Post-750 -by Gautam Shah
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The Sun is an extremely powerful energy source, as a result of the nuclear fusion reactions. Some of this (a small fraction) is transmitted to the Earth through the space, as electromagnetic radiation. The intensity of solar radiation at the Earth’s surface is actually quite low, because the Earth’s atmosphere and its clouds absorb or scatter as much as half of all the incoming sunlight. The strength of solar radiation at the outer edge of the Earth’s atmosphere (the solar constant), is 1.37 kW per sq.m. The intensity of energy actually available at the Earth’s surface is less than the solar constant, because of absorption and scattering of radiant energy. The process Climate starts with the arrival of radiant energy (radiation) from the Sun, near our planet.
Energy enters into the precinct of Earth from many sources and in different forms. Earth receives electromagnetic energy from other bodies in space and it also experiences gravitational energy associated with their masses. However, the most important is the Solar energy. Earth receives only 0.002 % of the total radiation emitted by the sun and yet it provides the main energy input for the Earth system. The solar radiation from Sun consists of, on average 7% ultraviolet (short wavelengths), 50% visible wave bands and 43% infrared (long wavelengths) radiation. Almost all of the absorbed energy is matched by energy emitted back into space, forming a Balance. Some residual energy can lead to global warming. This has in past one century, increased, from 0.6 watts/sq mt to 0.79.
During its passage through the space, the solar radiation loses little energy. But, on entering the atmosphere, it encounters molecules of gases, liquids and solids. Ozone and water vapour are major absorbers of radiation, but affect specific parts of the solar spectrum. Ozone absorbs ultraviolet radiation, having a profound effect on the development of life. Water vapour absorbs an infrared sector. Gases and suspended matter disperse the incident solar radiation, into multi-directional radiation, some of which passes back to the space. In the visible spectrum the blue light is scattered to a greater extent than other wave lengths, resulting in predominantly blue sky. Scattering by suspended materials is termed as diffused selection. The amount of scattering that takes place depends on the size of the particles, particle density in the air and the distance radiation travels in the atmospheric layer containing the particles. Sahara dust storms can reduce the solar radiation transmission by 30% and causing a fall of 6.0 C.
Atmosphere absorbs approximately 17 units out of the total 100 units of the solar radiation. This small component contributes to an increase in the internal energy store of the atmosphere. Approximately 29 units are lost to the space by reflection, of which 6 units are lost by scattering and 23 units are lost by cloud reflection. 54 units are transmitted to the Earth’s surface of which 36 units arrive as direct radiation and 18 units by diffuse radiation through the scattering.
On average, the Earth receives 340.4 watts /square meter. All sunshine falls on the daytime side, and the numbers are much higher at local noon. Of this 340.4 watts per square meter: 99.9 watts are reflected back into space by clouds, dust, snow and the Earth’s surface. The remaining 240.5 watts are absorbed (about a quarter by the atmosphere and the rest by the surface of the Earth). Earth’s surface gets direct sunshine that is only, half of what the warmed atmosphere sends. But together (energy from sun and from the atmosphere) add up to 504 watts/sq mt. This radiation is transformed into thermal energy within the Earth system.
Solar radiation interacts with the atmosphere. Some energy is absorbed, re-radiated and reflected while some is transmitted to the surface of the Earth. The radiation that penetrates the surface and is absorbed and heats up the surface, evaporate the water, melt the snow, generates winds, and causes a variety of chemical reactions. Natural collection of solar energy occurs in the Earth’s atmosphere, oceans, and plant life. Approximately 30% of the solar energy reaching the outer edge of the atmosphere is consumed in the hydrological cycle, which produces rainfall and the potential energy of water in mountain streams and rivers.
