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Hazardous Weather

Global Circulation System

  • Winds are large scale movements of air caused by differences in air pressure
  • Differences in air pressure are caused by temperature differences between the equator and the poles
    • Winds move from the areas of high pressure to the areas of low pressure
  • Winds are part of global atmospheric circulation loops called cells
    • These loops have warm rising air which creates a low-pressure belt, and cool falling air which creates a high-pressure belt
  • In each hemisphere, there are three cells, the Polar cell at 90-60°, the Ferrel cell at 60-30° and the Hadley cell at 30-0°
  • The global circulation system is the movement of winds
    • At the equator the sun warms the earth, which transfers heat to the air above causing it to rise
      • This creates a low-pressure belt with rising air, clouds and rain
    • As the air rises, it cools and moves out at 30° north and south
    • At 30° north and south of the equator, the cool air sinks
      • This creates a high-pressure belt with cloudless skies and very low rainfall
    • The cool air reaches the ground surface and moves as surface winds either back to the equator or towards the poles
      • Surface winds blowing towards the equator are called trade winds
        • At the equator, these trade winds meet and are heated by the sun, causing them to rise and form clouds
      • Surface winds blowing towards the poles are called westerlies
    • At 60° north and south of the equator, the warmer surface winds meet colder air from the poles
      • The warmer air is less dense than the colder air, so it rises, creating a low-pressure belt
    • Some of the air moves back towards the equator, and the rest moves towards the poles
    • At the poles, the cool air sinks, creating a high-pressure belt
      • The high-pressure air is drawn back towards the equator as surface winds
  • The three cells shift north and south of the Equator during the year as the position of Earth changes in relation to the Sun
    • This results in seasonal weather and is why the band of rain caused by low pressure moves
  • The pressure belts caused by global atmospheric circulation control temperatures, and influences precipitation and the prevailing winds, creating distinctive climate zones
    • Polar climates have low temperatures all year round
    • Temperate climates have moderate summers and winters
      • Usually they are at about 60° north and south, where rising air from the Polar cell and Ferrel cell means rainfall is frequent
    • Tropical climates have high temperatures all year round and high rainfall
      • Usually they are near the equator, where rising air from the Hadley cells meeting causes low pressure and lots of rainfall
    • Arid climates have very low rainfall most of the year and temperatures are warm
      • Usually they are 30° north and south, where sinking air from the Ferrel cell and Hadley cell meeting causes high pressure and prevents rainfall

