Weather hazards (WJEC Eduqas GCSE Geography B): Revision Note

Specification links

The notes on this page cover part 2.3.3 of the WJEC Eduqas B specification.

2.3.3 - How are weather hazards distributed at a global scale and how does this pattern change over time?

• How global circulation creates areas of high and low pressure.

• The distribution and location of these pressure systems should be discussed.

• Typical weather patterns and the extreme weather hazards associated with high- and low-pressure systems should be discussed.

• Coverage should include:

  • The use of weather charts.

  • Temporal and spatial changes in extreme weather to include

    • Seasonal and longer-term changes in the tropics which result in drought (note the link to changing patterns of desertification in 3.4.1)

    • The seasonal distribution of cyclones/hurricanes.

Global circulation – high and low pressure

Wind formation

• To circulate the warm air around the Earth, specific wind and pressure patterns exist

• Air always moves from high pressure to lower pressure

• This movement of air generates wind 

• Pressure differences occur because the Earth's surface is heated unevenly

• It starts at the equator, which is the hottest part of the Earth

  • Air rises at the equator, leading to low pressure and rainfall at the surface

  • Once the rising air hits the edge of the atmosphere, it travels north and south

  • As the air becomes cold, it begins to sink, creating high pressure and dry air as it descends

• The cool air will 'rush' from the high-pressure zone to the low-pressure zone at the equator to be warmed again by the Sun, at the same time creating wind

Pressure differences

• Air moves in the atmosphere either towards the ground (subsidence) or up into the atmosphere (convection)

• These movements influence air pressure and rainfall

• The sea and land heat up differently

  • Sea:

    • Forms high pressure in summer and low pressure in winter

    • It takes longer to heat and cool

    • Air is denser and cooler in summer but warmer in winter

  • Land:

    • Generally, it forms areas of lower pressure in summer and higher pressure in winter

    • It heats quickly in summer and the air is lighter and rises

    • It cools quickly in winter

The influence of air movement on weather conditions

Subsidence (sinking air)

• This occurs in areas with lower levels of solar radiation, such as the mid-latitudes, the poles or at high altitudes where the air is very cold

• Air becomes denser and sinks towards the ground, forming high-pressure areas

• As air sinks, it begins to warm and can hold more moisture, preventing clouds from forming

• High pressure brings clear skies or very thin clouds

• It creates arid or semi-arid conditions due to very little precipitation.

Convection (rising air)

• This occurs in areas with higher levels of solar radiation, such as the equator

• The ground heats the air above and rises, where it begins to cool and condense into water droplets, which form clouds

• Low-pressure areas are created as air moves upward

• Thick, heavy cloud cover with heavy rainfall creates wet tropical regions

 Global pressure belts 

• The horizontal bands of the Hadley, Ferrel, and Polar cells create a large pattern of high- and low-pressure belts around the Earth

• The pattern is not the same in each hemisphere, even though the cells are mirrored

• This is because the location of these pressure zones is affected by the amount of land and sea

  • There is more land in the northern hemisphere and more sea in the southern hemisphere

Coriolis effect

• Winds are influenced by the Coriolis effect

• The Coriolis effect is the appearance that global winds and ocean currents curve as they move

• The combination of pressure cells, the Coriolis effect, and the three cells produces wind belts in each hemisphere

  • The trade winds blow from the subtropical high-pressure belts (30 ° N and S) towards the Equator's low-pressure zones and are deflected by the Coriolis force

  • The westerlies blow from the sub-tropical high-pressure belts to the mid-latitude low areas, but again, are deflected by the Coriolis force

  • The easterlies: Polar easterlies meet the westerlies at 60° S

Weather patterns and extreme weather

• Global atmospheric circulation affects the Earth's weather patterns

• It causes either a depression or an anticyclone

  • The UK has a lot of low-pressure weather systems (depressions) that are blown in from the Atlantic Ocean on south-westerly winds, bringing wet and windy weather

• These pressure systems bring extreme weather events to many regions of the world and include:

  • storms/gales

  • heavy rain

  • thick fog

  • heatwave

  • heavy snowfall

  • drought

  • cyclones/hurricanes

Weather fronts

• Weather fronts form when different masses of air with varying temperatures and amounts of moisture meet

• Fronts are the boundaries between air masses

• The area behind the front is known as a 'sector'

• Weather fronts are shown on charts using lines and symbols and they point in the direction the front is moving in:

  • Cold fronts are shown by blue lines with triangles as a steep, rounded face

    • Cold air is advancing and pushing underneath warmer air

    • The tips of the triangles indicate the direction of movement of the cold air

  • Warm fronts are shown by red lines with semicircles as a gradually sloping face

    • Warm air is advancing and rising up over cold air

    • The edges of the semicircles indicate the direction of movement of the warm air

  • Occluded fronts are shown by purple lines with alternating triangles and semicircles

    • A cold front 'catches up' with a warm front

    • The cold front lifts the warm air up from the surface, which is then ‘hidden’

    • An occlusion has the characteristics of both warm and cold fronts

• The amount of rain depends on the strength of a front and the temperature difference between the air masses that they separate

• As a warm front moves past, a sector of warm air follows behind it, making the temperature rise

• The temperature drops when a cold front goes by, as it brings colder air with it

Depression

• A depression is an area of low atmospheric pressure (<1004 mb). It is usually characterised by:

  • Winds spiral towards the centre and are strongest along the cold front

  • Isobars are close together

  • It is wettest along the cold front

  • The lowest rainfall occurs in the warm sector

  • The temperature is typically warmer in the warm sector behind the warm front and cooler in the cold sector behind the cold front

