Weather

• Global atmospheric circulation


  The interconnected circular air movements called cells that affect global weather
• Effects of global atmospheric circulation

  • In the UK
  • In deserts
  • At the Equator
• Atmosphere


  The air above our heads on which we depend for life
• Atmospheric circulation


  1. Sinking cold air creates high pressure

  2. Rising warm air creates low pressure

  3. Surface winds move from high to low pressure

  4. Winds curve due to Earth's rotation

  5. Winds change seasonally due to tilt and rotation of Earth
• The atmosphere becomes thinner with increasing altitude
• Cells of atmospheric circulation

  • Polar cell
  • Sub-polar low
  • Sub-tropical high
  • Equatorial low
  • Hadley cell
• Global atmospheric circulation


  Drives the world's weather
• Cloudy and wet in the UK because 50-40°N is close to where cold polar air meets warm subtropical air, bringing warm and wet weather
• Hot and dry in the desert because most deserts are found at about 30°N and S where sinking air means high pressure, little rain, hot days and cold nights
• Hot and sweaty at the Equator because low pressure marks where the sun is directly overhead, causing hot, humid air to rise, cool and condense, resulting in heavy rain
• Tropical storms are huge storms called hurricanes, cyclones and typhoons in different parts of the world
• Most tropical storms form 5-15 north and south of the Equator, in summer and autumn, where ocean temperatures are highest (above 27°C) and the Coriolis effect is very high
• Unstable rising air in the equatorial regions

  Encourages air to rise, condense and form storm clouds
• Winds blow from high-pressure belts (sub-tropical) to low (equatorial)


  These trade winds are responsible for the predominantly east-west storm paths
• Formation of tropical storms

  1. Rising air draws huge quantities of evaporated water from the ocean surface

  2. Condensation releases heat which powers the storm

  3. Individual thunderstorms join to form a rotating storm

  4. Coriolis forces spin the storm at over 120km/h

  5. Prevailing winds drift the storm over the ocean surface
• Structure of a tropical storm

  • Eye - calm, clear sky
  • Eye wall - most severe conditions with very strong winds
  • Cumulonimbus clouds surrounding the eye
• Tropical storms are associated with extremely strong winds, often gusting over 200km/h, and very high rainfall totals that can exceed 400mm in a day, causing flooding and landslides
• Low-lying coastal areas can be inundated by storm surges up to 3m high, driven onshore by the strong winds
• In a warmer world

  Tropical storms may be affected by climate change
• Over recent decades, sea surface temperatures have increased by 0.5-0.8°C
• In 2020, scientists concluded that climate change has changed the global distribution (frequency of occurrence) of tropical cyclones over the last 40 years
• The IPCC cautiously concludes that the frequency of tropical storms will decrease in future but there is expected to be an increase in the number of very intense storms
• There is increasing evidence that the intensity of tropical storms has increased in recent years and that this is a result of climate change
• Cyclone Idai is Africa's deadliest tropical cyclone on record
• Cyclone Idai made landfall in Mozambique in March 2019 as a Category 2 storm
• Strong winds, torrential rainfall and powerful storm surges brought widespread destruction in Mozambique, Zimbabwe and Malawi
• Primary effects of Cyclone Idai

  • 1300 killed, mostly drowned
  • Over 3 million people affected, many homeless
  • 90% of Beira destroyed by strong winds and storm surge
  • 600 houses destroyed in Zimbabwe
  • Agricultural fields and houses flooded in Malawi, leaving people without food and shelter
• Immediate responses to Cyclone Idai

  • Boats and helicopters used to rescue stranded people and drop food
  • International crisis agencies supplied emergency provisions
  • The UK government provided food, water and shelter kits
  • The WHO provided 500,000 cholera vaccinations and anti-mosquito nets
  • 140 evacuation centres set up in Mozambique
• Secondary effects of Cyclone Idai

  • 3000 km² flooded by overflowing rivers in Mozambique and Zimbabwe, 700,000 ha of crops destroyed, and roads and bridges washed away
  • Mudslides hampered rescue efforts in Zimbabwe
  • Dams collapsed and two HEP plants damaged in Malawi
  • Lack of sanitation and clean water caused 4000 cases of cholera in Beira
• Long-term responses to Cyclone Idai

  • Dutch engineers helped restore water supplies in Beira
  • CAFOD rebuilt schools and supported local farmers
  • 90,000 survivors were relocated to 66 safer settlements in Mozambique
  • Early warning system improved
• The UK government provided food, water and shelter kits
• The WHO provided 500,000 cholera vaccinations and anti-mosquito nets
• 140 evacuation centres set up in Mozambique
• 3,000 km² flooded by overflowing rivers in Mozambique and Zimbabwe - 700,000 ha of crops destroyed, and roads and bridges washed away
• Mudslides hampered rescue efforts in Zimbabwe
• Dams collapsed and two HEP plants damaged in Malawi
• Lack of sanitation and clean water caused 4,000 cases of cholera in Beira
• Dutch engineers helped restore water supplies in Beira
• CAFOD rebuilt schools and supported local farmers
• 90,000 survivors were relocated to 66 safer settlements in Mozambique