|
|
| Line 1: |
Line 1: |
| The surface of the [[Sun]] radiates [[Electromagnetic radiation|light and heat]] at approximately 5,500 °C. The [[Earth]] is much cooler and so radiates heat back away from itself at much longer wavelengths, mostly in the [[infrared]] range. The '''idealized greenhouse model''' is based on the fact that certain gases in the Earth's atmosphere, including [[carbon dioxide]] and [[water vapour]], are transparent to the high-frequency, high-energy solar radiation, but are much more opaque to the lower frequency infrared radiation leaving the surface of the earth. Thus heat is easily let ''in'', but is partially trapped by these gases as it tries to ''leave''. Rather than get hotter and hotter, [[Kirchhoff's law of thermal radiation]] says that the gases of the atmosphere also have to re-emit the infrared energy that they absorb, and they do so, also at long infrared wavelengths, both upwards into space as well as downwards back towards the Earth's surface. In the long-term, thermal equilibrium is reached when all the heat energy arriving on the planet is leaving again at the same rate. In this idealized model, the greenhouse gases cause the surface of the planet to be warmer than it would be without them, in order for the required amount of heat energy finally to be radiated out into space from the top of the atmosphere.<ref>{{cite web
| | by Nas, is very fitting and the film agrees with it. Medical word press themes give you the latest medical designs. Wordpress Content management systems, being customer friendly, can be used extensively to write and manage websites and blogs. s ultimately easy to implement and virtually maintenance free. Also our developers are well convergent with the latest technologies and bitty-gritty of wordpress website design and promises to deliver you the best solution that you can ever have. <br><br>These websites can be easily customized and can appear in the top rankings of the major search engines. Infertility can cause a major setback to the couples due to the inability to conceive. A Wordpress plugin is a software that you can install into your Wordpress site. They provide many such popular products which you can buy for your baby. For a Wordpress website, you don't need a powerful web hosting account to host your site. <br><br>Digital photography is a innovative effort, if you removethe stress to catch every position and viewpoint of a place, you free yourself up to be more innovative and your outcomes will be much better. The following piece of content is meant to make your choice easier and reassure you that the decision to go ahead with this conversion is requited with rich benefits:. Possibly the most downloaded Word - Press plugin, the Google XML Sitemaps plugin but not only automatically creates a site map linking to everyone your pages and posts, it also notifies Google, Bing, Yahoo, and Ask. Our skilled expertise, skillfulness and excellence have been well known all across the world. Purchase these from our site, or bring your own, it doesn't matter, we will still give you free installation and configuration. <br><br>The disadvantage is it requires a considerable amount of time to set every thing up. Quttera - Quttera describes itself as a 'Saa - S [Software as a Service] web-malware monitoring and alerting solution for websites of any size and complexity. When we talk about functional suitability, Word - Press proves itself as one of the strongest contestant among its other rivals. If you choose a blog then people will be able to post articles on your site and people will be able to make comments on your posts (unless you turn comments off). In case you cherished this information in addition to you desire to acquire guidance relating to [http://invisible-players.com/index/users/view/id/58877 wordpress backup plugin] i implore you to go to our internet site. If your blog employs the permalink function, This gives your SEO efforts a boost, and your visitors will know firsthand what's in the post when seeing the URL. <br><br>As a open source platform Wordpress offers distinctive ready to use themes for free along with custom theme support and easy customization. Sanjeev Chuadhary is an expert writer who shares his knowledge about web development through their published articles and other resource. Just download it from the website and start using the same. This is because of the customization that works as a keystone for a SEO friendly blogging portal website. As with a terminology, there are many methods to understand how to use the terminology. |
| |url=http://oceanservice.noaa.gov/education/pd/climate/factsheets/whatgreenhouse.pdf
| |
| |title=What is the Greenhouse Effect?
| |
| |publisher=Intergovernmental Panel on Climate Change
| |
| |year=2007}}</ref>
| |
| | |
| The [[greenhouse effect]] can be illustrated with an idealized planet. This is a common "textbook model":<ref>[http://www.geo.utexas.edu/courses/387H/Lectures/chap2.pdf Chapter 2, The global energy balance], UT course ''Physical Climatology''</ref> the planet will have a constant surface temperature T<sub>s</sub> and an atmosphere with constant temperature T<sub>a</sub>. For diagrammatic clarity, a gap can be depicted between the atmosphere and the surface. Alternatively, T<sub>s</sub> could be interpreted as a temperature representative of the surface and the lower atmosphere, and T<sub>a</sub> could be interpreted as the temperature of the upper atmosphere. In order to justify that T<sub>a</sub> and T<sub>s</sub> remain constant over the planet, strong ocean and atmospheric currents can be imagined to provide plentiful lateral mixing. Furthermore, any daily or seasonal cycles in temperature are assumed to be insignificant.