• As a depression travels, the following occur:

  1. Before the warm front arrives

     • Cloud cover: Increases to thicker, lower clouds

     • Pressure: Is relatively high before it starts to fall steadily

     • Temperature: Remains cool or mild

  2. At the warm front

     • Cloud cover: Continues to thicken

     • Precipitation: Steady, continuous rain or drizzle begins

     • Temperature: Starts to rise as the warm air mass moves in

  3. Between the warm and cold fronts

     • Cloud cover: Scattered or clear with dry weather

     • Pressure: Reaches its lowest point before beginning to rise again

     • Temperature: Remains warm

  4. At the cold front

     • Cloud cover: Develops into tall, heavy clouds

     • Precipitation: Heavy rain showers and thunderstorms occur as the cold air forces the warm air to rise rapidly

     • Temperature: Drops noticeably

  5. After the cold front passes

     • Cloud cover: Breaks up

     • Pressure: Continues to rise

     • Temperature: Remains cool

AWAITING IMAGE

Characteristics of a depression

Anticyclone

• An anticyclone is an area of high atmospheric pressure (>1008 mb) where air is sinking

• As the air sinks, no clouds or rain are formed

  • The air gets warmer as it sinks, so it can hold more water

• Winds spiral outwards from the centre and are usually light with settled, dry and bright conditions

• Isobars are far apart

• In the summer, anticyclones bring dry, hot weather

• In the winter, clear skies bring cold nights and frost

  • In cold weather, anticyclones can bring fog and mist because the cold causes air moisture to condense at low altitudes

Weather (synoptic) charts

• Meteorological readings are plotted on weather charts

• They can show some or all of the following:

  • wind speed

  • wind direction

  • pressure patterns with isobars

  • weather fronts

  • cloud cover

  • temperatures

  

• Anticyclones (high-pressure systems) can be identified on a synoptic chart due to:

  • Widely spaced isobars

  • No fronts/clouds

  • Isobars show pressure increasing (above 1008mb) outwards from the centre

• Depressions (low-pressure systems) can be identified on a synoptic chart due to:

  • Closely spaced isobars

  • Fronts/clouds

  • Isobars show pressure decreasing (below 1004mb) towards the centre

Temporal and spatial changes in extreme weather

• Extreme weather is changing over time and space

• Many events are happening more often and with more force

• There is a trend towards heavier rain, hotter heatwaves, and longer dry spells around the world

• However, the effects are different depending on location and the time of year

• For example, depending on latitude and longitude, some places have wetter winters and drier summers, while others are seeing damaging winds and heavy flooding

Temporal changes (changes over time)

• Studies show there is an increased frequency and intensity of high- and low-pressure systems bringing heavy rain and flooding or heatwaves over time

• In some places, the number of dry days in a row has decreased, but the length of the dry spell has been increasing, resulting in droughts

• Overall rainfall totals do not show a clear change, but some places are seeing wetter winters and drier summers

Spatial changes (changes across locations)

• Latitude influences the type and severity of extreme weather events in a given area

  • For example, people living nearer to the Equator often suffer from life-threatening conditions such as tropical cyclones and droughts

• Tropical cyclones:

  • Hurricanes, typhoons, and cyclones form between 5° and 30° latitude north and south of the equator, where warm ocean water (above 27°C) provides the necessary energy

  • Tropical storms are rotating, intense low-pressure systems (below 950mb)

  • They are known as:

    • Typhoons in the South China Sea and the western Pacific Ocean

    • Hurricanes in the Gulf of Mexico, the Caribbean Sea and the west coast of Mexico

    • Cyclones in the Bay of Bengal, the Indian Ocean and northern Australia

  • While the total number of storms may not increase, warmer sea surface temperatures are intensifying them, leading to more powerful and damaging events

  • Cyclones are seasonal events that occur between June and October in the northern hemisphere and December and March in the southern hemisphere

• Temperature extremes:

  • Polar regions experience lower-angle solar energy, resulting in colder climates and extreme cold weather events

  • However, tropical and subtropical zones are prone to more frequent and intense heatwaves, especially in mid-latitude and semi-arid regions

  • This is due in part to a band of low pressure in the atmosphere known as the tropical rain belt

  • This belt is where the two hemispheres' trade winds meet and is called the Intertropical Convergence Zone (ITCZ)

    • Warm, moist air has to rise at the Equator because of this meeting, which causes heavy rain, thunderstorms, and the formation of cloud bands

    • The ITCZ moves with the seasons because it follows the area of highest solar heating, making tropical areas have distinct wet and dry seasons

• Monsoons

  • In mid-latitude regions, temperatures change over a seasonal cycle, and weather events last just a few days

    • This might make it cool and rainy one day and warm and sunny the next

  • However, in tropical latitudes, temperatures don’t change much throughout the year

  • The tropical season is a shift between dry and wet periods brought about by monsoons

  • A monsoon is not a storm but a large pattern of winds and rain that covers a geographic area like a continent

• The summer monsoon has large amounts of rain

• The winter monsoon has dry conditions

  • During the winter, air descends, causing high pressure

  • The dry conditions during winter can even lead to drought if they are too intense or too long

Climate change and tropical storms

• Global temperatures are set to rise as a result of global warming

• More of the world's oceans will be above 27° C; therefore, more places across the world will experience tropical storms

• Oceans will stay at 27°C or higher for longer during the year, which will increase the annual number of tropical storms 

• Higher temperatures will mean storms will be stronger and more frequent and cause more damage