Tropical storms

Question 1

What is the basis for storms forming?

Answer 1

Develop over warm water. As warm, moist air rises and condenses, it releases energy that increases wind speed.

Question 2

What conditions do tropical storms need?

Answer 2

• Disturbance near the sea-surface that triggers storm (area of low pressure)
• Warm sea water (27degrees + 50m below surface) - evaporation
• Convergence of air in lower atmos. - within ITCZ or along boundary between warm and cold masses - air rises
• At least 5 degrees from Equator - don’t form either side because Coriolis force is not strong enough

Question 3

What is the Coriolis effect?

Answer 3

Force caused by earths rotation. Deflects the path of winds but is weak near the equator.

Question 4

What are the characteristics of tropical storms?

Answer 4

AKA cyclones, hurricanes and typhoons. Can extent to 500km in diameter and cause extensive damage and loss of life in many coastal regions on the tropics. Must have av. wind speeds over 75mph.
The eye wall, bank of cloud that rings the central eye, is the most powerful and damaging part. Cloud and rain extend in a series of waves that extend beyond the eye wall. Tornados form within storms
Highly localised = difficult to predict and highly destructive.

Question 5

What is the distribution of tropical storms?

Answer 5

Vast majority form in the tropics close to the Equator on either side.
They do extend to other places like China, Japan and eastern seaboard of USA.

Question 6

What are the factors that can effect distribution?

Answer 6

• Oceans
• High temperatures
• Atmospheric instability
• Rotation of the earth
• Uniform wind directions at all levels

Question 7

What happens as storms form?

Answer 7

• It develops a clear and distinct rotation.
• Warm moist air rises rapidly in its centre to be replaced by air drawn in at the surface.
• A central cortex develops as more and more air is drawn in and rises. The eye often characterised by column of dry sinking air.
• Air cools as it rises - condensation and towering cumulonimbus clouds form. Sometimes a no. of isolated storms will join to form 1 large one.

Question 8

What happens when condensation forms in a storm?

Answer 8

Latent heat is released. This effectively powers the storm.

Question 9

When do tropical storms begin to die down?

Answer 9

Continue to grow and dev. as they are driven by prevailing winds across ocean.
When it reaches land and supply of energy and moisture is cut off storms die down. Back over ocean - reinvigorated.

Question 10

What are the storm hazards?

Answer 10

• High winds
• Coastal flooding
• Storm surges
• River flooding
• Landslides

Question 11

Storm hazards: strong winds

Answer 11

Average wind speeds of 75 mph (120 km/h). Speeds of up to 250km/h have been recorded at the eye wall.
Capable of significant damage - tearing off roofs, breaking windows, uproot trees, carry debris and damage communication networks.
Debris form flying missiles, strewn on roads = transport disruption.
Damaged power lines - electricity cuts and occasionally fires.

Question 12

Storm hazards: Storm surges

Answer 12

Surge of water 3m high inland flooding low-lying areas. Caused by intense low atmos. pressure of TS w/ powerful driving surface winds.
Major cause of widespread devastation and loss of life. Inundate agricultural land w/ saltwater and debris, pollute freshwater supplies and destroy housing and infrastructure. Enhanced coastal erosion = undermining of buildings and highways.

Question 13

Storm hazards: Coastal and river flooding

Answer 13

Warm humid air can generate torrential rainfall, over 200mm in a few hrs. Flash flooding at the coast, urban areas where surface water overwhelms drainage. Urbanisation increases flood hazard.
As moves inland, weakens as moisture supply is cut off. Still may result in sig. flooding due to intensity and quantity of rain falling on river basin.

Question 14

What is an example of flooding due to a tropical storm?

Answer 14

August 2011, Hurricane Irene caused widespread flooding throughout New Jersey resulting in evacuation of over 1 mil ppl and over $1 bil in damage.

Question 15

Storm hazards: Landslides

Answer 15

Up to 90% of landslides a yr caused by heavy rainfall and many triggered by TS. Intense rainfall increases pore water pressure, weakens cohesion and triggers slope failure. Additional weight of water exacerbates problem.
Possible that load release caused by TS-induced landslides may trigger earthquakes in tectonically stressed regions.

Question 16

What is the Saffir-Simpson scale?

Answer 16

TS can be classified using the Saffir-Simpson scale. Developed in 1971 as a means by which of enabling storms of diff. magnitudes to be compared. It is a scale based on sustained wind speeds and has 5 categories.

Question 17

Why is the Saffir scale limited?

Answer 17

Doesn’t take rainfall into account or area affected by storm. Therefore a low-category TS that hits a densely populated area will have more impacts than a high-category TS hitting a remote region.

Question 18

TS: magnitude and frequency

Answer 18

Currently no clear evidence that no.s or intensity of TSs are increasing w/ global temp.s.
In last 2 decades there have been several yrs w/ high no. of TSs but the pattern is erratic. No evidence they are becoming more intense.
Some logic in expecting warmer atmos. to hold more water but this countered by increase wind shear that acts as neg. feedback loop nullifying TS formation.

Question 19

TS: regularity and predictability

Answer 19

Can be predicted to some extent - restricted to tropics and not usually close to equator. Mostly occur late summer into autumn w/ peak from August through to October.

Question 20

What does the NOAA use as indicators for TSs?

Answer 20

The NOAA publishes predictions of TS activity for forthcoming season.
Uses indicators like sea-surface temp.s, atmos. conditions, and short term climatic cycles like El Nino.
Not likely to predict landfall or levels of activity at particular locations - these can only be predicted few days before.

Question 21

What is a hurricane return period?

Answer 21

The frequency at which a certain intensity of hurricane can be expected within a given distance of a given location.

Question 22

What is the danger of attempting to make predictions?

Answer 22

Though they tend to follow similar tracks, this is an over simplification. Each storm dev.s own characteristics and responds uniquely to atoms. and oceanographic conditions. Some tracks are extremely eccentric.