AS Geography Atmosphere & Weather Energy Budgets.

AS Geography

Atmosphere & Weather

Energy Budgets

• 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

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.

ExamplesSurface 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.