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Unit 2 Section C - Causes and effects of tropical storms and responses to them

Page history last edited by K J Hutchinson 14 years, 1 month ago

Lesson 1 - What is a tropical storm? Where do they form?


Learning objectives:

- to know what a tropical storm is

- to be able to explain the physical processes leading to the formation of a tropical storm

- to be able to describe the locations in which we find tropical storms

- to know how the Saffir-Simpson scale is used to classify tropical storms


Note: Depending on where you are in the world, you may call these weather systems by a different name. In the USA, once wind speeds reach 74 mph they are referred to as hurricanes; in Bangladesh, they are called tropical cyclones; in Japan, people speak of typhoons; in Australia, they have willy-willies. All of these terms refer to the same weather event.


Tropical storms are areas of intense low pressure. On a satellite image, a tropical storm will show as a huge, swirling mass of cloud. The eye (calm area) is at the centre. You can see this in the image below. It shows Tropical Storm Edzani over the Southern Indian Ocean and was taken on 7th January 2010.



There are several stages in the formation of a tropical storm. These storms occur over water in areas where the sea surface temperature is at least 27 degrees. Essentially, this means that they form between the Tropic of Cancer and the Tropic of Capricorn (hence the name 'tropical' storm). They rarely occur within 5 degrees either side of the equator because the Coriolis force is too weak here.



Tropical cyclones use warm, moist air as fuel. That is why they form only over warm ocean waters near the equator. The warm, moist air over the ocean rises rapidly upward from near the surface and becomes saturated with evaporated moisture. This means that there is less air left at the surface (i.e. low pressure). Air from surrounding areas with higher air pressure pushes in to the low pressure area to try to equalise the pressure. Then that “new” air becomes warm and moist and rises, too. As the warm air continues to rise, the surrounding air swirls in to take its place. Trade winds cause the moist air to spin inwards. As the warmed, moist air rises and cools off, the water in the air forms towering cumulonimbus thunderclouds (because there is a huge amount of condensation). The whole system of clouds and wind spins and grows, fed by the ocean’s heat and water evaporating from the surface. The diagram below shows a cross section through a tropical storm.



Storms that form north of the equator spin counterclockwise. Storms south of the equator spin clockwise. As the storm system spins faster and faster, an eye forms in the centre. It is very calm and clear in the eye, with very low air pressure. Higher pressure air from above flows down into the eye.


The video clip below explains this process in images.


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When the winds in the rotating storm reach 39 mph, the storm is called a tropical storm. When the wind speeds reach 74 mph, the storm is officially a tropical cyclone, or hurricane. The strength of a storm is measured using the Saffir-Simpson scale, as shown in the table below.



Tropical cyclones usually weaken when they make landfall (i.e. hit the land) because they are no longer being fed by the energy from the warm ocean waters. In the northern hemisphere they track (i.e. move) westwards due to the Coriolis effect. The video clip below shows Hurricane Dean, from birth to death.


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Useful weblinks:

BBC animation - the formation of a hurricane

NASA animated guide to the development and structure of tropical storms

Make a tropical storm - simulator

Investigate how winds work together to determine where a hurricane will strike - simulator

BBC Bitesize revision notes about the formation of tropical storms


Lesson 2 - The effects of tropical storms


Learning objectives:

- to be able to distinguish between primary and secondary effects of tropical storms

- to be able to categorise the effects of tropical storms into social, economic and environmental (SEE)

- to understand why storm surges can accompany the landfall of a tropical storm


We have already seen that the effects of earthquakes and volcanoes can be classified into primary and secondary effects. The same is true of the effects of tropical storms. Primary effects are those that result directly from the event itself. Secondary effects are those that result from the primary effects. The exam specification states that you must be able describe effects in both of these categories.


Primary effects of tropical storms


Primary effects of tropical storms include high winds, torrential rain and storm surges at landfall (more about this one later on). There may also be localised tornadoes and waterspouts. These are all physical effects.


