall leading to severe river
floods and mass movement, and high winds.
Hazards from small scale atmospheric disturbances: intense
precipitation (rain and hail), high winds, and pressure
imbalances. Primary and secondary impacts on lives and
property. Prediction, preparedness and monitoring of large
and small scale atmospheric disturbances and perception of risk.
A tropical disturbance forms over
waters of at least 26.5C.
A tropical disturbance is defined as
an area of organized thunderstorm
activity 100 - 300 miles in diameter
which maintains its identity for 24
hours or more.
In meteorology, a tropical
disturbance can be defined as a very
weak, or incipient (just beginning),
A tropical cyclone is a rotating storm system characterized
by a low-pressure centre, a closed low-level atmospheric
circulation, strong winds, and a fast spiral arrangement of
thunderstorms that produce heavy rain.
Depending on its location and strength, a tropical cyclone is
referred to by different names, including hurricane, typhoon,
tropical storm, cyclonic storm, tropical depression, and
A hurricane is a tropical
cyclone that occurs in the
Atlantic Ocean and North
Eastern Pacific Ocean,
and a typhoon occurs in
the North Western
Pacific Ocean; while in
the south Pacific or
Indian Ocean, comparable
storms are referred to
simply as “tropical
cyclones” or “severe
are given different
names in different
parts of the world.
TROPICAL STORMS AS HAZARDS
A tropical storm is a hazard that brings heavy rainfall,
strong winds and other related hazards such as mudslides and
Tropical storms usually form between approximately 5° and
30° latitude and move westward due to easterly winds. The
Coriolis force sends them spinning towards the poles.
In most areas, tropical storms are given names.
The names are alphabetical and alternate between male and
female. This makes storms easier to identify, especially when
they are close together.
It is hard to predict the path of a tropical storm, and
therefore difficult to manage an adequate evacuation of an
area if needed.
Apparent force, due to the spinning of the Earth, which
deflects movement of particles and wind.
HOW DO TROPICAL STORMS FORM?
Hurricanes need a lot of heat to form, which is why they
usually occur over tropical seas (at least 26°C). The sun is close
to the equator, providing energy to heat the ocean.
The warm ocean heats the air above it causing it to rise
rapidly. Water evaporates quickly from the hot surface of the
ocean, so the rising air contains great amounts of water
vapour. The rising air starts to spin (clockwise in the
The centre of the storm - the eye - is calm. As the air rises it
cools, condenses and forms towering cumulonimbus clouds.
The rapidly rising air creates an area of intense low pressure.
The low pressure sucks in air, causing very strong winds. Once
the storm moves over land it starts to lose energy and fades.
Tropical storms bring intense rainfall and very high winds,
which may in turn cause storm surges and coastal flooding,
and other hazards such as flooding and mudslides.
They develop as intense low-pressure systems over tropical
Winds spiral rapidly around a calm central area known as the
eye. The diameter of the whole tropical storm may be as much
as 800km, although the very strong winds that cause most of
the damage are found in a narrower belt up to 300km wide.
In a mature tropical storm pressure may fall to as low as 880
millibars. This, and the strong contrast in pressure between
the eye and outer part of the tropical storm, lead to very strong
winds of over 118km/h.
For tropical storms to form, a number of conditions are needed:
- Sea temperatures must be over 27 C to a depth of 60m
(warm water gives off large quantities of heat when it is
condensed – this is the heat which drives the tropical storm).
- The low-pressure area has to be far enough away from the
equator so that the Coriolis force (the force caused by the
rotation of the Earth) creates sufficient rotation in the rising
Tropical storms is a generic term that includes Hurricanes
(North Atlantic), Cyclones (Indian Ocean and the Bay of
Bengal), and Typhoons (Japan).
A tropical storm is a low pressure system up to 600km in
diameter with wind speeds of up to 300km/h (typically
160km/h) and bringing up to 30-50cm of rainfall.
There is no need to
learn all of the details
of the Saffir-Simpson
scale (measuring the
scale of tropical
storm strength and
impacts), but it is
good to know about
the lower, middle and
upper ends of the
Category 1: winds 119-153km/h, storm surge
generally 1.2-1.5m above normal.