| |
| | |
| == The model ==
| |
| | |
| {{see also|Radiative forcing}}
| |
| The model will find the values of T<sub>s</sub> and T<sub>a</sub> that will allow the outgoing radiative power, escaping the top of the atmosphere, to be equal to the absorbed radiative power of [[sunlight]]. When applied to a planet like Earth, the outgoing radiation will be longwave and the sunlight will be shortwave. These two streams of radiation will have distinct emission and absorption characteristics. In the idealized model, we assume the atmosphere is completely transparent to sunlight. The planetary [[albedo]] α<sub>P</sub> is the fraction of the incoming solar flux that is reflected back to space (since the atmosphere is assumed totally transparent to solar radiation, it does not matter whether this albedo is imagined to be caused by reflection at the surface of the planet or at the top of the atmosphere or a mixture). The flux density of the incoming solar radiation is specified by the [[solar constant]] S<sub>0</sub>. For application to planet Earth, appropriate values are S<sub>0</sub>=1366 W m<sup>−2</sup> and α<sub>P</sub>=0.30. Accounting for the fact that the surface area of a sphere is 4 times the area of its intercept (its shadow), the average incoming radiation is
| |
| S<sub>0</sub>/4.
| |
| | |
| For longwave radiation, the surface of the Earth is assumed to have an emissivity of 1 (i.e., the earth is a black body in the infrared, which is realistic). The surface emits a radiative flux density F according to the [[Stefan-Boltzmann law]]:
| |
| | |
| : <math>
| |
| F=\sigma T^4
| |
| </math>
| |
| | |
| where σ is the [[Stefan-Boltzmann constant]]. A key to understanding the greenhouse effect is [[Kirchhoff's law of thermal radiation]]. At any given wavelength the absorptivity of the atmosphere will be equal to the emissivity. Radiation from the surface could be in a slightly different portion of the infrared spectrum than the radiation emitted by the atmosphere. The model assumes that the average emissivity (absorptivity) is identical for either of these streams of infrared radiation, as they interact with the atmosphere. Thus, for longwave radiation, one symbol ε denotes both the emissivity and absorptivity of the atmosphere, for any stream of infrared radiation.
| |
| | |
| [[Image:IdealizedGreenhouseEmissivity78.png|thumb|400px|Idealized greenhouse model with an isothermal atmosphere. The blue arrows denote shortwave (solar) radiative flux density and the red arrow denotes longwave (terrestrial) radiative flux density. The radiation streams are shown with lateral displacement for clarity; they are collocated in the model. The atmosphere, which interacts only with the longwave radiation, is indicated by the layer within the dashed lines. A specific solution is depicted for ε=0.78 and α<sub>p</sub>=0.3, representing Planet Earth. The numbers in the parentheses indicate the flux densities as a percent of S<sub>0</sub>/4.]]
| |
| [[Image:IdealizedGreenhouseEmissivity82.png|thumb|400px|The equilibrium solution with ε=0.82. The increase by Δε=0.04 corresponds to doubling carbon dioxide and the associated positive feedback on water vapor.]]
| |
| [[Image:IdealizedGreenhouseEmissivity00.png|thumb|400px|The equilibrium solution with no greenhouse effect: ε=0]]
| |
| | |
| The infrared flux density out of the top of the atmosphere:
| |
| : <math>
| |
| F\uparrow =\epsilon \sigma T_a^4 + (1-\epsilon) \sigma T_s^4
| |
| </math>
| |
| In the last term, ε represents the fraction of upward longwave radiation from the surface that is absorbed, the absorptivity of the atmosphere. In the first term on the right, ε is the emissivity of the atmosphere, the adjustment of the Stefan-Boltzmann law to account for the fact that the atmosphere is not optically thick. Thus ε plays the role of neatly blending, or averaging, the two streams of radiation in the calculation of the outward flux density.
