5.5: Tropical Cyclone Hazards

Question 1

What are tropical revolving storms?

Answer 1

(Including hurricanes, cyclones, typhoons). Are low pressure weather systems that develop in the tropics.

Usually measuring 200-700km in diameter.

Question 2

How are tropical revolving storms formed?

Answer 2

Begins with an area of low pressure, resulting from surface heating, into which warm air is drawn into in a spiralling manner.

Such small scale disturbances can enlarge into tropical depressions with rotating wind systems and these may continue to grow into a much more intense and rapidly rotating system: the rapidly rotating storm.

Question 3

What conditions are needed for a tropical storm to be present?

Answer 3

• an oceanic location with sea temperatures above 27*C
• ocean depth of at least 70m.
• location of 5* north or south of the equator
• low level convergence of air in the lower atmospheric circulation system (winds have to come together near the centre of low pressure zone)
• rapid outflow of air in the upper atmospheric circulation

Question 4

Why do sea temperatures ended to be at least 27*C for tropical revolving storm to occur?

Answer 4

Because this provides a continuous source of heat in order to maintain rising air currents.

Question 5

Why is an ocean depth of 70m needed to form tropical revolving storms?

Answer 5

Because the moisture provides latent heat; rising air causes the moisture to be released by condensation and the continuation of this drives the system.

Question 6

Why is a location of at least 5* north or south of the equator needed to form a tropical revolving storm?

Answer 6

So that the Coriolis force can bring about the maximum rotation of the air. (The Coriolis force is weak at the equator and will stop a circular air flow from developing).

Question 7

Why is a rapid flow of air in the upper atmospheric circulation needed to form a tropical revolving storm?

Answer 7

The rapid flow of air in the upper atmospheric circulation pushed away the warm air which has risen close to the centre of the storm.

Question 8

What happens when a tropical revolving storm reaches maturity?

Answer 8

A central eye develops (10-15km in diameter).

Here, there are calm conditions, clear skies, higher temperatures and descending air.

Question 9

How does a tropical revolving storm end?

Answer 9

Once the storm reaches land or cooler waters pole wards, it will decline as the source of heat and moisture is removed.

Question 10

What is the distribution of tropical revolving storms?

Answer 10

Between 5* and 20* north and south of the equator. Once generated, they tend to move westwards. Eg to the Gulf of Mexico area (called hurricanes), off SE Asia (called typhoons), or off NW Australia (called willy-willies).

Question 11

How are tropical revolving storms measured?

Answer 11

On the Saffir-Simpson scale.

Question 12

Outline the Safari-Simpson scale.

Answer 12

A 5 point scale based upon central pressure, wind speed, storm surge and damage potential.

Question 13

What’s the average life span of a tropical storm?

Answer 13

7-14 days

Question 14

How many tropical storms develop per year?

Answer 14

80-100

Question 15

Why does a tropical storm end when it reaches land?

Answer 15

Because it’s supply of energy (prevailing winds) and moisture is cut off, so storm starts to decay

Question 16

Why are tropical revolving storms likely or form at the ITCZ?

Answer 16

Because this is where two limbs of the Hadley Cell converge to form low pressure on the ground.

Question 17

The vulnerability of people to storm events depends on…

Answer 17

• the intensity of the storm
• speed of movement (the length of time over an area)
• distance from the sea
• preparations made by a community
• physical geog of the coastal area (width of coastal plain, and location of any mountain ranges relative to the coast)

Question 18

What are the major impacts (hazards) associated with tropical storms?

Answer 18

• strong winds
• heavy rainfall
• storm surges

Question 19

Outline strong winds as a hazard associated with tropical storms?

Answer 19

Winds that exceed 150km/hr.

Such high winds cause structural damage to buildings, roads and bridges etc.
Can also bring down electricity transmission lines and devastate agricultural areas. Debris also flung around - risk to humans.

Question 20

Outline storm surges as a hazard associated with tropical storms.

