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Case Study 7: Hurricane Katrina 'A case study of one extreme weather event: the causes, effects on different groups of people, and responses to the hazard.' Causes: How hurricanes form Hurricanes need a lot of heat to form and a sea surface temperature of at least 26°C, which is why they usually occur over tropical seas. They also need to be between 5 and 20° north or south of the equator. It works like this: 1. When this warm and wet air rises, it condenses to form towering clouds, heavy rainfall. It also creates a low pressure zone near the surface of the water. 2. Rising warm air causes the pressure to decrease at higher altitudes. Warm air is under a higher pressure than cold air, so moves towards the ‘space’ occupied by the colder, lower pressure, air. So the low pressure ‘sucks in’ air from the warm surroundings, which then also rises. A continuous upflow of warm and wet air continues to create clouds and rain. 3. Air that surrounds the low pressure zone at the centre flows in a spiral at very high speeds - anti-clockwise in the northern hemisphere - at speeds of around 120 km/h (75 mph). 4. Air is ejected at the top of the storm – which can be 15km high – and falls to the outside of the storm, out and over the top, away from the eye of the storm. As this happens, it reduces the mass of air over the ‘eye of the storm’ - causing the wind speed to increase further. Some ejected air also cools and dries, and sinks through the eye of the storm, adding to the low pressure at the centre. 5. The faster the winds blow, the lower the air pressure in the centre, and so the cycle continues. The hurricane grows stronger and stronger. 6. Seen from above, hurricanes are huge circular bodies of thick cloud around 450 km (300 miles) wide. The cloud brings heavy rain, thunder and lightning. 7. In the centre is the eye of the hurricane, about 45 km across (30 miles) across. Often there will be no clouds in the eye. Seen from below it will seem calmer, with a circle of blue sky above. The eye is formed because this is the only part of the hurricane where cold air is descending. 8. In the northern hemisphere, the prevailing easterly tropical winds tend to steer hurricanes toward land - although their course is unpredictable. As hurricanes move inshore, their power gradually reduces because their energy comes from sucking up moist sea air.
Transcript of case_study_7 (1)
Case Study 7: Hurricane Katrina
'A case study of one extreme weather event: the causes, effects on different groups of people, and responses to the hazard.'
Causes:
How hurricanes formHurricanes need a lot of heat to form and a sea surface temperature of at least 26°C, which is why they usually occur over tropical seas. They also need to be between 5 and 20° north or south of the equator. It works like this:1. When this warm and wet air rises, it condenses to form towering clouds,
heavy rainfall. It also creates a low pressure zone near the surface of the water.
2. Rising warm air causes the pressure to decrease at higher altitudes. Warm air is under a higher pressure than cold air, so moves towards the ‘space’ occupied by the colder, lower pressure, air. So the low pressure ‘sucks in’ air from the warm surroundings, which then also rises. A continuous upflow of warm and wet air continues to create clouds and rain.
3. Air that surrounds the low pressure zone at the centre flows in a spiral at very high speeds - anti-clockwise in the northern hemisphere - at speeds of around 120 km/h (75 mph).
4. Air is ejected at the top of the storm – which can be 15km high – and falls to the outside of the storm, out and over the top, away from the eye of the storm. As this happens, it reduces the mass of air over the ‘eye of the storm’ - causing the wind speed to increase further. Some ejected air also cools and dries, and sinks through the eye of the storm, adding to the low pressure at the centre.
5. The faster the winds blow, the lower the air pressure in the centre, and so the cycle continues. The hurricane grows stronger and stronger.
6. Seen from above, hurricanes are huge circular bodies of thick cloud around 450 km (300 miles) wide. The cloud brings heavy rain, thunder and lightning.
7. In the centre is the eye of the hurricane, about 45 km across (30 miles) across. Often there will be no clouds in the eye. Seen from below it will seem calmer, with a circle of blue sky above. The eye is formed because this is the only part of the hurricane where cold air is descending.
8. In the northern hemisphere, the prevailing easterly tropical winds tend to steer hurricanes toward land - although their course is unpredictable. As hurricanes move inshore, their power gradually reduces because their energy comes from sucking up moist sea air.
Tropical storms
Otherwise known as hurricanes, cyclones, typhoons, and wily willies (the Australian name!). These are very intense areas of very low pressure, approximately 500 to 1000 kilometres in diameter.
Tropical storms form over water that is above 26.5°C in temperature. The warm water heats the air above it, causing it to rise rapidly. Cooler air from elsewhere rapidly moves in to replace the rising air and the process starts again.The rapidly rising moist air, cools as it ascends and forms tall cumulo-nimbusclouds. These slowly spin around the central point. Jet streams above thestorm clouds continue to suck the air up.Cooling air sinks down through the spiralling mass of clouds to produce acentral eye of the storm.Tropical storms move quickly over water, as long as it is warms enough to keep the storm energised. Once over land, where there is less moisture and the surface is cooler, tropical storms quickly calm down.Impacts
Katrina was a category 4 storm. Storm surges reached over 6 metres in height. New Orleans was one of the worst affected areas because it lies below sea level
and is protected by levees. These protect it from the Mississippi River and Lake Ponchartrain. The levee defences were unable to cope with the strength of Katrina, and water flooded into the city.
Despite an evacuation order, many of the poorest people remained in the city. People sought refuge in the Superdome stadium. Conditions were unhygienic, and
there was a shortage of food and water. Looting was commonplace throughout the city. Tension was high and many felt vulnerable and unsafe.
1 million people were made homeless and about 1,200 people drowned in the floods.
Oil facilities were damaged and as a result petrol prices rose in the UK and USA.
There was much criticism of the authorities for their handling of the disaster. Although many people were evacuated, it was a slow process and the poorest and most vulnerable were left behind.$50 billion in aid was given by the government.The UK government sent food aid during the early stages of the recovery process.The National Guard was mobilised to restore and maintain law and order in what became a hostile and unsafe living environment.
Source: BBC & S-cool geography
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