a tornado is a violent, rapidly rotating and fast moving, narrow, funnel shaped
column of cloud that extends from base of a cumulonimbus cloud to the ground.
the wind makes it most violent type of storm. stronger winds than cyclones, 500
km/h. have very low pressure and usually only last a few mins. pressure falls
rapidly as tornado approaches, low in centre and rises quickly afterwards (like
tropical cyclone). but there’s differences in scale with both. average central
pressure isn’t known with confidence as tornado destroys recording instruments.
850mb, lower than cyclone, is lowest recorded central pressure and was measured
dropping rapidly by 100mb to 850mb in a few mins. majority of tornadoes are less
than 600m in diameter and have a path width of less than 50m. most touch ground
for less than 4km, record contact was for just over 350km.
most tornados become darkened by debris they pick up. most dangerous tornados
occur when approaching hazard isn’t noticed as the funnel shaped cloud is hidden
by rain or dust..
vary in size and shape. stovepipe tornadoes are narrow and cylindrical whereas
wedge tornadoes are very wide ones.
tornadoes are too small to be affected by Coriolis force, but most do rotate
anticlockwise in northern hemisphere and clockwise in southern. rotation is
believed to be started by wind shear. they generally move in USA from south west
to north east, they can take strange unpredictable paths and can be stationary or
race across ground at 80km/h. lightening, hail or heavy rain can accompany them.
processes leading to the formation of tornadoes
tornadoes aren’t fully understood and are studied in hope of being able to give
approach warning as country has 1200 a year. they aren’t becoming more frequent
but more people are living in their paths as populations rise.
they originate when a cold air mass from Canada moves south into warm moist air
from Gulf of Mexico, causing instability and turbulence. they occur at the cold front
boundary between the two air masses.
they also form from rotating supercell thunderstorms and always come from a
cumulonimbus cloud. only 20% of supercell thunderstorms develop tornadoes.
form in warm moist air usually with a temperature in excess of 18ºC. supercell
thunderstorms are very violent and have large updrafts extending to top of cloud.
these rotating up-currents (mesocyclones) can be up to 16km in diameter. about
half become tornadoes by extending downwards, spiralling to reach ground level,
narrowing as they do so.
rotation may be started by a wind shear — winds blowing in different direction or
speeds. e.g. when a strong horizontal upper air wind meets a violent updraft. would
cause a horizontal rotation, which is then forced to rise with the strong updraft.
first stage in life cycle in a tornado occurs when heavy rainfall in cumulonimbus
cloud drags a column of rapidly descending air down and then that drags the
mesocyclone towards the ground.
as mesocyclone lowers, pulls in warm moist air from kilometres around and humid
air from the rainy area of the cloud base. moisture in this quickly condenses to
form a rotating wall cloud, isolated cloud that projects below the rest in the rain
free part of the storm. increasing outflow of cold air concentrates the mesocyclone
base into an increasingly smaller area from which to draw its air intake. updraft
grows in intensity, creating low pressure near surface. pulls mesocyclone down as
a funnel cloud. when downdraft reaches ground 10-20 mins after the wall cloud
forms, creates a gust that causes severe damage over a considerable distance.
soon after, the funnel cloud itself becomes damaging, dust/debris gets drawn in
tornado grows in intensity while its supplied with warm moist air which expands in
the low pressure and cools, releasing latent heat when water vapour condenses.
when funnel cloud has grown to its widest extent and is vertical, tornado is in its
most damaging mature stage, often with very destructive hailstones (grapefruit
size)
eventually cold air from downdraft spreads at the ground surface and cuts off
supply of moist warm air that was fuelling the tornado. then rapidly weakens and
funnel becomes wavy and rope like, before disappearing.
a classification of tornadoes
measured on the fujita-pearson scale which is three independent scales measuring
wind speed, path length and path width.
frequency with which tornadoes occur decreases down the scale
tornado hazards
main hazard is being hit by flying debris or massive hailstones. or being lifted up
and blown through air until hitting an object or being crushed, either by falling
trees or collapsing buildings parts. broken power lines are a secondary hazard.
case study: super tornado in Moore, a suburb of Oklahoma City, USA, 20
May 2013
tornado was one of the worst in recent years in USA.
risk perception: Oklahoma department of emergency management advises
inhabitants to; keep informed about the weather by listening to local weather
forecasts and to have battery operated NOAA weather radio, which has a warning
alarm feature, sign up for free cell phone or email alerts, plan well ahead, knowing
what to do to keep safe, get inside a strong building when a tornado threatens, go
to lowest floor (basement). keep far away from windows doors and outside walls,
cover up to protect from flying or falling debris, if possible, wear a hard hat, avoid
being in a mobile home or vehicle. many homes have reinforced underground
storm shelters but they’re too expensive for some, schools have weather safety
shows and annual drills. 75% of warnings are false so not everyone listens as don’t
like wasting time. likely that most people in Moore would have taken the warning
seriously as fastest wind ever recorded was in Moore in 1999, 36 killed, 486km/h.