| |
|
| |
| Zero net radiation leaving the top of the atmosphere requires:
| |
| : <math>
| |
| -\frac{1}{4}S_0(1-\alpha_p)+\epsilon \sigma T_a^4 + (1-\epsilon) \sigma T_s^4= 0
| |
| </math>
| |
| Zero net radiation entering the surface requires:
| |
| : <math>
| |
| \frac{1}{4}S_0(1-\alpha_p)+\epsilon \sigma T_a^4 - \sigma T_s^4 = 0
| |
| </math>
| |
| Energy equilibrium of the atmosphere can be either derived from the two above equilibrium conditions, or independently deduced:
| |
| : <math>
| |
| 2 \epsilon \sigma T_a^4 - \epsilon \sigma T_s^4 = 0
| |
| </math>
| |
| Note the important factor of 2, resulting from the fact that the atmosphere radiates both upward and downward.
| |
| Thus the ratio of T<sub>a</sub> to T<sub>s</sub> is independent of ε:
| |
| : <math> T_a = { T_s \over 2^{1/4} }
| |
| = { T_s \over 1.189 }
| |
| </math>
| |
| Thus T<sub>a</sub> can be expressed in terms of T<sub>s</sub>, and a solution is obtained for
| |
| T<sub>s</sub> in terms of the model input parameters:
| |
| : <math>
| |
| \frac{1}{4}S_0(1-\alpha_p)=\left( 1-\frac{\epsilon}{2} \right) \sigma T_s^4
| |
| </math>
| |
| or
| |
| : <math>
| |
| T_s=\left[ \frac{S_0(1-\alpha_p)}{4\sigma} \frac{1}{1-{\epsilon \over 2}} \right]^{1/4}
| |
| </math>
| |
| The solution can also be expressed in terms of the [[effective temperature|''effective emission temperature'']] T<sub>e</sub>, which is the temperature that characterizes the outgoing infrared flux density F, as if the radiator were a perfect radiator obeying F=σT<sub>e</sub><sup>4</sup>. This is easy to conceptualize in the context of the
| |
| model. T<sub>e</sub> is also the solution for T<sub>s</sub>, for the case of ε=0, or no atmosphere:
| |
| : <math>
| |
| T_e \equiv \left[ \frac{S_0(1-\alpha_p)}{4\sigma} \right]^{1/4}
| |
| </math>
| |
| With the definition of T<sub>e</sub>:
| |
| : <math>
| |
| T_s= T_e \left[ \frac{1}{1-{\epsilon \over 2}} \right]^{1/4}
| |
| </math>
| |
| For a perfect greenhouse, with no radiation escaping from the surface, or ε=1:
| |
| : <math>
| |
| T_s= T_e 2^{1/4} = 1.189 T_e \qquad T_a=T_e
| |
| </math>
| |
| | |
| Using the parameters defined above to be appropriate for Earth,
| |
| : <math> T_e = 255 ~\mathrm{K} = -18 ~\mathrm{C} </math>
| |
| For ε=1:
| |
| : <math> T_s = 303 ~\mathrm{K} = 30 ~\mathrm{C} </math>
| |
| For ε=0.78,
| |
| : <math> T_s = 288.3 ~\mathrm{K} \qquad T_a = 242.5 ~\mathrm{K} </math>.
| |
| This value of T<sub>s</sub> happens to be close to a very widely quoted (but unattributed) 288 K claimed for the average global "surface temperature". ε=0.78 implies 22% of the surface radiation escapes directly to space, consistent with the statement of 15% to 30% escaping in the [[greenhouse effect]].
| |
| | |
| The [[radiative forcing]] for doubling carbon dioxide is 3.71 W m<sup>−2</sup>, in a simple parameterization. This is also the value endorsed by the [[Intergovernmental Panel on Climate Change|IPCC]].
| |
| From the equation for <math>F\uparrow</math>,
| |
| : <math> \Delta F\uparrow = \Delta\epsilon \left( \sigma T_a^4 -\sigma T_s^4 \right) </math>
| |
| Using the values of T<sub>s</sub> and T<sub>a</sub> for ε=0.78 allows for <math> \Delta F\uparrow </math>= -3.71 W m<sup>−2</sup> with Δε=.019. Thus a change of ε from 0.78 to 0.80 is consistent with the radiative forcing from
| |
| a doubling of carbon dioxide. For ε=0.80, | |
| : <math> T_s = 289.5 ~\mathrm{K} </math>
| |
| Thus this model predicts a global warming of ΔT<sub>s</sub> = 1.2 K for a doubling of carbon dioxide. A typical prediction from a [[Global climate model|GCM]] is 3 K surface warming, primarily because the GCM allows for positive feedback, notably from increased water vapor. A simple surrogate for including this feedback process is to posit an additional increase of Δε=.02, for a total Δε=.04, to approximate the effect of the increase in water vapor that would be associated with an increase in temperature. This idealized model then predicts a global warming of ΔT<sub>s</sub> = 2.4 K for a doubling of carbon dioxide, roughly consistent with the IPCC.
