Atmospheric Processes

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Global Energy Balance

  • This is an open system where Earth receives heat/solar energy from Sun and is stored and transferred continuously between Earth and atmosphere
  • Cloud and dust particles in atmosphere both absorb and reflect energy
  • Some gases such as ozone and CO2 can absorb short wave energy
  • Reflective index of clouds is known as albedo
  • Ave. surface albedo value is 6%
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The Heat Budget

  • Heat Budget Diagram shows that due to variations in day length and seasons, some regions receive an energy surplus over a year while others have a net deficit
  • Anywhere between 40°N and 40°S of Equator, there is surplus energy as incoming solar energy is greater than outgoing energy
  • Beyond this, from 40°N to North Pole and from 40°S to the South Pole, there is an annual deficit of heat energy as incoming energy is less than outgoing energy
  • If this happened all the time then the Polar regions would be getting colder and tropics would be getting hotter- this doesn't happen due to vertical and horizontal heat transfer

Global Patterns of Insolation and Temperature

  • Latitude is the most important factor in pattern of temperature across the Earth
  • Within tropics day length is always 12 hours and sun is always high in the sky
  • Higher latitudes, day and night varies over the year, and height of sun is lower, meaning energy is spread over a larger surface area
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Factors Affecting Temperature Patterns

Latitude- Position of sun in the sky

Altitude- Temps. fall with height above sea level at a rate of 0.6°C every 100m

Land and Sea- Sea can absorb insolation to greater depth than land surface- sea warms up and cools slowly

Prevailing Winds and Ocean Currents- Cause local heating or cooling

Cloud Cover

Aspect- Angle of slopes in relation to the sun

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Vertical Heat Transfer

  • Heat can be transferred from surface of Earth to atmosphere having a cooling effect on Earth while warming air

1. Radiation- Land radiates heat back to space through long-wave radiation

2. Convection- Warm air is forced to rise as part of convection currents. Rising warm air is replaced by cold descending air

3. Conduction- Energy transferred by contact

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Horizontal Heat Transfer

Ocean Currents- Currents that flow away from the Equator are warm and formed in Western basins and carry warm water towards the poles. Currents that flow towards the Equator from the poles carry cooler water towards tropics and cool currents in the Eastern basins eg North Atlantic Drift brings warm water from Caribbean to Europe while Labrador Current brings cold Arctic water to Canada

Winds- Carry surplus energy from tropics as heat, this is air that is warmed by hot tropical land or sea surface and heat is transferred as latent or stored energy in water vapour carried by air. Warm winds blow over sea and pick up moisture

Hurricanes- Can sometimes occur involving ocean currents, warm sea and tropical winds, which lack latent heat and transfer it gradually across the globe

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What is wind?

  • This is the movement of air that results from the atmosphere redressing pressure difference

Factors Controlling Wind Direction

  • Pressure Gradient- Air flows from high to low pressure along a gradient eg Trade Winds blow from high to low pressure area
  • Coriolis Effect- Earth's rotation deflects flow of air in Northern hemisphere to the right (left in Southern hemisphere). More northern a place greater impact of Coriolis
  • Friction- Friction between air and surface reduces wind speed and limits impact of Coriolis 

Factors Controlling Wind Speed

  • Pressure Gradient- Bigger the gradient more powerful the wind- closer isobars mean stronger wind
  • Friction- Wind can be slowed down with contact from other surfaces eg land and water
  • Turbulence- Internal friction is caused by uneven surfaces
  • Local Factors- Buildings and other local factors can influence wind patterns and slow down down wind creating a "wind tunnel" effect
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Tricellular Model

Hadley Cell

  • Atmospheric engine get its energy from solar heating
  • Heat is transferred to air above where it cool. This rising air means pressure is low at the surface- ITCZ
  • The rising air diverges flowing towards the poles where it then sinks again around 30° N and S of the Equator which then warms giving high pressure
  • Air get diverted to Equator at a much lower level while some escapes into the mid-latitudes

Ferrel Cell

  • Some air from Hadley Cell moves across the surface at the Northern and Southern westerlies
  • At 60°N and S low pressure forms as warm air from Equator rises and meets blocks of cold air from the poles

Polar Cell

  • Extreme cold air at the poles subsides which then moves away from high pressure across 60°N and S where the air begins to rise
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  • Humidity is a quantity representing the amount of water vapour in the atmosphere 
  • Absolute Humidity is the actual amount of water vapour per unit volume of air
  • Relative Humidity is the amount of water vapour in the air expressed as a % of the total water vapour air can hold at a particular temp. When the RH reaches 100% the air becomes saturated and dew point is reached. RH is controlled by temp, so that if air is heated, its capacity for holding more water vapour also increases
  • Dew Point is the temp. at which a parcel of unsaturated air becomes fully saturated and achieves a RH of 100%- water vapour then condenses bringing clouds and rain will form
  • Condensation- process by which water vapour changes from a gas to liquid. It requires cooling (radiation, advection or uplift) which will then lead to clouds and precipitation
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Types of Rainfall

Orographic Rainfall

  • As a parcel of air is moved by wind from the sea towards land it is forced upwards by hills which cause a physical barrier
  • As the air is forced to rise it expands, cools adiabatically and RH increases
  • At 100% RH dew point is reached, resulting in condensation which produces clouds and then precipitation

Convectional Rainfall

  • Convection currents form when air above Earth's surface is heated by the sun
  • As air is now warm it is forced to rise, then expands, cools adiabatically and RH increases
  • At 100% RH dew point is reached, resulting in condensation which producing clouds and then thunderstorms and in some cases a hurricane

Frontal Rainfall

  • Occurs when two bodies of air meet which can happen at the polar front when warm air meets cold air
  • At a front air can become unstable, because warm and cold air can't mix
  • Warm air rises over colder, denser air and as it does this it expands, cools adiabatically and RH increases
  • At 100% RH dew point is reached, resulting in condensation which produces clouds and then precipitation
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