Answer 20

High sea levels result when the wind-driven waves pile up and the ocean heaves upwards as a result of the lower atmospheric pressure.

These storm surges can have a devastating effect on low-lying coastal areas eg river deltas where the flooding can extend a long way inland.

Question 21

Outline heavy rainfall as a hazard associated with tropical storms.

Answer 21

Rainfall can exceed 300mm, bringing about severe landslides.

The intense rainfall increases pore water pressure (hydrostatic pressure within a slope) which weakens cohesion and triggers slope failure.

Question 22

Outline prediction as a way of managing the tropical revolving storm hazard.

Answer 22

To some extent they can be predicted, as they’re mostly restricted to the tropics and don’t usually occur close to the equator.

Most usually occur from late summer - autumn (Aug-Oct).

Question 23

Outline how the NOAA can attempt to predicts tropical revolving storm hazards.

Answer 23

Uses indicators to suggest the number to storms to be expected eg:

• sea surface temperatures
• atmospheric conditions
• short term climatic cycles (eg El Niño)

Question 24

Why is it important that evacuation warnings are correct?

Answer 24

Because there’s a high economic cost associated with evacuation.

And people might become complacent and refuse future advice.

Question 25

What is one way of potentially preventing tropical revolving storm hazards?

Answer 25

There’s been research into the effect of cloud seeding in order to cause more precipitation. Because, if the cyclone could be forced to release more water over the sea, then this would be a weakening of a system as it approaches land

Question 26

Outline how protection can be used to manage the tropical revolving hazard.

Answer 26

• land use planning can identify the areas at greatest risk, and certain types of development can be limited in such areas.
• sea walls, breakwaters and flood barriers can be built and houses put on stilts.
• some structures can be retrofitted (adjusting the building to make it resistant to winds ) to ensure greater safety during an event.
• cyclone/hurricane drills can be practised. Eg in Florida, there is a ‘Project Safeside’: a hurricane awareness programme that’s composed of precautionary drills for use in schools and the Emergency Operations centre of state.

Question 27

Outline how preparedness can be used to manage the tropical revolving storm hazard.

Answer 27

Preparedness is one way in which many communities are able to use their abilities to mitigate the effects of tropical storms.

Eg in North Carolina, local communities have put together ‘the Outer Banks Mitogation Plan’. In order to save lives, money, property, and reduce future vulnerability.

Question 28

What is the location of tropical storms?

Answer 28

Within the tropics, between the Tropic of Cancer (23N) and Tropic of Capricorn (23S), as this is the only area with warm water (over 27*C).

Question 29

Outline some characteristics of a tropical storm.

Answer 29

Spinning storms with strong winds (up to 250km/hr).

They’re circular in shape and usually last 7-14 days).

Question 30

How are tropical storms formed?

Answer 30

1. As warm air rises over tropical oceans, it cools, condenses and forms storm clouds.
2. Being 27*C and 50m deep encourages huge amounts of evaporation.
3. This results in an area of low pressure.
4. As warm air rises, cooler air is drawn towards the centre of low pressure from the stratosphere. This area of air convergence forces the rising warm air upwards.
5. ITCZ (where 2 limbs of the Hadley cell converge to form is pressure on the ground provides perfect conditions for the formation of tropical storms).

Question 31

Why don’t tropical storms form at the equator?

Answer 31

Because the Coliolis effect is weak at the equator.

(This is a force caused by the Earth’s rotation, and deflects the path of converging winds causing the winds to rotate).

Question 32

How does the ICTZ affect tropical storms?

Answer 32

This is where two limbs of the Hadley cell converge to form low pressure on the ground - which provides a perfect condition for the formation of tropical storms.

Question 33

When do tropical storms finish?

Answer 33

They lose strength as they move over land because the supply of moist warm air is cut off.

Question 34

Tropical storms occur frequently (100per year). But why don’t this many develop into a hazard?

Answer 34

Because they don’t reach land.