The upper surfaces of clouds are extremely good reflectors of short-wave radiation. Cloud reflection can account for a direct turning back into space of form 30-60% of total incoming radiation while at the same time absorbing 5% to 20%. The average figures for cloud reflection and absorption are 21% and 3% respectively.
Cloud cover also serves to retain much of the heat that would be lost from the earth by radiation. Thus, the presence of cloud cover prevents extremes of temperature throughput the day and night.
d. Effects of Surface Covers:
Snow and ice covers may be found over cold temperate regions throughout the year. They have high albedos and hence much of the incoming solar radiation in such areas is reflected back to space.
The surfaces of the land and ocean reflect some short-wave radiation directly back into the atmosphere. This small quantity, about 6% as a world average, may be combined with cloud reflection in evaluating total reflective losses.
Water has a tendency to store the heat it receives. Land quickly returns it to the atmosphere. The difference between land and sea help to produce continentality. It implies that:
Latitude determines the annual distribution of insolation by controlling the angle at which the sun's rays strike the surface. As shown the figure below, when the sun is low in the sky (high latitudes in winter), the sun's rays impinge on the earth's surface at an oblique angle. Such rays deliver on the energy at the ground than vertical rays, when the sun is high in the sky (low latitudes) This is because:
- the same amount of insolation is spread over a larger surface;
- the same solar beam undergoes more severe atmospheric dilution by
reflection, scattering and absorption in passing through a thicker layer of air.
For the year as a whole, the angle at which the sun's rays strike the surface becomes more oblique pole wards of the equator. As a result, there is a general latitudinal decrease from the equator to the poles in the observed distribution of solar energy received at the earth's surface
f. Effect of Elevation and Aspect:
Some slopes are more exposed to the sun than others, while high elevations which have a much smaller mass of air above them receive considerably more insolation under clear skies than locations near sea-level. On the average in middle latitudes the intensity of incident solar radiation increases by 5-15% for each 1000m increase in elevation in the lower troposphere.
VII. The Energy Budget / The Heat Budget:
On the average only less than half of the sun's energy is actually received by the earth, because of reflection by the earth surface and absorption by the atmosphere.
1. Insolation (Short-wave radiation):
Let there are 100 units energy come from the sun.
i. The total reflection of the earth is 32 units (albedo).
ii. Reflection from clouds and water droplets . The amount of reflection depends on weather and climatic conditions. The amount of clouds in desert is small, and thick clouds in humid conditions may reflect up to 80% of the total incoming solar radiation.
iii. Scattering of light by dust particles and gas molecules, which diffuse the light in all directions. It gives the sky a blue colour.
iv. Reflection from ground. Generally, land surfaces have higher albedo, while water has lower reflection value. The colour of the ground and the type of natural vegetation all contribute to the different amount of reflection.
v. About 8 units are absorbed by the atmosphere directly (Aad). (Ozone, water vapour, CO2...)
vi. About 10 units are absorbed by the atmos. indirectly (Aai). (Ozone, water vapour, CO2...)
Only 50 units can be reached the earth's surface.
26 absorbed by surface directly (ASd)
18 absorbed by surface indirectly (ASi)
6 scattering from the atmosphere (Si)
2. Terrestrial Radiation (Long-wave radiation):
i. The surface receives 77 units from atmos. through counter-radiation (CR).
ii. About 8 units are lost directly back into space through Atmospheric Window (Aw).
iii. About 90 units are radiated to the atmosphere (RL).
iv. About 20 units are emitted to the atmosphere by Latent Heat (E).
v. About 9 units are emitted to the atmosphere by Sensible Heat (H).
vi. About 60 units are lost into space from the atmosphere (RO). Most of them are radiated from the top of clouds.
3. The Green House Effect:
Note that, the earth receives only 68 units of insolation (50+10+8), but there are 119 units (90+8) are radiated out from the earth's surface. The atmosphere is largely heated from below, that is, it is warmed by trapping long wave terrestrial energy producing the green house effect. Of the long-wave radiation from the ground, a portion is radiating back to the earth's surface, the process called counter-radiation. Thus, the atmosphere receives heat by an indirect process in which the radiant energy in short-wave form is permitted to pass through, but that in long-wave form is delayed in making its escape. Therefore, the lower atmosphere with its water vapour and carbon dioxide acts as a blanket which returns heat to the earth and helps to keep surface temperatures from dropping excessively during the night or in winter.