Secondary effects of tropical storms


Secondary effects of tropical storms are very wide-ranging. We tend to divide them into social, economic and environmental effects (the SEE classification). To remember this, think about how a good geographer must open their eyes and SEE the world! 

Why not watch the video clips below (about Hurricane Andrew and try to make a list of the primary and secondary effects of the storm. You should also try to classify them into social, economic and environmental. Use the video clips to give you ideas, but do not limit yourself to the information included in the clips!


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Social effects of tropical storms


There are likely to be food and water shortages.

As a result of extensive flooding, people may catch water-borne diseases. This may eventually lead to death.

Communities are displaced from their homes, and may be broken up if the area is not restored. Many people are made homeless.

People suffer from stress due to loss of possessions and housing.

There may be looting of properties - domestic and commercial.

People may lose their jobs if they work in an industry that has been badly affected.

If insurance premiums rise in the future, some people may not be able to afford them and will consequently not be financially protected against future storms.

People might be stranded due to flooding - this will cause trauma.


Economic effects of tropical storms


There are the obvious costs of repairing any damage caused.

Insurance claims will be made, and this may cause the cost of insurance premiums to rise in the future.

Whilst businesses are closed, earnings (and profits) will be lost.

Crops may be damaged and exports lost. These may be a key source of income for the local economy.

Oil prices may increase (this was a significant effect in the aftermath of Hurricane Katrina).


Environmental effects of tropical storms (sometimes referred to as physical effects)


A huge amount of water is released in a tropical storm so there will be extensive flooding.

Flooding might led to swer systems being flushed out and spreading disease.

There will be structural damage to buildings and they may have to be pulled down and rebuilt. This is very costly. Other buildings may have broken windows, chimneys etc.

Roads and other infrastructure such as railways may be destroyed. This can lead to communication problems.

Electricity lines might be blown down and, as a result, people could be without power supplies.

Sensitive ecosystems may be destroyed and plant and animal habitats lost.

Sea fish are often killed because of silting, and freshwater fish may be killed in storm surges.

Fishing boats and other craft may be damaged.

Crops and livestock may be damaged or destroyed.

Mudslides become common because the soil is saturated. They will flow quickly down hillsides and may bury houses, crops and livestock (or even people).


Storm surges


Storm surges occur where the sea level along a shoreline rises as the result of a storm. This is shown in the diagram below.



There are two main causes of storm surges (described below and shown on the accompanying diagram):


1. The eye pile. The low pressure draws the water beneath it upwards to form the eye pile. It gets dragged along with the eye. The eye pile grows in height as it is forced into shallower water. It splats onto the land as the storm makes landfall.


2. Wind-blown water. Zonal winds push the water in the same direction that they are moving. If the winds blow long enough and hard enough, the water banks up and floods the land when the winds blow onshore.



The shape of the coastline will affect the likelihood of there being a damaging storm surge. The steepness of the shoreline affects the manner in which the waves strike it, as shown on the two animations below.




Storm surges can cause considerable damage to property and to people!


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Useful weblinks: 

An experiment to see how the shape of the coastline affects storm surges - you will need playdough, sugar cubes and a hair-dryer!


Lesson 3 - MEDC case study: Hurricane Katrina, New Orleans, USA


Learning objectives:

- to develop a case study factfile for Hurricane Katrina

- to begin to consider how the effects of tropical storms might vary among countries with different levels of economic development


The exam specification requires you to study the effects of tropical storms in both MEDCs and LEDCs so that you can compare how they vary among countries with different levels of economic development.


Hurricane Katrina struck in August 2005. It tracked over the Gulf of Mexico and devastated most of the coastline from Louisiana to Alabama (see map below). It arrived as a category 4 storm with winds of over 140 mph and a storm surge of approximately 6 metres. The city of New Orleans was very badly affected because it is mostly below sea-level and is surrounded by water. The city was protected by defence walls called levees. However, the levees were overwhelmed by the extra water from the storm surge and rainfall, and many collapsed allowing water to flood into the city. About 80% of the city was flooded to depths of up to 6 metres.




