AS Geography

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AS Geography

• Atmosphere Weather
• Energy Budgets

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• Meteorology is the study of the atmosphere.
• Weather is the short term conditions of the
  atmosphere.
• Climate is the longer-term average conditions in
  the atmosphere (temperature, humidity,
  precipitation).

Instrument Measures Unit
Thermometer Temperature Celsius/ Fahrenheit
Hygrometer Humidity
Barometer Air Pressure Mb (milibars)
Anemometer Wind Speed Km or Miles/hour
Weather Vane Wind Direction Compass directions
Rain Gauge Rainfall/precipitation mm
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Structure of the atmosphere
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Incoming Outgoing Energy

• Energy enters the atmosphere as short wave solar
  radiation (insolation).
• It may leave as
• Reflected solar radiation
• Outgoing long-wave (infra-red) radiation
• There is a balance between the energy arriving
  leaving.
• Positive heat balance at tropics
• Negative heat balance at polar regions

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Energy Budgets

• Some parts of the earth receive a lot of solar
  energy (surplus), some receive less (deficit).
• In order to transfer this energy around, to
  create some sort of balance, the earth uses
  pressure belts, winds and ocean currents.
• The global energy budget is an account of the key
  transfers which affect the amount of energy gain
  or loss on the earths surface.
• The energy budget has a huge effect on weather
  and climate.

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The six-factor day model
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1. Incoming solar radiation

• Atmospheres main energy input
• Strongly influenced by cloud cover and latitude
• At the equator, the suns rays are more
  concentrated than at the poles.

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2. Reflected solar radiation

• The proportion of reflected solar radiation
  varies greatly with the nature of the surface.
• The degree of reflection is expressed as either a
  fraction on a scale of 0 to 1, or as a
  percentage.
• This fraction is referred to as the albedo of the
  surface.

• Albedo
• This is simply the proportion of sunlight
  reflected from a surface.
• Fresh snow ice have the highest albedos,
  reflecting up to 95 of sunlight.
• Ocean surfaces absorb most sunlight, and so have
  low albedos.

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Examples
Surface or object Albedo ( solar radiation reflected)
Fresh snow 75-95
Thick clouds 60-90
Thin clouds 30-50
Ice 30-40
Sand 15-45
Earth atmosphere 30
Mars (planet, not bar) 17
Grassy field 25
Dry, ploughed field 15
Water 10
Forest 10
Moon 7
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3. Surface absorption

• Energy arriving at the surface has the potential
  to heat that surface
• The nature of the surface has an effect, e.g.
• If the surface can conduct heat rapidly into the
  lower layers of the soil its temperature will be
  low.
• If the heat is not carried away quickly it will
  be concentrated at the surface result in high
  temperatures there.

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4. Latent heat (evaporation)

• The turning of liquid water into vapour consumes
  a considerable amount of energy.
• When water is present at the surface, a
  proportion of the incoming solar radiation will
  be used to evaporate it.
• Consequently, that energy will not be available
  to raise local energy levels and temperatures.

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Energy transfers of state
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5. Sensible heat transfer

• This term is used to describe the transfer of
  parcels of air to or from the point at which the
  energy budget is being assessed.
• If relatively cold air moves in, energy may be
  taken from the surface, creating an energy loss.
• If warm air rises from the surface to be replaced
  by cooler air, a loss will also occur.
• This process is best described as convective
  transfer, and during the day it is responsible
  for removing energy from the surface and passing
  it to the air.

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6. Longwave radiation

• This is emitted by the surface, and passes into
  the atmosphere, and eventually into space.
• There is also a downward-directed stream of
  long-wave radiation from particles in the
  atmosphere
• The difference between the 2 streams is known as
  the net radiation balance.
• During the day, since the outgoing stream is
  greater than the incoming one, there is a net
  loss of energy from the surface.

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Simple daytime energy budget equation

• Energy available at surface
• Solar radiation receipt
• (reflected solar radiation surface
  absorption latent heat sensible heat
  transfer longwave radiation)

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The four-factor night model
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1. Longwave radiation

• During a cloudless night, little longwave
  radiation arrives at the surface of the ground
  from the atmosphere
• Consequently, the outgoing stream is greater and
  there is a net loss of energy from the surface.
• Under cloudy conditions the loss is reduced
  because clouds return longwave radiation to the
  surface, acting like a blanket around the earth
• With clear skies, temperatures fall to lower
  levels at night.

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2. Latent heat (condensation)

• At night, water vapour in the air close to the
  ground can condense to form dew because the air
  is cooled by the cold surface.
• The condensation process liberates latent heat,
  and supplies energy to the surface, resulting in
  a net gain of energy.
• However, it is possible for evaporation to occur
  at night. If this happens on a significant scale
  a net loss of energy might result.

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3. Subsurface supply

• The heat stored in the soil and subsoil during
  the day can be transferred to the cooled surface
  during the night.
• This energy supply can offset overnight cooling,
  and reduce the size of the night-time temperature
  drop on the surface.

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4. Sensible heat transfer

• Warm air moving to a given point will contribute
  energy and keep temperatures up.
• By contrast, if cold air moves in energy levels
  will fall, with a possible reduction in
  temperature.