Extreme Weather

  • The global circulation of the atmosphere causes extremes in weather conditions in different parts of the earth
    • The Lut Desert in Iran had the hottest recorded temperature on Earth of 70.7°C because there is intense sunshine and the surface absorbs heat
    • Antarctica is the coldest place on Earth with surface temperatures of up to -89.2°C because the Sun’s rays are weak, and the ice reflects heat
    • Mount Washington in the USA is one of the windiest places on Earth, with winds up to 372km/h because it forms a barrier to the westerly trade winds
    • Antarctica is also one of the windiest places on Earth, with winds over 320km/h because of the large ice sheets sloping gently into the sea, causing katabatic winds
    • The Khasi Hills in India had the most recorded rainfall in one month of 9300mm because it lies on the Tropic of Cancer, and clouds are blown in from the Bay of Bengal and over the flat lands of Bangladesh, they then rise very rapidly resulting in relief rainfall
    • The Atacama in Chile is the driest hot desert on Earth with an average annual rainfall of about 15mm because it lies on the Tropic of Capricorn where it is the rain shadow of the Andes and has a cold ocean current flowing alongside it
    • Antarctica is the driest place on Earth because the air is so cold that it holds very little water vapour
  • Wind is air moving from areas of high to low pressure
    • This means that atmospheric circulation causes winds, making some parts of the world windier than others
    • Winds are weak in high- and low-pressure belts
    • Winds are strong between pressure belts
    • When the difference in pressure between areas is large, winds can be extremely strong
  • Wind speed and direction in a location can be shown using a rose chart
    • Anemometers are used to measure wind speed by counting the number of times they rotate in a minute
    • Wind speed is measured using the Beaufort scale, which ranges from 0 (calm) to 12 (hurricane)
  • There are different types of wind
    • Trade winds blow from high pressure belts to low pressure belts
    • Katabatic winds are caused by air flowing downhill
    • Jet streams blow high in the atmosphere and are very strong
  • Temperature
    • The equator receives the most energy from the sun and the poles receive the least
    • Heat drives atmospheric circulation as warm air from the equator moves towards the poles
    • Temperatures can be very high in high pressure areas, around 30° north and south
      • There are few clouds due to the sinking air, so there is little to block the Sun’s energy
    • In contrast, the temperatures in the polar regions of the Arctic and Antarctic are very low
  • There are other factors affecting temperature
    • Albedo effect is the amount a surface reflects the Sun’s rays back into space
    • Cloud cover affects the reflection of the Sun’s rays
    • Surface winds move heat around the world
    • Ocean currents move heat easily
    • On land, insolation is concentrated on the surface and at sea, it reaches deeper because water is transparent
    • The different altitudes mean that higher up, air pressure is lower, which makes temperatures colder
  • Precipitation is rain or snow etc. and it occurs when warm, wet air rises and cools causing water vapour to condense
    • Air rises in low pressure belts
      • This means that precipitation is frequent and often intense in these areas
      • Rainforests are usually in low pressure belts
    • Air sinks in high pressure belts
      • This means that precipitation is extremely low
      • Deserts are usually near high pressure belts
    • The exact location of high- and low-pressure belts varies slightly over time
  • Precipitation differs around the world
    • On a global scale, rain falls heavily in a band around the equator, but the tropics are much drier
    • On a regional scale, the coasts of continents can be particularly wet
    • On a local scale, precipitation is influenced by altitude
  • Rainfall comes in three different types
    • Convectional rainfall occurs when thick clouds form because of warm air rising quickly
      • This is often heavy rain at the Equator because of high temperatures
    • Frontal rainfall occurs when warm air rises over cool air
      • This is common in the UK because it is situated where warm air from the tropics meets cool air from the North Pole
    • Relief rainfall occurs when clouds rise over mountains
      • In the UK some mountains are wetter on their western sides and drier on the east

Extreme Weather in Contrasting Countries

  • Australia and the UK are contrasting countries with different extremes in weather conditions associated with wind, temperature and precipitation
  • Australia has stronger extreme winds than the UK
    • Australia is affected by tropical cyclones which can cause very strong winds of over 118km/h
    • In the UK, gales are rare; most places only have a few days of gales a year
    • The strongest wind recorded in Australia is 407km/h, recorded on Barrow Island during tropical cyclone Olivia in 1996
    • The strongest sea-level wind recorded in the UK is 229km/h, recorded in Fraserburgh, Scotland in 1989
  • Australia is warmer than the UK – it has hotter summers and milder winters
    • In Darwin, the average maximum summer temperature is 33°C
      • Temperatures over 40°C are extremely hot
    • In London, the average maximum summer temperature is 23°C
      • Temperatures over 30°C are extremely hot
    • Australian summers are approximately 10°C warmer than UK summers and
      • Extreme temperatures in summer are approximately 7°C warmer than the average temperature for both countries
    • The highest extreme was 51°C in Australia and 38.5°C in the UK
    • The lowest extreme was -23°C in Australia and -27.2°C in the UK
  • Australia has much lower precipitation than the UK
    • The average annual rainfall in Australia is 465mm
      • An extremely wet year is over 550mm
      • An extremely dry year is less than 360mm
    • The average annual rainfall in the UK is 1154mm
      • An extremely wet year is over 1210mm
      • An extremely dry year is less than 950mm
    • The annual rainfall in the driest year ever was 314mm in Australia and 835mm in the UK
    • The annual rainfall in the wettest year ever was 760mm in Australia and 1337mm in the UK

