Weather Hazards

• What are winds? 

  The wind is the movement of air on a large scale. The air is made up of gases 
• What causes winds? 

  Differences in air pressure cause winds 
• What causes differences in air pressure? 

  • Like with convection currents, different temperatures cause movement  
  • The equator is warmer than the North Pole or the South Pole 
  • This difference causes different air pressures 
• How does air pressure affect winds? 

  Winds move from high-pressure areas of the Earth to lower-pressure areas of the Earth  
• What are global atmospheric circulation cells? 

  • Winds combine to help create global atmospheric circulation cells  
  • Global atmospheric circulation cells are made up of: 
  • Cool sinking air – forms high-pressure belts of wind 
  • Warm rising air – forms low-pressure belts of wind  
  • The Northern hemisphere has 3 global atmospheric circulation cells, and the Southern hemisphere has 3 global atmospheric circulation cells 
  • These cells are called the Hadley, Ferrel and Polar cells  
• Surface winds

  At 30° north and south, the air from the equator cools and falls. When the cool air reaches the Earth’s surface, surface winds blow the cool air either towards the equator, or away from the equator towards the Poles 
• Trade winds 

  • Trade winds are surface winds that blow from 30° north or 30° south back towards the equator  
  • In the southern hemisphere trade winds will blow from south-east to north-west until they reach the equator  
  • In the northern hemisphere, trade winds blow from north-east to south-west until they reach the equator  
  • Trade winds from the northern & southern hemisphere meet at the equator, where they are heated, and the cycle begins again  
• Westerlies

  • Westerlies are surface winds that blow from 30° north towards the North Pole or from 30° south out towards the South Pole 
  • In the southern hemisphere, westerlies blow from north-west to south-east until they reach the South pole 
  • In the northern hemisphere, westerlies blow from south-west to north-east until they reach the North Pole 
• What is the global atmospheric circulation model?

  The global atmospheric circulation model describes how air circulates between low and high-pressure belts as a result of differing conditions at different latitudes. This involves the transfer of heat via circulation cells  
• What happens at the equator? 

  • The Earth’s surface is warmed by the sun at the equator. The Earth transmits this heat to the nearby air. This air gets hotter and rises. The rising, hot air forms a low-pressure belt. As the hot air rises, the air will begin to cool, with water vapour condensing creating rain and clouds  
  • The rising air cools and moves away from the equator towards 30° north and 30° south
• What happens at 30° north and south? 

  • At 30° north and south, the air is further from the equator and the cool air falls. This results in a high-pressure belt with minimal rainfall and no clouds.  
  • When the cool air reaches the Earth’s surface, surface winds blow the cool air either towards the equator, or away from the equator towards the Poles 
• What happens at 60° north and south? 

  • At 60° north and south, cold air blown from the poles meets warm air surface winds. Because the warmer air is less dense, it rises and forms a low-pressure belt 
  • The air splits, with some returning back towards the equator and the rest heading to the poles  
• What happens at the Poles? 

  • At the North Pole and the South Pole, cool air will sink, forming a high-pressure belt  
  • This high-pressure belt will move back towards the Earth’s equator as a surface wind  
• Polar  

  Temperatures are low throughout the year in polar regions 
• Temperate

  • Temperate regions experience moderate summers and winters  
  • Rainfall is frequent because of a low pressure belt caused by rising air from 2 cells meeting at about 60° north and south of the equator  
• Arid

  • Temperatures are high and rainfall is low for most of the year in arid (dry) regions  
  • Rainfall is low because of a high pressure belt caused by sinking air from 2 cells meeting at about 30° north and south of the equator  
• Tropical

  • Temperatures and rainfall are high throughout the year in tropical regions  
  • Rainfall is frequent because of a low pressure belt caused by rising air from 2 cells meeting near the equator  
• What does the global atmospheric circulation system cause?

  The global atmospheric circulation system causes extreme winds, precipitation and temperature in some parts of the world  
• Temperature

  • Temperatures can be extremely cold or extremely hot  
  • The hottest temperature on Earth was recorded in California in 1913 at 56.7°C (the accuracy of this reading is not confirmed) 
  • The Sun’s energy is strongest at the equator. This energy causes warm air to move out towards the poles. This movement is the driver of the global atmospheric circulation system 
  • In high pressure areas at about 30° north and south, temperatures can be very high. These areas are close to the equator and there are not many clouds in these areas  
• Winds

  • Winds that are extreme are usually fast-moving winds instead of slow-moving winds or a lack of wind  
  • Winds are weak in pressure belts but strong in between belts  
  • Extreme winds happen when there is a big difference in pressure between a low-pressure area and a high-pressure area  
• Precipitation

  • Extreme precipitation can be a lot of rain (causing flooding) or a severe lack of rain (causing drought) 
  • Precipitation happens when warm, moist air rises from the Earth’s surface and then cools. When it cools, water vapour condenses  
  • Low pressure belts cause air to rise. Heavy rainfall (or snowfall) is more common in low pressure belts  
  • High pressure belts cause air to sink. Precipitation is less common in high pressure belts  
  • Pressure belts can move small distances over time, but they are usually in roughly the same place  
• Weather in Australia

