9 Hazardous environments
Now test yourself
Tested
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?
Answers on p.219
9.3 Hazards resulting from atmospheric disturbances 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 800 km, although the very strong winds that cause most of the damage are found in a narrower belt up to 300 km 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 118 km/h. For tropical storms to form a number of conditions are needed: l Sea temperatures must be over 27°C to a depth of 60 m (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).
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 600 km in diameter with wind speeds of up to 300 km/h (typically 160 km/h) and bringing up to 30–50 cm of rainfall.
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
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Category
Description
Category 1: Winds 119–153 km/h; storm surge generally 1.2–1.5 m above normal
No real damage to building structures. Damage primarily to unanchored mobile homes. Also, some coastal road flooding and minor pier damage.
Category 3: Winds 178–209 km/h; storm surge generally 2.7–3.6 m above normal
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.
Category 5: Winds greater than 249 km/h; storm surge generally greater than 5.5 m above normal
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 8–16 km of the shoreline may be required.
Cambridge International AS and A Level Geography Revision Guide
