Global Hazards

• Global pattern of air circulation

  • Atmospheric circulation is the large-scale movement of air by which heat is distributed on the surface of the Earth
• Cells of atmospheric circulation

  • Hadley cell
  • Ferrel cell
  • Polar cell
• Hadley cell


  Largest cell which extends from the Equator to between 30° to 40° north & south
• Ferrel cell


  Middle cell between 30/40° and 60° to 70° latitude. Air flows pole-wards
• Polar cell


  Smallest & weakest extends between the poles and the Ferrel cell
• High and Low Pressure belts

  • High Pressure
  • Low Pressure
• High Pressure


  Caused by cold air sinking. Causes clear and calm weather
• Low Pressure


  Caused by hot air rising. Causes stormy, cloudy weather
• Climate Zones


  • Temperate Climate
  • Tropical Climate
  • Polar Climate
  • Sub Tropical (Desert) Climate
• Temperate Climate


  Mid-latitude, 50° - 60° north & south of the Equator. Here air rises and cools to form clouds and therefore frequent rainfall. e.g. UK. Moderate temp and rainfall. [Ferrel Cell]
• Tropical Climate


  Found along the Equatorial belt, this zones experiences heavy rainfall and thunderstorms. E.g. Brazil. [Hadley cell]
• Polar Climate


  Within the polar zones cold air sinks causing dry, icy and strong winds. E.g. Antarctica.
• Sub Tropical (Desert) Climate

  30° north and south of the equator, sinking dry air leads to high temperatures without conditions for rainfall. E.g. Libya. [Hadley cell]
• Wind


  The movement of air from an area of high pressure to one of low pressure
• Types of wind


  • Katabatic Winds
  • Trade Winds
  • Jet Streams
• Katabatic Winds


  Winds that carry air from the high ground down a slope due to gravity. e.g. Antarctic.
• Trade Winds


  Wind that blow from high pressure belts to low pressure belts
• Jet Streams


  These are winds that are high in the atmosphere travelling at speeds of 225km/h
• Precipitation


  When water vapour is carried by warm air that rises. As it gets higher, the air cools and the water vapour condenses to form a cloud. As water molecule collide and become heavier, the water will fall to Earth as precipitation.
• Types of precipitation


  • Convectional Rainfall
  • Frontal Rainfall
  • Relief Rainfall
• Convectional Rainfall


  When the land warms up, it heats the air enough to expand and rise. As the air rises it cools and condenses. If this process continues then rain will fall.
• Frontal Rainfall


  When warm air meets cool air an front is formed. As the warm air rises over the cool air, clouds are produced. Eventually steady rain is produced.
• Relief Rainfall


  When wind meets mountains, the warm air is forced to rise quickly and cool. This leads to condensation and eventually rainfall. When the air descends however, very little rainfall falls, creating a rain shadow.
• Extremes in weather conditions

  • Wellington, New Zealand (windy)
  • Vostok, Antarctica (coldest)
  • The Atacama desert, Chile (driest)
  • Mawsynram, India (wettest)
• Tropical Storms


  They are known by many names, including hurricanes (North America), cyclones (India) and typhoons (Japan and East Asia). They all occur in a band that lies roughly between the Tropics of Cancer and Capricorn and despite varying wind speeds are ferocious storms. Some storms can form just outside of the tropics, but generally the distribution of these storms is controlled by the places where sea temperatures rise above 27°C.
• Formation of Tropical Storms

  1. The sun's rays heat large areas of ocean in the summer & autumn. This causes warm, moist air to rise over these particular spots.

  2. Once the temperature is 27⁰, the rising warm moist air results in low pressure. This cools, condenses & clouds form eventually it turns into a thunderstorm. This causes air to be sucked in from the trade winds.

  3. With trade winds blowing in the opposite direction and the rotation of earth (Coriolis effect), the thunderstorm will eventually start to spin.

  4. When the storm begins to spin faster than 74mph, a tropical storm (such as a hurricane) is officially born.

  5. With the tropical storm growing in power, more cool air sinks in the centre of the storm, creating calm, clear condition called the eye of the storm.

  6. When the tropical storm hit land, it loses its energy source (the warm ocean) and it begins to lose strength. Typical storms last 6-14 days
• El Nino effect


  Normally, warm ocean currents off the coast of Australia cause moist warm air to rise and condense causing storms and rain over Australia. In an El Niño year (every 5-7 years) the cycle reverses. Cooler water off the coast of Australia reverses the wind direction leading to dry, sinking air over Australia causing hot weather and a lack of rainfall. Central America gets wetter.
• Distribution of Droughts


  • Drought can occur anywhere throughout the world but are more frequent between the tropics of Cancer and Capricorn. Many countries in Africa suffer from severe drought, such as Ethiopia but Australia, China & Mexico also suffer.
• Changing pattern of Tropical Storms


  Scientist believe that global warming is having an impact on the frequency and strength of tropical storms. This may be due to an increase in ocean temperatures. Others think it is a cycle.
• Changing pattern of Droughts

  The severity of droughts has increased since the 1940s. Possibly due to changing rainfall & evaporation patterns related to gradual climate change.
• Structure of the Earth

  • The Crust (Continental & oceanic. Varies in thickness (7km beneath the ocean, 35 below land). Made up of several large plates.)
  • The Mantle (Widest layer (2900km thick). The heat and pressure means the rock is in a semi liquid state. Convection currents operate here.)
  • The Inner and outer Core (Hottest section (5000 degrees). Mostly made of iron and nickel; 4x denser than the crust. Inner section is solid whereas outer layer is liquid.)
• Convection Currents


  1. Radioactive decay of some of the elements in the core and mantle generate a lot of heat.

  2. When lower parts asthenosphere heat up they become less dense and slowly rise.

  3. As they move towards the top they cool down, become more dense and slowly sink.

  4. These circular movements of semi-molten rock are convection currents. They are partly responsible for plate movement
• The Crust


  Continental & oceanic. Varies in thickness (7km beneath the ocean, 35 below land). Made up of several large plates.
• The Mantle


  Widest layer (2900km thick). The heat and pressure means the rock is in a semi liquid state. Convection currents operate here.
• The Inner and outer Core

  Hottest section (5000 degrees). Mostly made of iron and nickel; 4x denser than the crust. Inner section is solid whereas outer layer is liquid.
• Convection Currents


  1. Radioactive decay generates heat

  2. Lower asthenosphere heats up, becomes less dense and rises

  3. As they move up they cool, become more dense and sink

  4. Circular movements of semi-molten rock
• Plate movement theory


  Partly based on convection currents
• Ridge push


  New crust rises as it is warm and thin, pushes older crust away from the ridge
• Slab pull


  Old plate sinks
• Destructive Plate Margin


  • Denser plate subducts beneath the other, friction causes it to melt and become molten magma
  • Magma forces its way up to the surface to form a volcano
  • Responsible for devastating earthquakes