The Solar energy received on Earth varies from location to location, due to, the solar flares and solar spots, relative position and so the distance of the Earth on the elliptical orbit around the Sun. The Earth receives slightly more radiation in January than in July (+ or -3.4%). Solar radiation received on Earth depends on the angle of an incidence of solar rays, which is determined by the tilt of the Earth’s axis with respect to its orbital plane or the angle of latitude. Regions beyond 23 N and 23 S, are exposed to Sun only for a part of the `Season’, due to the tilt of the Earth’s axis of rotation. The amount of solar energy that can be collected also depends on the orientation of the collecting object.
The upper surface of clouds are good reflector of the solar radiation. The amount of reflection depends on the cloud cover, type and thickness. A dense cloud may reflect 50% where as a heavy storm cloud may reflect 90% of the radiation. If there is persistent cloud cover, as exists in some equatorial regions, much of the incident solar radiation is scattered back to space and very little is absorbed by the Earth’s surface. Water surfaces have low reflectivity (4-10%) and are the most efficient absorbers. Snow surfaces, on the other hand, have high reflectivity (40-80 percent) and so are the poorest absorbers. High-altitude desert regions consistently absorb higher than average amounts of solar radiation because of the reduced effect of the atmosphere above them.
Oceans like the atmosphere, play a very important role in redistribution of heat energy. Oceans in latitudes greater than 30 (N or S) gain energy, while oceans in other latitudes lose energy. Water absorbs a substantial amount of such energy and stores it for many different time limits, couples of moments to several thousand years. The oceans also represent a form of natural collection of solar energy. As a result of the absorption of solar energy in the ocean and ocean currents, temperature gradients occur in the ocean.
Earth’s body is a very poor conductor of heat, therefore, surface energy influx does not significantly affect the interior. Daily variation seldom exceeds 1 C at a depth of 1 m, and seasonal temperature variations rarely affect depths below 30 m. Waters at a temperature of 4 C increases its mass, and being lighter, the cold water and ice float at the top. Even in arctic conditions, water rarely turns into ice below 2.4 mts depth. A cold current flows out toward a warmer region, either the ocean bottom or a tropical area.
Energy equal to what is received from Sun is transferred back to the space as radiation. Over a period of time a delicate balance is achieved. When such a balance is disturbed, ice ages or green house type of effect set in. Minor variations in radiation inputs on day to day, season to season or year to year basis provides a small but very important change in the climate. When such small variations persist over a long period of time, they cause vast climatic and related changes.
The radiation, as reflected and generated by the Earth, are absorbed by the atmosphere, as an insulating blanket (the green house effect). Without this insulation the loss of energy to the outer space would be substantial and the temperatures on Earth would be lower by 30 C at night time. Earth also receives energy from its core as geo thermal heat flow, but the quantity is very small compared to the energy received from the space. The radiation from the Earth’s surface is infrared or long wave type. The surface of the Earth is an imperfect emitter and absorber of radiation. Ocean surfaces have an emission between 0.92 and 0.96, while land surfaces have lower than 0.90.
Energy balance is a global phenomenon, but regional climates occur due to different levels of solar input on various locations of the Earth. In Northern zone countries, due to high reflection from snow and ice, radiation absorption is of low level. Whereas on an equator region, if the sky is cloud covered, considerable reflection (re-radiation) occurs. The tropical areas get cooled as they export energy to mid and high latitude regions, which thus gain energy and are warmed. The transport between latitudes is accomplished by horizontal energy transfers using both the atmospheric (air -winds) and oceanic (sea water currents) circulation.
Plants and other vegetation convert a substantial amount of solar energy into food through photosynthesis. The fossils of such vegetation also provide energy at another time and space.
The potential for solar energy is enormous. Each day, the Earth, receives from sun energy, equal to about 200,000 times the total world electrical-generating capacity. Even though solar energy itself is free, the high cost of its collection, conversion, and storage has limited its exploitation. Even in sunny parts of the world’s temperate regions, for instance, a collector must have a surface area of about 430 square feet (40 square m) to gather enough energy to serve one person for one day. Solar energy utilization devices are called passive or active devices depending on the stages of conversion that take place between collection and actual use. Solar energy devices are often categorized depending on how the energy is collected, that is diffused (normal) and concentrating collection systems.
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