Download Thursday Atmosphere Weather2

Cambridge AS Geography @
Ballakermeen
Atmosphere & Weather
Local Energy Budgets
Thursday 12A Friday 12D
Structure of
the
atmosphere
• Energy enters the
atmosphere as short wave
solar radiation (insolation).
Incoming &
Outgoing• It may leave as:
solar radiation
Energy –– Reflected
Outgoing long-wave
(Global Energy (infra-red) radiation
is a balance
Budget) • There
between the energy
arriving & leaving.
• Positive heat balance at
tropics
• Negative heat balance at
polar regions
Local Energy Budgets
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 earth’s
surface.
• The energy budget has a huge effect on weather and climate.
The six-factor day model
Factor 1. Incoming solar radiation
• Atmosphere’s main
energy input
• Strongly influenced
by cloud cover and
latitude
• At the equator, the
sun’s rays are more
concentrated than at
the poles.
Factor 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.
Albedo 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
Factor 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.
Factor 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.
Energy &
transfers of
state
Factor 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.
Factor 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.
Simple daytime energy budget equation
• Energy available at surface =
Solar radiation receipt (below)
(reflected solar radiation + surface absorption + latent heat +
sensible heat transfer + longwave radiation)
Got it?
Now, what happens at night?
The four-factor night model
Factor 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.
Factor 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.
Factor 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.
Factor 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.
Got it?