suburbs population was better prepared from that.
primary and secondary impacts on lives and property: the F5 touched down
outside Oklahoma City 16 mins after the warning was issued. mayor estimated the
warning saved 1000 lives. after touching down and tossing horses in air, headed
ENE into Moore where it did big damage. most tornadoes are very localised, this
one was an unusually wide wedge type so it managed to flatten whole housing
estates and reduce 2 and 3 story buildings to piles of sticks. also destroyed 2
primary schools. 1st was so damaged but no one died. tornado strengthened to
320km/h when it reached 2nd with no shelter, caused greatest loss of like when
roof tore off and wall collapsed, destroying the hall (safest area). Moore
hospital ,cinema and bowling alley also destroyed and fires broke out. cars tossed
in air and parts stripped from buildings. power lines left lying across
roads/pavements, created obstacles to movement and danger from power outages.
tornado was on ground for 45 mins, path of devastation was 27km long and 2.4km
wide. 24 deaths (7 at school), 237 injuries, 2500 homes destroyed, 10,000
affected. gas and water supplies cut. state of emergency declared and disaster
assistance agreed by US government. emergency workers from nearby states
searched with sniffer dogs for survivors and cleared wreckage that lay over
underground shelters. hindered by loss of communications. power restored to
hospital fast. doctors and nurses brought in and volunteers with aid organisations
served food and helped people search. not accompanied by hailstorm but day
before, tornado with hailstones unto 10.8cm in diameter struck Shawnee, another
suburb of Oklahoma and destroyed a mobile home park.
prediction and monitoring of large- and small-scale atmospheric
disturbances and perception of risk: how the USA prepares for hurricanes
and tornadoes
hurricane prediction and monitoring
storms are tracked on satellite images to provide warning to put strong covers over
windows and evacuate.
satellites make it much more accurate.
meteorologists fly in planes with instruments across eyeballs and eye of hurricanes
to measure strength.
predictions on where it’ll hit isn’t as accurate as can quickly change course and
speeds can vary.
where the landfall happens won’t be known until it happens.
causes problems for evacuation details. makes sure places are prepared with
regular practice drills and various methods of evacuation.
tornado prediction and monitoring
prediction improving constantly.
when satellite imagery shows right situation to form, announcement to media and
more regular images sent.
the doppler radar will be monitored for supercell thunderstorm clouds that have a
hook echo at their rears (associated with tornadoes).
large rotating updraft (mesocyclone) from which a tornado may form also helps
meteorologists spot them with more time to warn.
radar can detect debris so tornado can be spotted in rain or night.
perception of risk
people who haven’t experienced the hazard won’t be willing to take necessary
action to avoid it. 1992 hurricane Andrew threatening south Florida, authorities
evacuated from coastal areas, hard decision as economy harmed by stopping
normal life so authorities criticised if disaster doesn’t occur. hurricane destroyed
80,000 homes and 15 deaths from non-reluctant people.
inappropriate risk perception also in hurricane Katrina. physical and economic
reasons for the 1800 deaths in New Orleans. in marshes on Mississippi delta, built
on soft, easily eroded sediment, city below sea level with seawater kept out by
concrete embankments. economic reasons also contributed to disaster, not
thought to be cost-effective to spend on protection against events considered
unlikely. embankments and flood walls designed for category 3 hurricanes, not 6m
high storm surge. they collapses and 80% of city flooded 3m. little wind damage as
buildings built to withstand hurricane winds.
risk perception leads to protective measures against hurricanes, depends on
effectiveness of flood defences. people deter from living in vulnerable areas, such
as floodplains, by increasing the cost of their insurance.
integration of hazard prevention measures into planning of new developments may
make it possible to avoid flood damage. land use zoning used to reduce cost and
inconvenience of flooding; floodplains used for playing fields, pasture and nature
reserves and buildings at higher levels.
case study: the Philippines: a multi hazard country
many typhoons as located to west of a huge area of warm water in Pacific Ocean.
same as Haiyan, experience problems in sustainably managing an environment
made hazardous by flooding and typhoons but have success in reducing danger
from volcanic eruptions.
problems of sustainable management: poor country and multi hazard location so
has to balance what to spend on disaster resistant infrastructure or basic services.
schools and hospitals not built to withstand hazards and many sustainable
management methods are too expensive. population risen from 19 to 100m in 70
years, increased deaths inevitable as increasing number of people live in
vulnerable locations. some of the severity of the frequent flooding has been
blamed on inadequate drainage systems, deforestation been more damaging.
original cover of tropical rainforest, barrier to landslides and mudflows, been
cleared for export income. after flooding landslides and typhoon deaths in 2011,
government banned any new cuttings but illegal logging still occurs. need to use
funds to restoration and recovery after each of the 20 + typhoons that afflict the
country each year continues the poverty cycle. no more resources to protect
against next disaster. country lacks funds to construct canalised channels which
would move flood waters away fast. few and spread out shelters. few people can
afford insurance to cover losses. doesn’t haven’t same amount of tech or
knowledge.