The National Hurricane Centre predicted accurately where Hurricane Katrina would make landfall and how strong it would be. This gave people the opportunity to prepare for the storm. The mayor of New Orleans ordered people to evacuate the city. About 80% of the city's residents did so, but about 20% remained. The majority of these were in the poorest areas of the city (people had little access to transport so couldn't leave in many cases).


Over 10,000 people sought refuge in the city's Superdome football stadium. Conditions here deteriorated quickly - food and water soon ran out and the toilet facilities were inadequate. The atmosphere in the stadium was described as 'very tense and unsafe'.




More than 80% of the city was submerged with floodwater and over 1200 people drowned. Approximately 1 million people were made homeless and thousands of businesses were destroyed. Thousands of jobs were lost and millions of dollars lost in tax income. There was a lot of looting. Criminal gangs roamed the streets, looting homes and businesses and committing other crimes.


Major highways were disrupted and some major road bridges were destroyed. Agricultural production was badly damaged by the tornaodes and also by flooiding. Cotton and sugar-cane crops were flattened.


Hurricane Katrina didn't just impact people in the USA. Many offshore oil facilities were damaged and supplies of oil were reduced. This caused the price of oil to rise on the global markets and the price of petrol in the UK rose as a result.


Estimates suggest that Hurricane Katrina has cost over $300 billion. This makes it one of the costliest hurricanes ever to hit the USA.


Useful weblinks: 

The Met Office's guide to Hurricane Katrina for GCSE students (this is well worth reading as a summary of the case study)

BBC Special in-depth report on Hurricane Katrina - lots of useful weblinks from this page

BBC animation showing Katrina's track and where she made landfall

BBC guide to how the levees were repaired in New Orleans


Lesson 4 - LEDC case study: Cyclone Sidr, Bangladesh


Learning objectives:

- to develop a case study factfile for Cyclone Sidr

- to begin to consider how the effects of tropical storms might vary among countries with different levels of economic development


Cyclone Sidr formed in the central Bay of Bengal and quickly strengthened to reach sustained winds of 160 mph, making it a category 5 storm. The storm eventually made landfall in Bangladesh on November 15, 2007. It weakened quickly after landfall.


The Joint Typhoon Warning Centre predicted the scale and location of landfall and so people were forewarned. Government officials were recalled from their weekly leave. Ports were closed. There was mass-scale evacuation of the coastal area, much of which is land below sea-level. The image below shows the storm surge prediction for the area. 2 million people were evacuated to emergency shelters. Over 40,000 Red Crescent volunteers were deployed to order residents in the 15 affected provinces into special cyclone and flood shelters (see image below). Relief organisations distributed seven-day emergency disaster kits of food, blankets and clothing for evacuated families.




Coastal districts of Bangladesh faced heavy rainfall as an early impact of the cyclone. The damage in Bangladesh was extensive, including tin shacks flattened, houses and schools blown away and enormous tree damage. The entire cities of Patuakhali, Barguna and Jhalokati District were hit hard by the storm surge of over 5 metres. About a quarter of the world heritage site Sunderbans were damaged. Researchers said mangrove forest Sunderban will take at least 40 years to recover itself from this catastrophe. Much of the capital city of Dhaka was also severely affected, as electricity and water service were cut and significant damage was reported there due to winds and flooding.The local agricultural industry was also devastated, as many rice crops — which have a December harvest — were lost.


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This presentation from the charity CAFOD shows some of the work they have done in the area since the cyclone hit.



Useful weblinks: 

BBC News - Cyclone Sidr in pictures

Guardian newspaper in-depth report

Interactive learninbg activities from Games4Geog


Lesson 5 - How can we reduce the damaging effects of tropical storms


Learning objectives:

- to know how tropical storms are monitored

- to be able to describe and explain some of the startegies used in LEDCs and MEDCs to reduce the damaging effects of tropical storms


There are three steps that people are urged to follow to reduce the damage from tropical storms:






Rich MEDCs, such as the USA, can invest in technology to predict the general area in which hurricanes will strike. This technology is not so readily available to LEDCs, although many countries receive information from organisations such as the Joint Typhoon Warning Centre (Pacific) or National Hurricane Centre (Atlantic) to help them prepare for storms. The track of a tropical storm is affected by a great many fators, so they are very unpredictable. Forecasters have a 20-25% chance of knowing exactly where a tropical storm will hit 48 in advance of landfall.


