Starter

Causes of tropical storms

• An oceanic location with sea temperatures above 26.5°C
• An ocean depth of at least 70 m
• A location at least 5° North or South of the Equator
• Low level convergence of air in the lower atmospheric circulation system
• Rapid outflow of air in the upper atmospheric circulation

Distribution

Where are tropical storms formed?

Measuring tropical storms

Saffir-Simpson scale

- a five-point scale based upon central pressure, wind speed, storm surge and damage potential.
A scale 5 event, for example, has:

• central pressure at 920 mb or below
• wind speeds at 250 km/hr (69 m/sec) or greater
• storm surge at 5.5 m or greater
• damage potential that refers to ‘complete roof failure of many buildings with major damage to lower floors of all structures lower than three metres above sea level. Evacuation of all residential buildings on low ground within 16–24 km of coast is likely

Impacts of tropical storms

The vulnerability of people to storm events depends upon a range of factors, both physical and human:

• the intensity of the storm (scale 1–5)
• speed of movement, that is, the length of time over the area
• distance from the sea
• physical geography of the coastal area – width of coastal plain or size of delta, and location of any mountain ranges relative to the coast
• the preparations made by a community
• warnings and community response.

Impacts of tropical storms

The major impacts that a tropical revolving storm can have on an area are described below:

• Winds: often exceed 150 km/hr (over 250 km/hour in a scale 5 event.
• Heavy rainfall: exceed 200 to 300 mm, bringing about severe flooding, landslides and mudslides.
• Storm surges: High sea levels result when the wind- driven waves pile up and the ocean heaves upwards as a result of the lower atmospheric pressure.

Frequency of tropical storms

• The average lifespan of a tropical storm is 7-14 days.
• Every year around 80-100 such storms develop, with approx. 80% going on to become tropical revolving storms.

Predictability of tropical revolving storms

• Tropical storms can be predicted if monitoring and warning systems are in place.
• Geostationary satellites and land and sea based recording centres and aircraft gather data.
• Information is compared to computer models so that a path can be predicted and warnings issued if needed.

Activities

1. Look at photo A

• Identify the eye of storm
• Which hemisphere is the storm in? How do you know?

Activities

2. a Study diagram B. Draw a sketch to show the different parts of a tropical storm and the conditions needed for it to form. Add labels to show:

• the main parts of the storm, including the spiraling clouds and the eye
• the temperature and the depth of the ocean
• the speed of the winds
• how much rain can fall in one day
• how wide storms can be
• where they form in relation to the Equator

Activities

2. b Write a paragraph to explain how a tropical storm forms

Activities

3. Study table D.

• Compare the death tolls of Hurricane Katrina and Hurricane Sandy, which both affected the USA
• Suggest reasons for the difference in the death tolls rot these two hurricanes
• Typhoon Haiyan affected parts of Asia including Philippines. Suggest why its death toll was higher than that of Hurricane Katrina whilst its economic lost was lower.

Practice questions

Describe the distribution of tropical storms. [5 marks]

Challenge: Outline the causes of the extreme weather conditions associated with tropical storms. (6 marks)

Practice questions

Describe the distribution of tropical storms. [5 marks]

Reasons for tropical storm formation

Tropical storms form where the conditions are suitable...

• The sun's energy
• Low wind shear
• Coriolis affect