eruption of Mt Pinatubo Philippines, June 1991: stratovolcano on the island on
Luzon where the south china sea plate, part of the Eurasian plate, sub-ducts
eastwards under the Philippine sea plate. densely populated island, 1m live within
30km of the volcano. hadn’t erupted in 500 yrs. 2nd biggest eruption of 20th C
caused a small life loss. First sign of activity was April 91 when many small
earthquakes accompanied the emission of 1000’s of tonnes of toxic sulphur dioxide
gas. 15th June there was a cataclysmic explosion of 5 cubic km of material. ash
cloud rose 35km in the air and typhoon blew ash in all directions, covering a wide
area. pyroclastic flows of hot gas and ash moved at a high speed down slopes and
giant lahars swept rapidly down valleys onto the lowlands as intense typhoon rains
mixed with ash deposits
impacts on lives and property: only 84 people killed, 300 by roofs collapsing under
weight of wet ash accumulations, 100 by lahars and rest from disease due to poor
sanitation in evacuation centres. 1m lost their homes and cost was $US 700m.
droplets of sulphuric acid formed in ash cloud and caused $US 100m damage to
flying aircraft. manila airport closed for a time. annual rainfalls up to 4000mm
occur on volcano, lahars removed half the deposits on slopes during next 4 rainy
seasons. caused more destruction than the eruption by burying towns and
lowlands under 3 cubic km of material in 4 years, making 200,000 homeless,
destroying the harvest roads and bridges. buried farmland was unusable for many
years.
reasons for the unexpectedly low loss of life: prediction monitoring and risk
perception kept the death toll small. March 1991 scientists installed seismometers,
tilt meters and other monitoring equipment on volcano. drew hazard maps and
geologists studied the area and discovered lahars had been a hazard in the past.
population was informed that a serious threat existed. daily bulletins about the
alert level were issued on TV, radio and newspapers. April 7, people living within
10km of volcano were evacuated. June 7, scientists warned that a major eruption
was imminent. people were evacuated from 10-20km zone. June 13, people
evacuated from 20-40km zone. 200,000 people evacuated to the velodrome in
quoin city, evacuee camp provided by government. soon after eruption, new lahar
hazard map produced and system to monitor and warn. monitoring and prep saved
5000 lives. large potential financial losses prevented by moving aircraft and other
expensive equipment to safe areas. total monitoring and prep costs were only $US
56m, benefits of it far outweighed the costs. illustrates that hazards in countries
like Philippines are best dealt with by accurate predictions whenever possible.
human factor in vulnerability to hazards
vulnerability and no of hazardous events has increased and will continue to rise as
population growth forcing people to live in vulnerable places. most vulnerable are
poorest, no choice as to where to live, poorly educated so don’t realise living in
vulnerable area. rich can quickly move out of danger and can regain normal life
after hazard. young and elderly are weaker, less able to migrate or withstand
hazard.
vulnerability also depends on: degree of technical ability to monitor hazard and
take preventative actions to minimise. prevention achieved by taking action to
avoid hazard impacts. mitigation attempts to limit adverse impacts. degree of
education and practising dills gives awareness of how to minimise danger
individuals less likely to be prepared than communities with organisations.
different economies have different vulnerability types; HICs have small loss of like
but high costs and LICs is opposite.
human responses to hazards are possible when risk is perceived
reduce vulnerability: predict the hazard and warn the community, prepare the
community by education, shelters, emergency drills, etc, plan land-use
appropriately
prevent/reduce hazard using technology: prevention; strengthen structures to
reduce the possibility of a hazard occurring. mitigation; design structures to reduce
damage when a hazard occurs
reduce the loss: provide aid for relief and reconstruction, insure people and
property
risk perception leads to improved hazard management
sustainable development is possible in high risk areas if public/private sectors act
to mitigate or prevent future hazardous events. risk assessments helps decisions
about whether redevelopment should take place or not.
construction measures and land use zoning save lives and prevent damage. hazard
mitigation is improving. loss of life during hurricanes has been reduced in
Caribbean by early warning systems. flood damage in some HICs reduced by
prohibiting building on floodplains, enforced by insurance conditions. hazard
mitigation is now seen to be cost effective. hazard management is most effective
when all sectors in the area are involved in integrated development planning
pronouncement by UN in November 2015 suggested the future is less optimistic.
death from hazard caused by climate change has increased. 90% of deaths in
Philippine hazards since 1990 resulted from storms and associated floods and
landslides, only 8% from tectonic hazards, but its taking a very simplistic view to
claim these deaths are caused by climate change alone. deforestation contributtion,
left only 3% of original protective forest cover and population increase of 40% since 1990 has been ignored. at same time, president announced plans to build 23
new coal fired power stations.
equation of disaster risk, R = (H x V)/C maintains that hazard risk increases as
peoples vulnerability increases and coping ability decreases.