Topic 1 - Global Hazards (draft)

global circulation, air patterns

• air moves (circulates) around the world as a system of winds

• these winds carry heat from warmer low latitudes to cooler high latitudes and back again, through a system of looped cells, either side of the equator

• this figure of 8 pattern, distributes heat around the globe

global circulation, wind formation

• air moves due to pressure differences, knows as a ‘pressure gradient’

• the bigger the difference, the stronger the winds

• at the equator, the Sun heats the Earth’s surface and air becomes warmer

• the warm air begins rising due to expanding air molecules, which are less dense (heavy) than the air around it

• as the air rises, it creates a low pressure zone below the rising air

• the area above the rising air, becomes an area of high pressure

• as more air rises, it pushes the air apart which begins to cool

• cool air is denser, and begins to sink

• this sinking air leaves behind an area of low pressure

• as it descends back to the Earth’s surface, it starts to form an area of high pressure at the surface

• there is now a pressure difference at the surface and this draws the wind back to the area of low pressure

• therefore, air always moves from areas of high pressure to areas of low pressure

insolation

• insolation that reaches the Earth’s surface is greater at the equator and at the poles

• this is due to the Earth’s natural curvature and its angle of tilt

• the irregular heating of the Earth’s surface, generates several pressure cells. each cells produces a different weather pattern

the 3-cell atmospheric wind model

• air movement within each cell is roughly circular and moves surplus heat from equatorial regions to other parts of the Earth

• the three-cell model shows global circulation: the Hadley, Ferrel and Polar cells

• each hemisphere contains three atmospheric cells, known as the Hadley, Ferrel and Polar cell

• these cells circulate air from the surface up to the high atmosphere and back down to the Earth’s surface

• Hadley cell is the largest cell that starts at the equator and reaches as far as 40° north and south (depending on time of year)

  • warm trade winds travel in an easterly direction from tropical regions to the equator

  • as these trade winds meet near the equator, warm air is forced upwards, which quickly cools and condenses forming tropical rainstorms

  • from the top of these storms, air flows towards the high latitudes, where it becomes cooler and sinks over subtropical regions

  • this brings dry, cloudless air, which is warmed by the Sun as it descends - the climate is warm and dry (hot deserts are usually found here)

• polar cell is the smallest and weakest that reaches from the edge of the Ferrel cell to the poles at 90° north and south

  • cold air sinks forming high pressure over high latitudes

  • this cold air flows at the surface, towards the low latitudes

  • the air is warmed slightly which encourages it to rise and return, at altitude, to the poles

• Ferrel cells sits in the middle at the edge of the Hadley cell between 60° and 70° north and south of the equator

  • unlike the Polar and Hadley cells, the Ferrel cell flows n the opposite direction (creating a figure of 8 type movement)

  • air joins the sinking air of the Hadley cell and flows at low atmospheric height to mid-latitudes where it then rises along the border with the cold air of the Polar cells

  • this is the reason why the UK frequently has unsettled weather

Coriolis effect

• each cell has prevailing winds associated with it

• these winds are influenced by the Coriolis effect

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

• the curve is due to the Earth’s rotation on its axis, and this forces the winds to actually blow diagonally

• the Coriolis effect influences wind direction around the world in this way:

  • in the northern hemisphere it curves winds to the right

  • in the southern hemisphere it curves them left

• the exception is when there is a low pressure system:

  • in these systems, the winds flow in reverse (anti clockwise in the northern hemisphere and clockwise in the southern hemisphere)

global wind belts, surface winds

• the combination of pressure cells, the Coriolis effect and the 3 cells produce wind belts in each hemisphere:

  • the trade winds: blows from the subtropical high pressure belts (30° north and south) 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° south

global circulations

• global atmospheric circulation affects the Earth’s climate

• it creates differences in air pressure and causes some areas to have certain types of weather more frequently than other areas:

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

  • the UK is classified as a temperate weather system as there are no extremes of weather

• pressure systems or atmospheric air pressure is measured in milibars

• normal range of air pressure is between:

  • 1050mb - high pressure

  • 980mb - low pressure

• the lower the pressure system, the higher/stringer the winds become

where are high pressure zones created

air descends exerting pressure on the Earth’s surface

climatic conditions at high pressure zones

air is stable, clear, and dry

winds are low/light

example of high pressure zones

arid - 30° north and south of the equator, high (Sahara Desert) or low (Atacama Desert) temperature with little to no precipitation

polar - 90° north and south with low temperatures, and little to no precipitation (Arctic and Antarctica)

where are low pressure zones created

air rises releasing pressure on the Earth’s surface

climatic conditions in low pressure zones

air is unstable, cloudy, wet with thunderstorms

winds are strong/high

example of low pressure zones

tropical - 0° equatorial region with high levels of rainfall, high temperatures and humidity (Singapore)

temperate - 60° north and south of the equator - moderate weather but can experience extremes of dry or wet weather occasionally (the UK)

where do extreme weather conditions occur

where atmospheric cells meet

extreme weather, winds

• winds are relatively weak within pressure belts/zones

• between pressure zones/belts, winds are strong

• the larger the pressure difference, the stronger the winds

extreme weather, precipitation

• precipitation occurs when warm, moist air rises, cools and condenses

• air rising in low pressure areas causes frequent precipitation

• in high pressure zones, little precipitation occurs - desert regions

extreme weather, temperature

• the Equator receives the highest levels of sunlight

• these areas are the warmest, but also the wettest

• in high pressure zones, temperatures are high during the day, as there is a lack of cloud cover

• however, at night temperatures can drop below freezing, due to te lack of cloud cover

• the polar regions are very low as little sunlight reaches these areas

UK temperature VS Australia

uk:

• cooler than Australia

• in London, the average maximum summer temperature is around 23°C

• anything above 30°c is considered extremely hot

australia:

• warmer than the UK, summers are around 10°c warmer than the UK

• in Darwin (northern Australia), the average maximum summer temperature is around 33°c

• anything over 40°c is considered extremely hot

UK precipitation vs Australia

UK:

• higher precipitation than Australia

• average annual rainfall is around 1150mm

• extremely wet years have over 1210mm

• extremely dry years have less than 950mm

Australia:

• lower precipitation than UK

• average annual rainfall is around 465mm

• extremely wet years have over 550mm

• extremely dry years have less than 360mm

UK wind vs Australia

UK:

• gales over 62km/h are rare, most places in the UK only have a few days of gales each year

• strongest ever sea-level wind was over 220km/h, recorded in Fraserbu1gh, Scotland in 1989