Weather hazards

• Air Circulates between High and Low Pressure Belts as Surface Winds


  1. The sun warms the Earth at the equator, causing the air to rise

  2. This creates a low pressure belt

  3. As the air rises it cools and moves away from the equator

  4. 30° north and south of the equator, the cool air sinks, creating a high pressure belt

  5. At the ground surface, the cool air moves either back to the equator (as trade winds) or towards the poles (as westerlies)

  6. These winds curve because of the Earth's rotation- this is called the Coriolis effect
• Global atmospheric circulation is the transfer of heat from the equator to the poles by the movement of air
• Air moves due to differences in air pressure-winds blow from high pressure areas to low pressure areas
• The global atmospheric circulation system is divided into loops (called cells)


  1. Each cell has warm rising air that creates a low pressure belt

  2. And cool sinking air that creates a high pressure belt
• At the equator, the sun is directly overhead-this means the Earth's surface receives a lot of solar radiation, so it's hot. Warm, moist air rises and forms clouds, so it rains a lot
• By the time air reaches 30° north and south of the equator, it has released most of its moisture as rain. The dry air means there are few clouds and little rainfall, so deserts are often found at this latitude
• The low pressure zone at 60 north- Warm rising air brings lots of cloud cover and rainfall, often as low pressure systems carried from the Atlantic by westerly winds
• Tropical storms (a.k.a. hurricanes in the North Atlantic and Northeast Pacific, typhoons in the Northam cyclones in the South Pacific and Indian Ocean)

  Low pressure weather systems with intense rain and winds
• Tropical storms


  • Develop between 5 and 30° north and south of the equator
  • When sea temperature is 27 °C or higher
  • And wind shear (the difference in wind speed) between higher and lower parts of the atmosphere is low
• Tropical storms develop


  1. The warm surface water evaporates, rises and condenses into clouds. This releases huge amounts of energy producing powerful storms

  2. The rising air creates an area of low pressure, which increases surface winds

  3. Low wind shear prevents clouds breaking up as they rise, so the storm stays intact
• Easterly winds near the equator move tropical storms towards the west
• The storms spin because of the Coriolis effect
• As the storm moves over the ocean


  The energy from the warm water strengthens the storm, so wind speeds increase
• Storms lose strength when they move over land or cooler water, because the energy supply from the warm water is cut off
• The majority of storms occur in the northern hemisphere from August to October, while in the southern hemisphere most storms occur from December to April
• Tropical storms


  • Circular in shape, hundreds of kilometres wide and usually last 7-14 days
  • Spin anticlockwise in the northern hemisphere, and clockwise in the southern hemisphere
• Eye


  The centre of the storm, up to 50 km across, caused by descending air with very low pressure, light winds, no clouds, no rain and a high temperature
• Eyewall


  Surrounds the eye, with spiralling rising air, very strong winds (around 160 km per hour), storm clouds, torrential rain and a low temperature
• Towards the edges of the storm the wind speed falls, the clouds become smaller and more scattered, the rain becomes less intense and the temperature increases
• Hurricane Katrina struck Mississippi and Louisiana, USA, in August 2005
• Primary Effects of Hurricane Katrina

  • More than 1800 people were killed
  • 300 000 houses were destroyed
  • Large area were flooded, including 80% of New Orleans
  • 3 million people were left without electricity
  • Some bridges collapsed
  • Coastal habitats were damaged
• Secondary Effects of Hurricane Katrina


  • Hundreds of thousands of people were made homeless
  • 230,000 jobs were lost from damaged businesses
  • Water supply was polluted with sewage and chemicals
  • The total cost of the damage was estimated at $150 billion
• Immediate Responses to Hurricane Katrina

  • 70-80% of New Orleans residents were evacuated before the hurricane reached
  • Mississippi and Louisiana declared states of emergency-they set up control centres and emergency shelters, and stockpiled supplies
  • The military, police, firefighters and sea rescue teams rescued over 50,000 people
  • Charities collected donations and provided aid including millions of bottles of water
• Longer Term Responses to Hurricane Katrina

  • The US government provided over $16 billion for the rebuilding of homes, and provided funds to repair other critical infrastructure
  • The US Army recommended that buildings should not be rebuilt at a very low-lying area
  • Repaired and improved flood defences for New Orleans costing $14.5 billion were completed in 2013
• The response to Hurricane Katrina was highly criticised - the poorest were left behind (black African-Americans), and the evacuation centres could not cope with so many people, lacking food and water for 3-4 days in some places
• Climate change may make tropical storms more severe, so we need to find ways to manage their impact
• Effects of climate change on tropical storms

  • Frequency: Oceans will stay at 27 °C or higher for longer each year, so there's a longer period when tropical storms can form, which may mean more storms each year
  • Distribution: As the average ocean temperature rises, more of the world's oceans could be above 27C, which may mean that tropical storms can form in areas that haven't experienced them before, e.g. at higher latitudes
  • Intensity: Higher sea surface temperatures are likely to result in more evaporation and increased cloud formation, so more energy is released, which could mean storms become more powerful
• In the Atlantic, the number of tropical storms each decade seems to have increased, and 16 of the last 24 years have had a higher than average number of hurricanes
• In the Atlantic, the number of major hurricanes (with winds over 178 km/h) has increased since 1970, and 2014-2018 had almost twice as many major hurricanes as 1970-1974
• Higher sea surface temperatures are likely to result in more evaporation and increased cloud formation, so more energy is released
• This could mean storms become more powerful
• In the Atlantic, the number of major hurricanes (Category 3 or higher) has increased since 1970
• 2014-2018 had almost twice as many major hurricanes as 1970-1975
• Prediction and Monitoring

  1. Storms can be monitored using radar, satellites and aircraft

  2. Computer models can then be used to calculate a storm's predicted path
• Predicting where and when a tropical storm is going to happen


  Gives people time to evacuate and protect their homes and businesses
• Planning

  1. Future developments, e.g. new houses, can avoid high-risk areas, such as low-lying coastal zones

  2. Governments can plan evacuation routes to ensure people can get away quickly

  3. Emergency services can prepare for disasters by practising rescuing people from flooded areas
• Protection


  1. Buildings can be designed to withstand tropical storms

  2. Flood defences can be built along rivers (e.g. levees) and coasts (e.g. sea walls
• Weather hazards are quite common in the UK-and it's not just about rain, either...
• Weather hazards in the UK

  • Strong Winds
  • Heavy Rainfall
  • Snow and Ice
  • Drought
  • Thunderstorms
  • Heat Waves
• Strong winds (gales)


  Can damage properties and disrupt transport