Category 3: winds 119-209km/h, storm surge
generally 2.7-3.6m above normal.
Category 5: winds greater than 249km/h,
storm surge greater than 5.5m above normal.
TROPICAL STORM MANAGEMENT
The unpredictability of tropical storm paths makes the
effective management of tropical storms difficult, while the
strongest storms do not always cause the greatest damage.
TRACKING TROPICAL STORMS
Information regarding tropical storms is received from a
number of sources including:
- Satellite images
- Aircraft that fly into the eye of the tropical storm to record
- Weather stations at ground level
- Radars that monitor areas of intense rainfall
PREPARING FOR TROPICAL STORMS
There are a number of ways in which national governments
and agencies can help prepare for a tropical storms.
These include risk assessment, land-use control (including
floodplain management) and reducing the vulnerability of
structures and organisations.
LAND USE ZONING
The aim is to control land use so that the most important
facilities are placed in the least vulnerable areas, including
REDUCING STRUCTURES’ VULNERABILITY
New buildings should be designed to be wind and water
Communication and utility lines should be located away from
the coastal area or installed underground.
Improvement of building sites includes raising the ground level
to protect against flooding and storm surges.
Protective river embankments, levees and coastal dykes should
be regularly inspected for breaches due to erosion.
Improved vegetation cover helps to reduce the impact of soil
erosion and landslides, and facilitates the absorption of rainfall
to reduce flooding.
Managing hurricane impact is difficult.
The unpredictability of tropical storm path makes the
effective management of tropical storms difficult.
The strongest storms do not always cause the greatest
People living in coastal areas face increased risks associated
with tropical storms.
WHAT TO DO BEFORE A TROPICAL STORM
Know where your emergency shelters are.
Have disaster supplies on hand.
Protect your windows.
Permanent shutters are the best protection.
A lower-cost approach is to put up plywood panels.
Trim back branches from trees.
Trim branches away from your home and cut out all the dead or
weak branches on any trees on your property.
Check your home and car insurance.
Make arrangements for pets and livestock.
Develop an emergency communication plan.
EMERGENCY SHELTERS AUCKLAND CENTRAL
ASB Stadium, Kohimaramara Rd, Kohimaramara (adjacent to Selwyn College)
City Impact Church, 385 Mt Wellington highway, Sylvia Park
Eden Park, Reimers Ave, Mt Eden
Ellerslie Events Centre, 80 - 100 Ascot Ave, Ellerslie
Ellerslie War Memorial Hall, 138 Main Highway, Ellerslie
Fickling Centre, 1011 Mt Eden Rd, Three Kings
Freemans Bay Community Hall, 52 Hepburn St, Freemans Bay
Victory Church, 98 Beaumont St, Freemans Bay
Glen Innes Hall, 102 Line Rd, Glen Innes
Mt Albert Senior Citizens Hall, Corner Wairere Ave and New North Rd, Mt Albert
Mt Albert War Memorial Hall, corner Wairere Ave and New North Rd, Mt Albert
Onehunga Community Centre, 83 Church St, Onehunga
Panmure Community Hall, 7-13 Pilkington Rd, Panmure
Te Huruhi Primary School, 7 Donald Bruce Rd, Waiheke
Waiheke High school, 1 Donald Bruce Rd, Surfdale
WHAT TO DO DURING A TROPICAL STORM
Listen to the radio or television for tropical storm progress
Check emergency supplies.
Make sure your car is full of fuel.
Bring in outdoor objects such as lawn furniture, toys, garden
tools, and anchor objects that cannot be brought inside.
Secure buildings by closing and boarding up windows.
Remove outside antennas and satellite dishes.
WHAT TO DO AFTER A TROPICAL STORM
Assist in search and rescue.
Seek medical attention for persons injured.
Clean up debris and effect temporary repairs.
Report damage to utilities.
Watch out for secondary hazards: fire, flooding, etc.
Tornadoes are small and short-
lived but highly destructive storms.