| |
| | |
| == Extensions ==
| |
| The simple one-level atmospheric model can be readily extended to a multiple-layer atmosphere. In this case the equations for the temperatures become a series of coupled equations. This simple model always predicts a decreasing temperature away from the surface, and all levels ''increase'' in temperature as "greenhouse gases are added". Neither of these effects are fully realistic: in the real atmosphere temperatures increase above the tropopause, and temperatures in that layer are predicted (and observed) to ''decrease'' as GHG's are added. This is directly related to the non-greyness of the real atmosphere.
| |
| | |
| == See also ==
| |
| {{wikibooks|Climate Change}}
| |
| {{wikiversity|Topic:Climate change}}
| |
| * [[Greenhouse effect]]
| |
| * [[Anti-greenhouse effect]]
| |
| * [[Climate change]]
| |
| * [[Climate forcing]]
| |
| * [[Earth's energy budget]]
| |
| * [[Earth's radiation balance]]
| |
| * [[Global dimming]]
| |
| * [[Global warming]]
| |
| | |
| == Footnotes ==
| |
| {{reflist}}
| |
| | |
| == References ==
| |
| *{{cite book |author=Bohren, Craig F.; Clothiaux, Eugene E. |title=Fundamentals of Atmospheric Radiation |publisher=John Wiley & Sons |location=Chichester |year=2006 |pages=31–41 |isbn=3-527-40503-8 |chapter=1.6 Emissivity and Global Warming}}
| |
| *{{cite book |author=Petty, Grant W. |chapter=6.4.3 Simple Radiative Models of the Atmosphere |title=A First Course in Atmospheric Radiation |publisher=Sundog Pub |location=Madison, Wisconsin |year=2006 |pages=139–143 |edition=2nd |isbn=0-9729033-1-3 }}
| |
| | |
| [[Category:Atmospheric radiation]]
| |
by Nas, is very fitting and the film agrees with it. Medical word press themes give you the latest medical designs. Wordpress Content management systems, being customer friendly, can be used extensively to write and manage websites and blogs. s ultimately easy to implement and virtually maintenance free. Also our developers are well convergent with the latest technologies and bitty-gritty of wordpress website design and promises to deliver you the best solution that you can ever have.
These websites can be easily customized and can appear in the top rankings of the major search engines. Infertility can cause a major setback to the couples due to the inability to conceive. A Wordpress plugin is a software that you can install into your Wordpress site. They provide many such popular products which you can buy for your baby. For a Wordpress website, you don't need a powerful web hosting account to host your site.
Digital photography is a innovative effort, if you removethe stress to catch every position and viewpoint of a place, you free yourself up to be more innovative and your outcomes will be much better. The following piece of content is meant to make your choice easier and reassure you that the decision to go ahead with this conversion is requited with rich benefits:. Possibly the most downloaded Word - Press plugin, the Google XML Sitemaps plugin but not only automatically creates a site map linking to everyone your pages and posts, it also notifies Google, Bing, Yahoo, and Ask. Our skilled expertise, skillfulness and excellence have been well known all across the world. Purchase these from our site, or bring your own, it doesn't matter, we will still give you free installation and configuration.
The disadvantage is it requires a considerable amount of time to set every thing up. Quttera - Quttera describes itself as a 'Saa - S [Software as a Service] web-malware monitoring and alerting solution for websites of any size and complexity. When we talk about functional suitability, Word - Press proves itself as one of the strongest contestant among its other rivals. If you choose a blog then people will be able to post articles on your site and people will be able to make comments on your posts (unless you turn comments off). In case you cherished this information in addition to you desire to acquire guidance relating to wordpress backup plugin i implore you to go to our internet site. If your blog employs the permalink function, This gives your SEO efforts a boost, and your visitors will know firsthand what's in the post when seeing the URL.
As a open source platform Wordpress offers distinctive ready to use themes for free along with custom theme support and easy customization. Sanjeev Chuadhary is an expert writer who shares his knowledge about web development through their published articles and other resource. Just download it from the website and start using the same. This is because of the customization that works as a keystone for a SEO friendly blogging portal website. As with a terminology, there are many methods to understand how to use the terminology.