Forecasts are made available to residents on the internet so that they can make decisions about whether they need to evacuate. This is obviously a much more useful means of communication in an MEDC where the percentage of residents with internet access is much higher than in LEDCs. Indeed, one of the major problems in preparing for a strike in an LEDC is communicating with the people in an area that is likely to be affected as the infrastructure may well be poorly developed.




Most countries affected by tropical storms now have education programmes to raise awareness on preparing for storms. There is an annual Hurricane Preparedness Week in the USA when schools, businesses and families are all encouraged to pland for possible strikes in the forthcoming hurricane season. Leaflets, posters and school lessons are common ways of spreading the message. New technologies - such as Twitter (see link below) are also being used to educate people.


The video clip below is a Public Service Announcement by Governor Haley Barbour encouraging people to get ready for the hurricane season.


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People living in areas affected by tropical storms are encouraged to have a hurricane emergency kit on standby and also to have thought in advance about how they will protect their home and belongings. This might include boarding up windows and doors. In MEDCS, people may also be able to afford windproof tiles, water-resistant windows and strengthened building structures.



People in LEDCs may not be able to afford this kind of protection. However, simple but cheap measures can be put in place to ensure that people are well prepared for tropical storms. These include educating people (particularly women) about what to wear in case they have to swim, building homes on stilts, building cyclone shelters and training local people to organise others in an evacuation.




When landfall has been forecast, action needs to be taken. This might include boarding up windows and doors, or evacuating the area. A mandatory evacuation is where the authorities command everyone in an area to leave. It is vital to ensure that only those peole who are in the danger area try to leave - otherwise the road system will get clogged up and people will be put at greater risk.




Useful weblinks: 

Twitter account for Atlantic Watch - keep up to date with forecasts and tips for preparing your home to survive a hurricane strike (unoffical Twitter for NHC)

FEMA guidance on how to prepare for a hurricane

Hurricane Strike! - a superb website with lots of interactives and simulations about preparing for a hurricane strike - user account required (free to sign up)

Stop Disasters! - an online game about preparing for a tropical storm


Lesson 6 - How and why might the distribution of tropical storms change?


Learning objectives:

- to recognise that scientists are still debating whether or not the distribution and frequency of tropical storms may change

- to be able to give reasons and evidence in support of the argument that global warming is increasing the number of storms

- to be able to explain the argument that there is a natural cycle

- to learn how to make an informed judgment as to whether or not the number and severity of tropical storms is changing


This is a controversial issue! Scientists are not agreed about how and why the distribution of tropical storms might change. The exam specification says that you must be able to 'explain why the distribution and frequency of tropical storms may change' and that you should include the impact of climate change in your explanation. However, as good geographers you will need to keep an open mind as to whether or not climate change is really at the heart of any changes to the intensity and distribution of tropical storms.


Why are opinions divided?


Satellite technology has only been used to monitor tropical storms since the late 1960s. Before this, accounts from ships' logs, aeroplane research flights, and simple weather recording instruments were used. Some scientists have argued that the number and strength of tropical storms in the pst may have been greater than was actually recorded at the time. They also cliam that there may have been some tropical storms occurring that we did not know about as they did not make landfall. The graph below shows the storm history of named tropical storms in the North Atlantic. It is divided into two periods - pre-satellites and post-satellites. Do you think that the number of storms is increasing?



Why do people think that climate change will lead to more tropical storms?


