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# Tropical Storms

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Hurricanes, typhoons and cyclones: these terms all conjure up an image of total destruction left by heavy winds and rain. While these are three different terms, they all mean the same thing: a tropical storm. Let's look at what tropical storms are, their formation and effects, and some examples.

## Tropical storms definition

Tropical storms aren't exactly your light breeze. Tropical storms are extremely powerful and can travel very fast, up to 40mph/65 km/h! Tropical storms are also accompanied by heavy rainfall and strong winds. Let's define them properly.

A tropical storm is a low-pressure area with winds moving in a spiral motion around the 'eye of the storm', which is a calm, central point.

Tropical storms occur when warm, tropical air rises, creating an intense low-pressure area. Then, as the moist, warm air reaches high altitudes, powerful winds swirl around the calm, central point. This creates the famous 'eye of the storm'. As the warm air cools, it condenses, resulting in heavy rainfall and thunderstorms.

The intensity of a tropical storm is recorded using the Saffir-Simpson hurricane wind scale (SSHWS). Note that "tropical storm" is a general term applied to all storms, but it is also a term for the category between tropical depression and Category 1 storm: a storm with winds of 39-73mph/63-118 km/h.

Fig. 1 - Saffir-Simpson Scale details

## Types of tropical storms

Now, when it comes to tropical storms, you have probably heard different terms. That would be correct..."huh"?! Well, don't fret; the explanation is actually very simple: all those terms are correct. There is no difference between them other than their location.

Different types of tropical storms
TypeFormation locationAffected areasExample
HurricanesNorth Atlantic Ocean and Northeast PacificUS and CaribbeanHurricane Katrina in 2005
CyclonesSouth Pacific and the Indian OceanSouth AsiaCyclone Pam in 2015
TyphoonsNorthwest Pacific OceanEast AsiaTyphoon Haiyan in 2013
Table 1

Tropical storms form at different times of the year, depending on their location.

Where and when do tropical storms occur?
LocationMonths occurringPeaks
North Atlantic Ocean, the Gulf of Mexico and the Caribbean SeaJune - Novemberearly to mid-September
Northeast Pacific basinLate May/early June - Late October/early Novemberlate August/early September
Northwest Pacific basinOccur all year round, but mainly July - Novemberlate August/early September
North Indian basicApril - DecemberMay and November
Southwest Indian basinLate October/early November - Maymid-January and mid-February/early March
Southeast Indian/Australian basinLate October/early November - Maymid-January and mid-February/early March
Australian/Southwest Pacific basinLate October/early November - Maylate February/early March

Table 2

## Global distribution of tropical storms

The global distribution, formation and development of tropical storms are related to global atmospheric circulation. The transportation of heat from tropical to polar latitudes is necessary, and the global atmospheric circulation is a global wind system providing that transportation.

Tropical storms occur in, you've guessed it, the tropics, in particular where the so-called intertropical convergence zone (ITCZ) is situated, where the northeast and southeast trade winds converge - roughly north of the Tropic of Capricorn and south of the Tropic of Cancer.

Most often, tropical storms form between 5o and 30o north and south of the Equator, i.e. low latitudes. Tropical storms most commonly occur in the warmest seasons, between summer and autumn. Furthermore, tropical storms need ocean temperatures of at least 27oC and a water depth of about 197-230 ft/60-70m to form a storm.

As you can see, tropical storms form under specific conditions. Tropical storms cannot develop along the Equator because the Coriolis effect is not strong enough here, which is needed for tropical storms to spin. Read more on the formation of tropical storms below.

For more in-depth information, read our explanation of global atmospheric circulation.

## Formation of tropical storms

Now that you know what tropical storms are, let's look at their formation, which happens in a specific sequence.

