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9.3 Hazards resulting from atmospheric disturbances
12 Suggest reasons why avalanches are clustered in the months between
January and March. Explain at least two of these reasons. 13 What is an avalanche? 14 What are the factors that increase the risk of an avalanche? 15 What were the conditions in Europe in February 1999 that led to widespread avalanches? 16 How and why may the threat of avalanches change in the next decades?
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Answers on p.219
Tropical storms (cyclones)
Revised
Tropical storms bring intense rainfall and very high winds, which may in turn cause storm surges and coastal flooding, and other hazards such as flooding and mudslides. They develop as intense low-pressure systems over tropical oceans. Winds spiral rapidly around a calm central area known as the eye. The diameter of the whole tropical storm may be as much as 800km, although the very strong winds that cause most of the damage are found in a narrower belt up to 300km wide. In a mature tropical storm pressure may fall to as low as 880 millibars. This, and the strong contrast in pressure between the eye and outer part of the tropical storm, lead to very strong winds of over 118km/h.
Tropical storm is a generic term that includes hurricanes (North Atlantic), cyclones (Indian Ocean and the Bay of Bengal) and typhoons (Japan). A tropical storm is a low-pressure system up to 600km in diameter with wind speeds of up to 300km/h (typically 160km/h) and bringing up to 30–50cm of rainfall.
For tropical storms to form a number of conditions are needed: l Sea temperatures must be over 27°C to a depth of 60m (warm water gives off large quantities of heat when it is condensed – this is the heat which drives the tropical storm). l The low-pressure area has to be far enough away from the equator so that the Coriolis force (the force caused by the rotation of the Earth) creates sufficient rotation in the rising air mass.
Tropical storms are measured using the Saffir-Simpson scale (Table 9.4).
Expert tip
There is no need to learn all of the details of the Saffir-Simpson scale, but it is good to know about the lower, middle and upper ends of the scale, as shown in Table 9.4.
Table 9.4 Saffir-Simpson scale of tropical storm strength and impacts
Category
Category 1: Winds 119–153 km/h; storm surge generally 1.2–1.5 m above normal Category 3: Winds 178–209 km/h; storm surge generally 2.7–3.6 m above normal
Category 5: Winds greater than 249 km/h; storm surge generally greater than 5.5 m above normal
Description
No real damage to building structures. Damage primarily to unanchored mobile homes. Also, some coastal road flooding and minor pier damage.
Some structural damage to small residences and utility buildings. Mobile homes are destroyed. Flooding near the coast destroys smaller structures with larger structures damaged by floating debris. Land below 1.5 m above mean sea level may be flooded inland 13 km or more. Evacuation of low-lying residences close to the shoreline may be necessary. Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or blown away. Complete destruction of mobile homes. Severe and extensive window and door damage. Lowlying escape routes are cut by rising water 3–5 hours before arrival of the centre of the tropical storm. Major damage to lower floors of all structures located less than 4.5 m above sea level and within 500 m of the shoreline. Massive evacuation of residential areas on low ground within 8–16km of the shoreline may be required.
Tropical storm management
The unpredictability of tropical storm paths makes the effective management of tropical storms difficult, while the strongest storms do not always cause the greatest damage.
Tracking tropical storms
Information regarding tropical storms is received from a number of sources, including: l satellite images l aircraft that fly into the eye of the tropical storm to record weather information l weather stations at ground level l radars that monitor areas of intense rainfall
Preparing for tropical storms
There are a number of ways in which national governments and agencies can help prepare for a tropical storm. These include risk assessment, land-use control (including floodplain management) and reducing the vulnerability of structures and organisations.
Risk assessment
l Living in coastal areas increases the risk associated with tropical storms. l The evaluation of risks of tropical storms can be shown in a hazard map.
These can be based upon: –analysis of climatological records to determine how often cyclones have struck, their intensity and locations –the history of winds speeds and frequencies, heights and locations of flooding and storm surges over a period of about 50–100 years
Land use zoning
The aim is to control land use so that the most important facilities are placed in the least vulnerable areas, including floodplains.
l New buildings should be designed to be wind and water resistant. l Communication and utility lines should be located away from the coastal area or installed underground. l Improvement of building sites includes raising the ground level to protect against flooding and storm surges. l Protective river embankments, levées and coastal dikes should be regularly inspected for breaches due to erosion. l Improved vegetation cover helps to reduce the impact of soil erosion and landslides, and facilitates the absorption of rainfall to reduce flooding.
In addition, there are many things that individuals can do to prepare for a tropical storm in terms of how to act during and after the event (Figure 9.4).
Now test yourself
17 Why is managing tropical storms difficult?
Answer on p.219
Tested
Before a tropical storm
• Know where your emergency shelters are. • Have disaster supplies on hand. • Protect your windows. • Permanent shutters are the best protection. A lower-cost approach is to put up plywood panels. • Trim back branches from trees. • Trim branches away from your home and cut out all dead or weak branches on any trees on your property. • Check your home and car insurance. • Make arrangements for pets and livestock. • Develop an emergency communication plan.
During a tropical storm
• Listen to the radio or television for tropical storm progress reports. • Check emergency supplies. • Make sure your car is full of fuel. • Bring in outdoor objects such as lawn furniture, toys, garden tools, and anchor objects that cannot be brought inside. • Secure buildings by closing and boarding up windows. • Remove outside antennas and satellite dishes.
Figure 9.4 What to do before, during and after a tropical storm
Tornadoes
Tornadoes are small and short-lived but highly destructive storms. Because of their severe nature and small size, comparatively little is known about them. Tornados consist of elongated funnels of cloud that descend from the base of a well-developed cumulonimbus cloud, eventually making contact with the ground beneath. Figure 9.5 shows how tornados form.
Jet stream
Thermal updraft
Cold winds moving fast
After a tropical storm
• Assist in search and rescue. • Seek medical attention for persons injured. • Clean up debris and effect temporary repairs. • Report damage to utilities. • Watch out for secondary hazards: fire, flooding, etc.
Revised
Typical mistake
Some students think that hurricanes and tornadoes are the same – they are very different in origin, scale, impact and potential for management.
A tornado is a violent, destructive weather system, with powerful rotating winds (up to 300km/h). Tornadoes are intense, low-pressure systems, and their development depends on instability in the atmosphere, convergence and strong updrafts in the air.
Inflow warm, moist Gulf air Heavy rain
General direction of travel is northeast
Figure 9.5 Formation of tornadoes in the USA
Many tornadoes have a short life. They can last from several seconds to more than an hour. ‘Favoured’ areas are temperate continental interiors in spring and early summer, when insolation is strong and the air may be unstable, although many parts of the world can be affected by tornado outbreaks at some time or another. The Great Plains of the USA, including Oklahoma, Texas and Kansas, have a high