As you already know, tropical storms form where the temperature of the water exceeds 27 degrees. Global warming is causing a rise in the ocean temperatures, and this will mean that ocean temperatures will exceed 27 degrees for longer periods of time. As a result, the tropical storm season will last longer. In addition, ocean temperatures will exceed 27 degrees in more places so there will be more tropical storms.


The map below shows the monthly sea surface temperature anomalies in January 2010 relative to the 1961-1990 base period.



The 'natural cycle' viewpoint


An alternative viewpoint is that there is a natural cycle of variations in the number of tropical storms and that this is not linked to global warming. A natural cycle is a series of events that happen over and over again. the AMO is a 20-20 year fluctuation between warmer and cooler than average ocean temperatures in the North Atlantic Ocean. We are currently in the warm phase of the AMO. Some scientists argue that this is why we are experiencing increased tropical storm activity. The graph below shows a clear cyclical pattern in AMO values.



Are tropical storms becoming more destructive?


In terms of the damage they cause, tropical storms do seem to be getting more destructive. 6 out of the 10 costliest hurricanes in the USA have happened since 1990. Why might this be?


Firstly, more people are living near the coast. As tropical storms' effects are concentrated in coastal areas, this means that more people are living in danger zones. There has been much more buidling in these aeas in recent years - so when a hurricane strikes, there is more infrastructure in its way and for it to damage! The value of property at the coast has increased rapidly, so the costs of clear-up have increased too.


El Nino


El Niño is a climate phenomenon affecting the Pacific, Atlantic and Indian Oceans, occurring approx every 3-7 years. Officially, it is a sustained sea surface temperature greater than 0.5°C more than normal across the central tropical Pacific Ocean. It is explained very clearly on the BBC Bitesize website - so please click here for more infomation. Thanks also to Slideshare user MissM for the PowerPoint embedded below. They are both far better explanations than I would be able to give you.



Normally, trade winds blow west and slightly north across the Pacific Ocean, causing surface water, warmed by the sun, to 'pile up' at the western end so that the sea surface is about half a metre higher here. The warm water means surface temperature is about 8 degrees higher. Sometimes the normal trade winds relax. This is the start of an El Niño. The warm pool of water in the Western Pacific is released. It 'floats' because it is less dense than the cold water around it, and flows across the surface of the ocean back east towards South America.


In general, El Nino events are characterised by more tropical storms in the eastern Pacific and less in the Atlantic, Gulf of Mexico and Caribbean Sea. This is shown in the table below.



There is some uncertainty as to what will happen to El Nino as the planet warms. 3-5 million years ago the carbon dioxide in the atmosphere was significantly higher than it is now. During this time the El Niño conditions we consider exceptional now were much more frequent, perhaps even normal. The Met Office has suggested than an increase in carbon dioxide in the atmosphere will increase the frequency and intensity of El Niño. This may well be a cause of an increase in global sea surface temperatures.


Exam Question


The specimen exam paper for Unit 2 includes a graph about the number of tropical storms. Candidiates are asked to describe the changes to the average number of tropical storms in the Atlantic Ocean between 1900-2005 (3 marks) and give reasons for this pattern (4 marks). The graph is shown below. You should try to answer this question. Allow yourself 8 minutes to complete your answer.



The first question (the describe part) is point-marked. You have to make 3 good points to get your 3 marks. You need to include figures from the graph in your description. Here are some things that you could write:

- Between 1900-1930 there was a low average number of storms (less than 10 per year)

- Between 1930-1990, the average number of storms increased slightly, fluctuating around 10 per year (between 9 and 11.5)

- Between 1990-2005, the average number of storms increased significantly and was greater than 10 storms per year throughout the period, reaching 15 per year by 2005


The second question (the explain part) is also point-marked. You can either make 4 good separate points, or 2 points that you expand on. Here are some ideas from the mark scheme:

Reference to global warming/climate change (1) + effects (1), e.g. Due to global warming (1), there has been a rise in sea-surface temperatures (1) which means that more storms go on to become major hurricanes (1) because there are longer periods when the temperatures exceed 27°C (1).


How did you do?


Useful weblinks:

A summary of the debate


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