Tropical Storms

  • Tropical storms are intense low-pressure weather systems with heavy rains and strong winds
    • Depending on where they occur, they are known as hurricanes, cyclones and typhoons
  • The number of tropical storms varies each year
    • In the Atlantic, the number of tropical storms has increased since 1984 but there is no overall trend over the last 130 years
    • Many scientists believe that global warming increases the temperature of the oceans, making tropical storms more likely
  • They develop when the sea temperature is 26.5°C or higher and it is at least 60m deep between 5-15° north and south of the equator, where the trade winds meet
    • The warm ocean temperature means there is lots of warm, moist air to cause extreme precipitation
    • They need to be at least 5° from the equator so that the Coriolis effect starts the storm spinning
      • The Coriolis effect is the way the spinning of earth makes winds move to the right in the northern hemisphere and to the left in the southern hemisphere
    • They only form if winds blow at a constant speed at ground level which allows the storm clouds to rise in a column
  • The storm gets stronger due to energy from the warm water, so wind speed increases
  • Extreme weather conditions are associated with tropical storms
    • Strong winds in tropical storms are caused by an area of very low pressure at the centre of the storm that creates a big pressure difference to the surrounding area
      • Tropical storms can have wind speeds of more than 120m/h
      • These winds are strong enough to damage and destroy buildings, plants, communication networks, transport networks and to move loose objects
    • Large amounts of warm moist air are sucked towards the centre of the storm due to the difference in pressure, and as this happens the air rises, cools and condenses, causing rain
      • Extremely high amounts of precipitation – up to 250mm in a day – can fall rapidly in tropical storms
      • Heavy rain can cause flooding, landslides and mudslides
    • Low air pressure means high tides can flood inland and causes storm surges

Drought

  • A drought is a long period when precipitation is less than the average for the area, leading to water shortages
  • In some places, droughts can lead to food shortages, hunger and death
  • Changes in atmospheric circulation, such as El Niño or La Niña, can mean that it doesn’t rain much in an area for months or years
    • Changes in atmospheric circulation can also make the annual rains, such as monsoon rains, fail
  • Droughts are also caused when high pressure weather systems block weather systems that cause rain
    • High pressure means fewer clouds and less precipitation; where there are also high temperatures, more water evaporates from plants, lakes and streams; so, more water evaporates than falls as rain
  • Areas most at risk from drought are central and southern Africa, the Middle East, Australia, eastern South America and parts of North America
    • They are most severe in northern and southern Africa, western South America, central Australia and parts of North America and Asia
    • The locations affected by drought vary overtime; since 1950, there have been more droughts in Africa, Asia and the Mediterranean and fewer droughts in the Americas and Russia
  • Globally, the frequency of droughts has varied from year to year but overall has not changed much since 1950
    • Some scientists suggest that droughts will become more frequent and more severe in future due to climate change

El Niño and La Niña

  • Air currents in the atmosphere and water currents in the ocean usually flow one way in the Pacific Ocean
    • Every few years, they weaken or reverse – this is an El Niño event
    • Sometimes, they become stronger – this is a La Niña event
    • Both cause weather pattern changes in surrounding areas
  • Normally, there’s low pressure over the western Pacific Ocean and high pressure over the east, with warm waters in the west and colder waters upwelling in the east
  • La Niña is when the normal conditions become more extreme
    • Trade winds blow to the west more strongly and more cold water rises in the Eastern Pacific
      • There is warm surface water being pushed to the west and cold water upwelling in the east due to strong ocean currents
    • It causes heavy rainfall and floods, due to rising warm air and low pressure in the west (Australia), and less rainfall and droughts, due to falling cool air and high pressure in the east (South America)
    • La Niña events occur every 2-7 years
  • El Niño is when the pressure rises in the western Pacific and falls in the east
    • This causes the trade winds, which normally blow from east to west, to weaken or change direction
      • This causes the warm water in the western Pacific to spread out so cold water in the east stops upwelling due to weak or reversed ocean currents
    • The sinking air in the high-pressure area over the western Pacific leads to unusually dry weather
      • This can cause drought due to less rainfall in areas like eastern Australia
    • The rising air in the low-pressure area over the eastern Pacific lead to unusually wet weather
      • This can cause serious floods in places that don’t normally get much rain, such as Peru and California
    • El Niño events occur every 3-4 years on average and last for 9-12 months
  • Scientists have found a strong connection between El Niño events and droughts around the world
    • The effects are felt most strongly on the Equator and in tropical areas where temperatures are higher
  • The influence of an El Niño or La Niña event goes beyond the Pacific Ocean
    • It causes changes to rainfall patterns around the world
    • It influences the frequency and distribution of tropical storms in the Atlantic as well as the Pacific
    • Asia is often hit hard during El Niño years with droughts causing food and water shortages, bush fires, poor air quality and power cuts in countries which rely heavily on hydro-electric power

