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 (% solar radiation reflected) 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 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.

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.