World At Risk - Hazards

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Natural Hazards are natural events or processes which affect people, e.g. causing loss of life or injury, economic damage, disruption to peoples lives or environmental degredation.

Natural hazards can be classified into two groups: hydrometeorological and geophysical.

  • Hydrometeorological hazards are those caused by water processes and/or weather patterns. They include: floods, tropical cyclones, wind storms (tornadoes), blizzards, bush fires. and drought.
  • Geophysical hazards are those caused by earth processes, there are two types: internal earth processes of tectonic origin e.g. earthquakes, tsunami and volcanic eruptions. External earth processes of geomorphological origin involving mass movements e.g. landslides and avalanche.

Dregg's Model

Dregg's model is a way of defining disasters. There needs to be an overlap of the hazard, and vulnerability before a hazard can become a disaster. A vulnerable population are susceptible to human and/or economic loss because of where they live. 


When does a natural hazard become a disaster?

  • In 1990, Swiss Re defined a diaster as an event in which at least 20 people died, or insured damage of over US$16 Million value was caused.
  • The EM-DAT international database suggests that a hazard becomes a disaster when at least 10 people are killed, more than 100 people are affected, a state of emergency is declared and international assistance is required.

Hazard researchers measure hazard vulnerability using a disaster risk formula: (


Hurricanes can also be called tropical cyclones and typhoons. They are a hydrometeorological and are inceasing in numbers because of the warming of the earth due to global warming. For a hurricane to form, sea temperatures must be at least 26 degrees. When the seas reach this temperature, warm, moist air rises and condenses which releases energy and increases windspeed, creating cumulus clouds.As the warm aire rises is creates a funnel of low pressure which begins to spiral anti-clockwise as it pushes the cooler air above it to one side causing clouds to spin. They can only take place between 5-30 degrees off the equator. This is because this is where the sea is the hottest, providing enough energy to form a hurricane. They don't form between 0-5 degrees of the equator because here, the coriolus effect isn't strong enough (the force that deflects the path of winds due to the earths rotation) to make the winds spin. Hurricanes always move westwards due to east-west winds. When hurricanes reach land, they start to die because they don't have enough energy from the sea anymore.

Hurricane Katrina - August 2005, New Orleans, Louisiana. 

Hurricane Katrina was the 3rd strongest recorded hurricane to make landfall over the USA. It formed in the south-eastern Bahamas, where it hit land in Florida, then gained energy again as it travelled through the Gulf of Mexico, where it then struck New Orleans. 

Effect on Infastructure - 

  • The communication infastructure failed, many telephones and mobiles did not work, internet access and local tv stations were disrupted. So nobody knew if their family members were alright and didn't know what was happening.
  • Most major roads into…



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