Because of their severe nature and
small size, little is known about
Tornados consists of elongated
funnels of cloud that descend from
the base of a well-developed
cumulonimbus cloud, eventually
making contact with the ground
WHAT IS A
A tornado is a violent,
destructive weather system,
with powerful rotating winds
(up to 300km/h).
Tornadoes are intense, low-
pressure systems, and their
development depends on
instability in the atmosphere,
convergence and strong
updrafts in the air.
Many tornadoes have a short life. They can last from several
seconds to more than an hour.
Favoured areas are temperate continental interiors in spring
and early summer, when insolation is strong and the air may be
unstable, although many parts of the world can be affected by
tornado outbreaks at some time or another.
The Great Plains of the USA, including Oklahoma, Texas and
Kansas, have a high global frequency, and are particularly prone
at times when cool, dry air from the Rockies overlies warm, most
About 1000 tornadoes hit the USA each year.
On average, tornadoes kill about 60 people per year, mostly
resulting from flying of falling (crushing) debris.
There are 3 damaging factors at work:
- The winds are strong so objects are removed or damaged
- Rotational movement tends to twist objects from their fixings
- Low atmospheric pressure near the vortex centre is a major source of
FUJITA TORNADO DAMAGE SCALE
The Fujita scale relates the degree of damage to the intensity of
the wind. It should be used with caution as it does not take into
account differences in building structure and materials.
Light damage (more than 117km/h)
Damage to chimneys, branches broken off
trees, sign boards damaged
Severe damage (254-332km/h)
Roofs and walls torn off well-constructed
houses, trains overturned, cars lifted off
Incredible damage (419-512km/h)
Strong-frame houses lifted off foundations
and swept away, cars flying around 100m
As yet there is no effective way of managing tornadoes. The
best advice is to stay indoor and, if possible, underground.
There is no proof that cloud seeding can or cannot change
tornado potential in a thunderstorm.
To what extent it is possible to manage the risk
of tornado damage?
As yet there is no effective way of managing tornadoes.
The best advice is to stay indoors and, if possible,
underground. There is no proof that cloud seeding can or
cannot change tornado potential in a thunderstorm.
Cloud seeding is a form of weather modification, a way of
changing the amount or type of precipitation that falls from
clouds, by dispersing substances into the air that serve as
cloud condensation or ice nuclei, which alter the microphysical
processes within the cloud.
CLOUD SEEDING – 1950’s NZ TRIALS
Over twenty countries around the world are actively involved
in weather modification trials.
New Zealand is not one of them.
According to NIWA, "So far we do not know enough about
clouds and how to seed them, to be sure of the effects. We
might make less rain when we want more, or more hail when
we want less. So cloud seeding is not done in New Zealand".
Trials were carried out in the 1950s using planes to drop dry
ice in the clouds to bring rain to dry farms.
The trial was unsuccessful.
seeding shows the
silver iodide or dry
ice being dumped
onto the cloud,
which then becomes
a rain shower. The
process shown in the
upper-right is what
is happening in the
cloud and the
condensation to the
CLOUD SEEDING – 1970’s NZ TRIALS
Back in the 1860s trials were conducted where cannons were
used to shoot clouds in the hopes it would alter rainfall.
In the 1970s the New Zealand Government dropped silver
iodide from planes over Marlborough in the hopes they could
create rain to ease droughts.
But if the air is dry and there are no clouds then there is no
point in seeding. It makes about as much sense as planting
plant seeds on top of concrete instead of in the soil.
Hurricane Katrina was a
devastating storm that hit
the area around New
Orleans, USA, on 25 August
It had social, economic and
1,800 people died.
300,000 homes were
3 million people were left
with no electricity.
People had to move out of
$300 billion of damage.
Oil platforms were
Shops were looted.
Fuel prices rose.
The storm surge flooded large
areas of the coast. 80% of New
Orleans flooded as man-made
levees, overwhelmed by extra
Cotton and sugar cane crops
were destroyed. Delicate coastal
habitats were destroyed.
Tornadoes were created.