Weather hazards

• How does global atmospheric circulation work?

  1. Diagram B shows global atmospheric circulation

  2. Involves a number of circular air movements called cells

  3. Cells join together to form the overall circulation of the Earth's atmosphere
• Air sinking towards the ground surface

  Forms areas of high pressure (e.g. at the North Pole)
• Air rising from the ground surface

  Forms areas of low pressure on the ground (e.g. at the Equator)
• Winds on the ground

  • Move outwards from areas of high pressure
  • Move towards areas of low pressure
• Winds on the ground

  • Distorted by the Earth's rotation
  • Curve as they move from areas of high pressure to areas of low pressure
• Surface winds


  • Very important in transferring heat and moisture from one place to another
• The patterns of pressure belts and winds are affected by seasonal changes
• The tilt and rotation of the Earth causes relative changes in the position of the overhead Sun
• Seasonal changes cause pressure belts and winds to move north during our summer and then south during our winter
• The UK is located at about 55 North just below the 60° N line of latitude


  This puts the UK close to the boundary of cold polar air moving down from the north and warm sub-tropical air moving up from the south
• The boundary between these two air masses is unstable


  There is rising air and low-pressure belts (the sub-polar low) on the ground
• Rising air cools, condenses and forms cloud and rain


  This is why it is often cloudy and wet in the UK
• Surface winds in these mid-latitudes come from the south-west


  These winds bring warm and wet conditions to the UK
• Cold polar air from the north moves down over the UK


  Bringing snow and very cold winter weather
• Most of the world's hot deserts are found at about 30° north and south of the Equator


  Here the air is sinking, making a belt of high pressure (the sub-tropical high)
• Air isn't rising here


  So there are few clouds forming and little rainfall
• The lack of cloud makes it very hot during the day very cold at night


  As heat is quickly lost from the ground
• At the Equator the air is rising


  There is another low pressure belt (the equatorial low)
• Equatorial regions experience hot, humid conditions

  It is often cloudy with high rainfall
• Tropical storms


  Huge storms that develop in the tropics (between the tropic of cancer (23) and Capricorn (83)
• Hurricanes


  • USA and the Caribbean
• Typhoons


  • Japan and the Philippines
• Cyclones


  • SE Asia and Australia
• Tropical storms occur


  • In the summer months in the northern hemisphere
  • In the winter months in the southern hemisphere
• Tropical storms form


  • Over warm oceans (above 27°C)
  • In summer and autumn (when sea temperatures are at their highest)
  • Most form from 5-15° north and south of the equator
• In tropical regions, the intense heat makes the air unstable


  Causing it to rise rapidly, causing the tropical storm
• How do tropical storms form?

  1. Water at 27°C causes warm air above to rise quickly, creating an area of low pressure

  2. As the air rises quickly, more warm moist air is drawn upwards above the ocean creating strong winds

  3. The rising warm air spirals up and cools, causing the water vapour to condense and form clouds

  4. These clouds form the eye wall of the storm
• On reaching land the storm's energy supply (evaporated water) is cut off


  Friction with the land slows it down and it begins to weaken
• If the storm reaches warm seas after crossing the land

  It may pick up strength again
• As the storm is carried across the ocean by the prevailing winds


  It continues to gather strength
• The storm now develops an eye at its centre


  • Where air descends rapidly
  • The outer edge of the eye is the eyewall where the most intense weather conditions (strong winds and heavy rain) are felt
• Several smaller thunderstorms join together


  To form a giant spinning storm
• When surface winds reach an average of 120km per hour (75 miles per hour)


  The storm officially becomes a tropical storm
• As the air condenses it releases heat


  Which powers the storm and draws up more and more water from the ocean
• This evaporated air cools as it rises


  And condenses to form towering thunderstorm clouds
• A strong upward movement of air


  Draws water vapour up from the warm ocean surface
• Beyond the eye wall there are further banks of clouds

  • With thunderstorms and occasionally tornadoes
  • There will also be strong gusty winds and heavy rain
• The central eye


  • Is a small area where relatively cold air sinks towards the ground and warms up
  • There are no clouds here and conditions are calm
• On either side of the eye is a tail bank of cloud called the eye wall


  Here there are strong winds in excess of 120km/h, heavy rain, thunder and lightning
• A tropical storm can release the energy of 10 atom bombs every second!