Greenhouse+effect

(8.5.1) Calculating the intensity of sun’s radiation incident on earth: The Earth, which is approximately 1.5 x 108 km from the Sun, the Sun’s radiant energy over empty space. The temperature of the Sun’s surface is about 6000 K. About 43% of it’s radiation is in the visible region of the electromagnetic spectrum with 49% in the infrared region and 8% in the ultra-violet region. The Earth intercepts only a small part of the Sun’s total radiation (about 0.5 of a billionth). The average **radiant power** radiated to an area placed perpendicular to the outer surface of the Earth’s atmosphere while the Earth is at its mean distance from the Sun defines the **flux density or the solar constant** at a particular surface. __It is defined as the amount of solar energy per second that falls on an area of 1m2 of the upper atmosphere perpendicular to the Sun’s rays.__ Its value is found to be equal to 1.35 x 103 Wm-2. The total solar irradiance has been monitored with absolute radiometers since 1978 on board five satellites. __The term “solar constant” is a misnomer because it is not constant.__ Since the Earth-to-Sun distance varies by about 6% over the course of a year from perihelion (nearest on January 3) to aphelion (furthest on July 3) due to the elliptical orbit of the Earth, the solar constant varies from 1038 Wm-2 to 1398 Wm-2. Furthermore, the energy radiated by the Sun has changed over its time of stellar evolution. As the solar constant applies “perpendicular” to the top of the atmosphere, and because the atmosphere reduces this flux considerably on a clear day, the value is reduced to about 1 KWm-2. On an overcast day this value could be as low as a few watts per square metre. The total solar radiation or irradiance reaching the top of the atmosphere is about 1.7 x 1017 W. Distributed over the whole globe, this amounts to about 170 Wm-2 averaged over a day and night. (8.5.2) Albedo: __The term “albedo” (__ __a__ __) (Latin for white) at a surface, is the ratio between the incoming radiation and the amount reflected expressed as a coefficient or as a percentage.__



(8.5.4) Greenhouse Effect: __The greenhouse effect can be divided into two sub-types, namely the “normal green house effect”, and the “enhanced green house effect” (green house effect due to anthropogenic causes).__ The green house gases form a layer around the Earth like an addition to the atmospheric layer’s components, and due to their properties only allow short wavelength and high energy waves to pass through them, blocking the longer wave length radiations. The part of the sun’s rays reflected back by the Earth’s surface is of longer wavelength type and is blocked by the green house gases layer, thereby raising the temperature of the earth’s surface. Normal green house effect is essential to sustain the rich biodiversity on earth. But the enhanced green house effect due to increased pollution and carbon gases emission into the atmosphere is the cause of concern now, by melting the ice and raising the global water body level, which in turn increases the albedo effect causing more ice to melt, forming a “positive feedback loop”.

Diagram shows shorter wavelength radiations coming in and longer leaving the Earth.

(8.5.5) Sources of green house gases:


 * S no. || Green house gas || Source (Natural/Human) ||
 * 1 || Water vapor || Both, with more from Natural causes like evaporation, condensation, etc. ||
 * 2 || CO2 || Both, with more from Human activities ||
 * 3 || CH4 || Both, with more from Natural activities ||
 * 4 || N2O || Both, with majorly from Human activities ||
 * 5 || O3 || Both (depending on the altitude its found at) ||

(8.5.6) molecular mechanism of IR absorption: Because of the Ultra Violet radiation being more energetic than infrared radiation, it tends to break bonds between atoms joined together. On the other hand, infrared radiation being less energetic tends to cause the atoms to vibrate in various ways. __When the frequency of the infrared radiation is equal to the frequency of vibration then resonance occurs. It just so happens that the natural frequency of vibration of the molecules of the greenhouse gases is in the infrared region.__ If resonance occurs and the “molecular dipole moment” undergoes a change, then the greenhouse gas will absorb energy from the albedo infrared radiation coming from a surface. Only certain energies for the system are allowed and only photons with certain energies will excite molecular vibrations. Therefore, vibrational motion is quantized and transitions can occur between different vibrational energy levels. The absorbed energy can then be re-radiated back into the biosphere. We have already learnt that energy is directly proportional to frequency or inversely propotional to wavelength (E = hf or E = hc / l ). If th wave number is the number of waves per centimeter (cm-1) we have a variable that is directly propotional to energy. When the energy of the infrared radiation from the instrument matches the energy of vibration of a molecule in the sample, radiation is absorbed, and the frequency given in wave numbers (cm-1) of the infrared radiation matches the frequency of the vibration. Each sample examined has its own individual spectrum and therefore a blueprint of the sample just like the DNA of an individual.

Blackbody: __A black body is a theoretical object that absorbs all energies and reflects back none, thereby appearing black. But it also emits a lot of radiations itself. For example, Sun is a black body.__ Black body radiation is the radiation emitted by a perfect emitter. The radiation is sometimes called temperature radiation because the relative intensities of the emitted wavelengths are dependant only on the temperature of the blackbody. An almost perfect black body can be made by painting the inside of an enclosed cylinder black and punching a small hole in the lid of the cylinder to allow different radiations to enter the box. A typical industrial “extended source plate” type black body.

As the temperature decreases, the peak of the blackbody radiation curve moves to lower intensities and loner wavelengths. The black-body radiation graph is also compared with the classical model of Rayleigh and Jeans.

Stefan-Boltzmann law: The total area under a spectral emission curve for a certain temperature T represents the total energy radiated per metre2 per unit time E and for that assigned temperature it has been found to be directly propotional to the fourth power T4. E a T4 E = s T4 This relationship is called Stefan’s law, and the constant is called the Stefan-Boltzmann constant an is equal to 5.67 x 10-8 Wm-2K-4. The power radiated by an area A of black body radiator is represented by: P = A s T4

Surface Heat capacity (Cs ): It is the energy required to raise the temperatureof a unit area of a planet’s surface by one degree Kelvin and is measured in Jm-2K-1. Cs = Q / A D T

Green-house effect on planets: The change of a planet’s temperature over a period of time is given by: (Incoming radiation Intensity-outgoing radiation Intensity) x time / surface heat capacity Suggest your own improvements.

same text and pictures in a downloadable word document:

For practice and understanding of the formulas and laws better, please see the powerpoint slide on phet! It will be of great help... http://phet.colorado.edu/teacher_ideas/view-contribution.php?contribution_id=497 Then you can practice the simulation link below to further strengthen your knowledge.
 * Simulation link || Purpose of simulation ||
 * http://phet.colorado.edu/simulations/sims.php?sim=The_Greenhouse_Effect || Affect of greenhouse gases, and photons' behaviour ||

For Climate predictions, update, and analysis of affecting and contributing factors, visit: http://climateprediction.net/content/international-baccalaureate