  • El Niño leads to less rainfall, less flooding, and more drought in the West of Australia 
  • Oodnadatta is the hottest city in Australia, temperatures are often 47°C. Darwin is another hot city with average temperatures of 31°C-33°C. Darwin is in the north of Australia which is closest to the equator  
  • Australia's high temperatures, low precipitation and high winds mean that it can be vulnerable to bushfires  
  • Unlike the UK, Australia has very strong winds and has tropical cyclones. Cyclone Yasi hit Australia in 2011. Winds can reach 400km/h  
• Weather in the UK

  • The south of the UK is hotter than the north of the UK because it is closer to the equator  
  • July, the average temperature in London is 19°C  
  • In the UK, flooding caused by lots of rainfall is more likely than a drought  
  • The UK’s average annual rainfall is 1,150mm 
  • The UK does not get extremely hot winds – usually winds of 80km/h are common (a lot weaker than tropical cyclones) 
• Extreme weather conditions of tropical storms

  The winds speed of a tropical storm can exceed 250km/h. This can destroy buildings and plants and destroy debris through the air. The amounts of rain can lead to flooding and mudslides  
• Form over warm water

  • Tropical storms form over water that is 26.5°C or warmer  
  • Lots of energy is released when air evaporates, rises or cools. This increases the speed and energy of a storm 
  • The strength of a storm falls as they move further away from warm water  
• Surface winds

  • Near the equator, surface winds are mainly trade winds moving east 
  • The trade winds and something called the Coriolis Effect mean that tropical storms move from east to west  
  • The rising warm air above warm oceans creates low pressure, which causes surface winds to strengthen  
• Spin

  The rotation of the Earth causes the Coriolis effect, which makes tropical storms spin by deflecting winds  
• Distribution

  • Climate change is expected to lead to increases in temperatures 
  • Currently, most tropical storms happen close to the equator, where the sea temperature is above 26.5°C  
  • More of Earth’s seas will be warmer than 26.5°C, so there will be more storms and those that happen will have more energy  
• Frequency

  • Oceans will be warmer than 26.5°C, so the frequency of storms will rise, and storm/hurricane/cyclone seasons will get longer  
  • The number of storms varies a lot, but warmer seas will lead to more storms  
• Intensity

  • The higher the temperatures of the oceans, the more energy tropical storms will accumulate, the greater the intensity of the storm  
  • More intense storms will cause more damage to buildings, infrastructure and people  
• Starting point  

  • Usually in the Pacific, there is an area of low pressure over the western Pacific Ocean and an area of high pressure over the eastern Pacific Ocean  
  • These conditions result in trade winds blowing west over the Pacific from South America to Australia  
  • Around south and central America, as the warmer water gets pushed away from the coast, it is replaced by cold water which is pulled up from deeper down in the ocean, this is a process called upwelling. This creates a temperature difference  
• What is La Niña? 

  • La Niña events happen when the air and water currents strengthen 
  • Air and water currents still blow west, but they both blow west more powerfully  
  • La Niña leads to more rainfall and flooding in the West (in Australia) 
  • La Niña leads to less rainfall and less flooding and more drought in the East (Latin America) 
  • Happens every 2-5 years  
• What is El Niño? 

  • El Niño events happen when these currents blowing westwards get weaker or change direction and go eastwards  
  • If pressure in the western Pacific increases and pressure in the eastern Pacific decreases, then air and water currents can change from westerlies to easterlies  
  • El Niño leads to less rainfall, less flooding and more drought in the West (Australia) 
  • El Niño leads to more rainfall and flooding in the East (Latin America) 
• What is a drought?

  Droughts are periods of time with below-average amounts of rainfall. Water supplies run low or run out during droughts. Drought is caused by a lack of rainfall.
• El Niño and La Niña  

  • El Niño and La Niña are examples of changes to atmospheric circulation that can cause areas to receive little rain from extended periods of time  
  • El Niño causes Australia to receive a lot less rainfall than it normally would  
• Blocked depressions

  • Depressions cause rain to happen  
  • Areas of high pressure can block depressions, stopping rain from reaching an area  
  • This is usually the cause of any drought in the UK 
• Examples of drought

  • Between 1931 and 1939, there was an 8-year drought in America’s ‘Dust Bowl’  
  • Syria experienced a drought between 2006 and 2010 (some think this helped trigger the civil war) 
• Where do droughts happen?

  • Ethiopia, Sudan, and Eritrea are the 3 most drought-prone countries  
  • Droughts usually affects Central Africa, Australia, the Middle East and western parts of North and South America 
  • Drought is often linked to water deficit, as countries do not receive enough rainfall. Most countries with water deficits are found in Africa and the Middle East. Qatar, Israel, Lebanon Libya and Mexico suffer from water deficits