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).
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.
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.
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.
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)
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.