When solar heating causes the ocean's surface temperature to increase to at least 27oC, the low-pressure conditions cause the warm air to rise, creating strong winds. Around a calm central eye of the storm, which can measure up to 30 mi/48 km across, the air spins upwards because of the Coriolis effect. As the warm air rises, it cools, and the water vapours condense to form cumulonimbus clouds, causing massive rainfall. When the air cools, the heat given off powers the storm. Cold air sinks, so when the air cools, it falls into the eye of the storm. That is why the eye of the storm has no clouds; it is drier and much calmer. The prevailing winds cause the storm to travel across the ocean, potentially making landfall at some point.

The Coriolis effect refers to the effect that the Earth's rotation has on weather patterns and ocean currents. This effect makes storms move counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Cumulonimbus clouds are tall, large, dense grey clouds. They produce rain, hail or snow.

Remember: when tropical storms reach a land surface, their energy will decrease because the ocean no longer provides the storm with its much-needed heat energy and moisture.

## Effects of tropical storms

Because tropical storms are so powerful, they almost always leave a path of destruction, especially when they are near or travel over land. Between 2001 and 2010, over 500 tropical storm disasters killed nearly 170,000 people, affected more than 250 million people and caused estimated damages at US$380 billion! There are primary and secondary effects, which can differ per location. Primary and secondary effects of tropical storms PrimarySecondary Strong winds often cause widespread damage to infrastructure, buildings, trees and crops.Heavy rains can lead to unstable slopes causing landslides/mudslides. Due to precipitation, strong winds and rising sea levels can cause storm surges, resulting in extensive coastal flooding.Water supplies can become polluted, causing diseases and potential further deaths. Widespread flooding due to heavy rainfall.Damages to houses can lead to uninhabitable homes. This forces people to live in make-shift shelters. Localised tornadoes can occur.Damaged infrastructure often means that basic services, such as powerlines, must be repaired. Road damage can also lead to basic provisions not reaching affected areas until they are fixed. Table 3 ### Tropical storms and climate change It should not come as a surprise that climate change also affects tropical storms, with the potential of tropical storms happening in more and different locations. An effect of climate change is the warming up of oceans. Warmer oceans can allow storms to extend further north and south of the Equator, causing bigger storms and putting more habitable land areas and people at risk. Low-lying coasts, where many people live, are more at risk than ever before. Regardless of an increase or decrease in the number of tropical storms happening, they will most likely increase in intensity, causing more damage and potential loss of life. Another effect of climate change is rising sea levels. This will make storms more destructive and increase coastal flooding. ## Examples of tropical storms With an average of 80-100 tropical storms per year, it is no surprise to learn there have been many devastating storms over many years. Let's look at a few examples. ### Cyclone Bhola The 1970 Bhola Cyclone occurred in East Pakistan (present-day Bangladesh) on 11 November 1970. The storm formed over the central Bay of Bengal on 8 November, from where it travelled north, gaining strength. On 10 November, the storm reached winds speeds peaking at 115 mph/185 km/h before making landfall on East Pakistan's coast. The following storm surge destroyed many offshore islands, wiping out entire villages and destroying crops. The storm eventually dissipated on 13 November. The Pakistani government, then led by Junta leader General Yahya Khan, was heavily criticised as relief operations were delayed. The Bhola Cyclone went down in history as the deadliest tropical cyclone ever recorded and one of the deadliest natural disasters in the world. At least 300,000 and possibly as many as 500,000 people lost their lives. Fig. 4 - Weather satellite image showing Cyclone Bhola at the Bay of Bengal on 12 November 1970 The 1970 Bhola Cyclone, also known as the Great Cyclone of 1970, was a Category 4 tropical cyclone on the SSHWS. ### Typhoon Nina Typhoon Nina formed on 30 July 1975 and gained strength as it moved west. Peak intensity was reached on 2 August, and on 3 August, it hit Taiwan. As the storm moved to China's southeastern shore, it weakened slightly. That said, it wreaked havoc in this region! The storm moved through central China at a slow pace with massive rainfall. This caused many dams to collapse, such as the Banqiao dam, causing the third-deadliest flood in history, affecting over 10 million people. At least 229,000 people died. Material damages were estimated at US$1.2 billion.

Fig. 5 - Map showing the path of Typhoon Nina

Typhoon Nina was a Category 4 super typhoon (SSHWS).