Case Study: Tropical Storm, Hurricane Katrina

  • Hurricane Katrina, a tropical storm, struck the south-east USA on 29 August 2005
    • Louisiana and Mississippi are in the Gulf of Mexico where sea temperatures are often 27°C or warmer
    • A storm formed 200 miles south-east of the Bahamas on 23 August
    • It moved north-west over the southern tip of Florida into the Gulf of Mexico
    • As it travelled over the warm water, it became stronger
    • On the morning of the 29th it struck land, bringing winds of around 200km/h and 200-250mm rainfall in Louisiana and a storm surge of up to 8.5m in Mississippi
  • Its consequences were severe socially, economically and environmentally
    • More than 1800 people were killed
    • 300 000 houses were destroyed and hundreds of thousands of people became homeless
    • Large areas were flooded, including 80% of New Orleans
    • 3 million people were left without electricity
    • Roads were damaged, and some bridges collapsed
    • Coastal habitats were damaged
    • 230 000 jobs were lost from damaged businesses
    • Water supplies were polluted with sewage and chemicals
    • The total cost of the damage was estimated it $150 billion
    • Rescue and recovery efforts were hampered by disagreements between national, state and local officials
  • Responses were effective in the main because the states were prepared
    • Approximately 75% of New Orleans residents were evacuated before the hurricane reached land
    • Mississippi and Louisiana declared states of emergency; they set up control centres and emergency shelters, and they stockpiled supplies
    • The coastguard, police, fire service and army rescued over 50 000 people
    • NGOs collected donations and provided aid, including millions of hot meals
    • The US government provided over $16 billion for the rebuilding of homes, and provided funds to repair other essential infrastructure
    • The US Army recommended that buildings are rebuild on stilts and not rebuilt at all in very low-lying areas
    • Repaired and improved flood defences for New Orleans costing $14.5 billion were finished in 2013

Case Study: Drought, UK

  • In 2012, much of England and Wales suffered one of the worst droughts recorded in the area
    • Less rainfall was caused by unusual wind patterns; dry winds came from Europe in the east rather than the usual wet winds from the Atlantic in the west
      • The area affected by the drought received 55-95% of its usual rainfall between April 2010 to May 2012
    • The weather was also warmer than usual, so more water evaporated from reservoirs than usual and soils dried out
    • Soils were dry and baked hard, so it was difficult for any rainfall to be soaked up by the ground
    • High water usage also caused the drought as people are used to using massive amounts of water every day and massive amounts are used in industry and farming, as well as water wasted by leaks
      • Around 1.7 billion litres of water are used each day
  • The drought had a severe effect on the UK
    • Water shortages made it difficult to find water for crops and livestock
    • The dry ground made it difficult to harvest crops in autumn 2011
    • Dry areas of moorland caught fire easily, with wild fires raging across parts of South Wales, Surrey and the Scottish Borders
    • River water was used to boost the water supply in some areas, reducing river levels and causing damage to plants and animals
    • Homeowners were still able to turn on the tap for water; but hosepipe bans meant that around 20million people weren’t allowed to use hosepipes to water their gardens or wash their cars
  • As the seriousness of the drought became clear, water companies acted to keep water flowing
    • Permits were granted to allow the water companies to extract water from the rivers
    • They issued hosepipe bans to conserve available water
    • There were campaigns to get ordinary people to use less water at home