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AS Geography
Atmosphere & Weather
Energy Budgets
• Meteorology is the study of the atmosphere.
• Weather is the short term conditions of the
atmosphere.
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
• Climate is the longer-term average conditions in
the atmosphere (temperature, humidity,
precipitation).
Structure of
the
atmosphere
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
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
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.
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.
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
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.
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
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.
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 –
(reflected solar radiation + surface
absorption + latent heat + sensible heat
transfer + longwave radiation)
The four-factor night model
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.
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.
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.
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.