On 22 October 2012, south of Jamaica in the western Caribbean Sea, Sandy developed as a tropical wave, where it quickly gained strength and was upgraded to a tropical storm a mere six hours later. On 24 October, the eye of the storm began to develop and later that day, the National Hurricane Center (NHC) upgraded the storm to a hurricane. In the evening of the same day, Hurricane Sandy, with winds of about 85 mph/105 km/h, made its first landfall near Kingston. When Sandy made its second landfall west of Santiago de Cuba on 25 October, the storm intensified into a Category 3 hurricane. Over the next few days, Sandy fluctuated between being a Tropical Storm, a Category 1 hurricane and a Category 2 hurricane before dissipating on 2 November. Sandy left a trail of destruction, with 233 people killed and material costs estimated at US$68.7 billion. Fig. 6 - Map showing the path of Hurricane Sandy Now that you know more about tropical storms learn more about managing them in our explanation on tropical storm management. If you want more case studies, read our explanations on Hurricane Katrina, Cyclone Nargis and Typhoon Haiyan. ## Tropical Storms - Key takeaways • Hurricanes, cyclones, and typhoons are all names for the same thing: tropical storms. Which term is used depends on where you live. • The global distribution, formation and development of tropical storms are related to global atmospheric circulation. • For a tropical storm to form, the surface water needs to be at least 27oC, and the water depth needs to be 197-230 ft/60-70m. • Tropical storms are affected by climate change. Climate change will be responsible for tropical storms occurring with ever-increasing intensity in more and different locations. • Examples of tropical storms are Cyclone Bhola, Hurricane Sandy and Typhoon Nina. ## Frequently Asked Questions about Tropical Storms Tropical storms are low-pressure areas with winds moving in a spiral motion around the 'eye of the storm'. Tropical storms form when the surface temperature is at least 27oC and the water is about 197-230 ft/60-70m deep. The low-pressure conditions lead to warm air rising, creating strong winds. As the warm air rises, it will cool and the water vapours will form cumulonimbus clouds. Cold air sinks into the eye of the storm. Tropical storms are caused by warm, tropical air rising, creating a low-pressure area. The moist, warm air reaches high altitudes with powerful winds swirling around the calm, central point. As the warm air cools, it condenses, resulting in a storm. Most often, tropical storms form between 5o and 30o north and south of the Equator, i.e. low latitudes. Tropical storms leave a path of destruction in their wake. This can result in loss of life and damaged houses, infrastructure, crops and land. Furthermore, tropical storms can have secondary effects such as mudslides/landslides and death and diseases due to polluted water supplies. ## Final Tropical Storms Quiz ## Tropical Storms Quiz - Teste dein Wissen Question Where did Hurricane Katrina develop? Show answer Answer Near Jamaica Show question Question What category storm was Hurricane Katrina when it hit Florida? Show answer Answer Category 1 Show question Question TRUE or FALSE: Hurricane Katrina caused tornadoes in Alabama, Georgia and Florida. Show answer Answer True Show question Question What is a storm surge? Show answer Answer A storm surge is a temporary rise in water above normal sea level as a result of a storm. Show question Question Which states were affected by Hurricane Katrina? Show answer Answer Alabama, Florida, Georgia, Louisiana and Mississippi Show question Question Which states experienced the greatest impacts from Hurricane Katrina? Show answer Answer Louisiana and Georgia Show question Question What category storm was Hurricane Katrina when it made landfall in Mississippi? Show answer Answer Category 1 Show question Question Which city received the greatest impact from Hurricane Katrina? Show answer Answer Miami, Florida Show question Question What was the total death toll for Hurricane Katrina? Show answer Answer 1833 Show question Question Which state had the highest number of deaths from Hurricane Katrina? Show answer Answer Alabama Show question Question What was the estimated total overall damage caused by Hurricane Katrina? Show answer Answer USD$70 billion

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TRUE or FALSE: New Orleans flooded because the levees protecting the city could not withstand the additional pressure caused by the 8-10 inches of rainfall and the 22ft storm surge. Therefore, they failed, causing flood water to flow into the city.

True

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How many people evacuated from New Orleans the day before Hurricane Katrina hit the city?

1.2 million

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TRUE or FALSE: Relief efforts were coordinated only by NGOs and international countries in the aftermath of Hurricane Katrina.

True

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How much money in aid was mobilized and deployed by the US federal government in the aftermath of Hurricane Katrina?

USD $62.3 billion Show question Question Cyclone Nargis formed in the _____. Show answer Answer Bay of Bengal Show question Question Cyclone Nargis initially tracked towards the _____, then turned and began to track towards the _____. Show answer Answer North-west North-east Show question Question True or False: Cyclone Nargis made landfall in Myanmar as a category 1 storm. Show answer Answer True Show question Question Cyclone Nargis hit the _____ Delta and affected _____ townships. Show answer Answer Irrawaddy 50 Show question Question True or False: Cyclone Nargis' storm surge affected areas up to 40 km/25 miles inland. Show answer Answer True Show question Question The impacts of Cyclone Nargis were exacerbated by high levels of environmental destruction, such as: Show answer Answer Deforestation of mangroves Over-exploitation of natural resources Soil erosion Show question Question What was the total value of the damages caused by Cyclone Nargis in Myanmar? Show answer Answer USD$18 billion

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How many hectares of mangroves and other trees did Cyclone Nargis destroy?

42,000

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Approximately how many people were significantly affected by Cyclone Nargis?

3 million

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How many people were estimated to have died in Myanmar due to Cyclone Nargis?

140,000

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True or False:

The government of Myanmar immediately allowed access to international aid after Cyclone Nargis.

True

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Name two organisations that provided aid to Myanmar in the aftermath of Cyclone Nargis.

World Food Programme (WFP)

Association of Southeast Asian Nations (ASEAN)

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Typhoon Haiyan was the _____ deadliest typhoon recorded in the Philippines, after Typhoon _____ in _____.

second

Haiphong

1881

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In the Philippines, Typhoon Haiyan was also known as?

Typhoon Yolanda

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Where did Typhoon Haiyan start?

Federated States of Micronesia (in the western Pacific Ocean)

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What category was Typhoon Haiyan?

Category 5

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True or False: Approximately 75% of Tacloban was destroyed?

False

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True or False: Around 600,000 hectares of farmland were affected.

True

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What effects did the oil leak at Estancia have?

All of the above

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True or False: the people who sought refuge in an indoor stadium in Tacloban died when the roof collapsed?

False

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True or False: the Aquino government got a lot of criticism for acting slowly in the relief efforts.

True

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Explain the long-term response 'Build Back Better'.

'Build Back Better' means that houses and buildings are not just rebuilt but also upgraded so they will better withstand future storms.

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Coconut, rice and sugarcane production accounted for _____% of the Philippines’ GDP before Typhoon Haiyan hit.

12.7

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What are the long-term responses to Typhoon Haiyan? (Select 3)

'‘Build Back Better'

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When did Typhoon Haiyan make landfall in Eastern Samar in the Philippines?

8 November

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True or False: There was a 5-metre storm surge in Leyte and Tacloban (Philippines).

True

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Why did the local government collapse?

Many local officials died during the storm. This had a significant impact as it took some time to get everything in (working) order.

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Tropical storm management is about strategies used to reduce the _____ of a _____ storm.

risk, tropical

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Which strategy does NOT help to manage a tropical storm?

Embracing

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_____ and _____ educate people on how to prepare and respond to tropical storms.

Governments, NGOs

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Which of the following are not necessary for a survival kit?

Television

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TRUE or FALSE: Building homes and businesses away from risk areas, such as the coast, can reduce the number of people at risk.

True

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Planning on a scale of city or country would involve planning _____ areas and _____ so that they can facilitate everybody.

evacuation, routes

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Which of the below do NOT help a house against tropical storms?

Plaster walls

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Which of the following infrastructures do NOT help with tropical storms?

Below sea level cycle paths

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