Greenhouse Effect Julia Porter, Celia Hallan, Andrew Vrabel Miles, Gary DeFrance, and Amber Rose
What is the Greenhouse Effect? The greenhouse effect is a natural occurrence caused by Earth's atmosphere and the reflective nature of Earth's surfaces. The greenhouse effect determines how much solar energy reaches Earth, how much is reflected away, and how much is trapped inside. It is responsible for keeping Earth's surface temperature warm enough to support life. Without the greenhouse effect, Earth's average surface temperature would be -18 C.
What Happens During the Greenhouse Effect (Part 1)? Earth's Atmospheric "Window" The transmissivity* of the atmosphere varies based on type of gas and the wavelength of the radiation. It has trouble absorbing wavelengths of 350 to 750 nanometers. Visible light waves generally have wavelengths from 400 to 750 nanometers, and so are not absorbed. Solar energy strikes the atmosphere. The radiation varies in wavelength and frequency. Contains visible light, ultraviolet, infrared, gamma rays, X-rays, and more 1/3 of the energy is absorbed by the atmosphere. Different gases absorb infrared and ultraviolet radiation of different wavelengths and convert the energy to heat. 1/3 of the energy is reflected away and lost to space. *Transmissivity is a measure of how much atmospheric gases allow radiation to pass through them. 1/3 of the energy, mostly in the visible spectrum, reaches the Earth
What Happens During the Greenhouse Effect (Part 2)? Heat waves are infrared, and can be absorbed by the atmospheric gases, trapping heat inside. The visible light waves make it through the atmosphere and hit Earth's surface. Re-Radiation When greenhouse gases absorb infrared and ultraviolet radiation, they re-radiate the energy in infrared heat waves with longer wavelengths. Some of these waves are radiated into space, and the rest are radiated back to Earth's surface. The main function of the greenhouse effect is not causing Earth to heat up faster, but causing it to cool down more slowly. Half of the energy is absorbed by rocks, soil, and other surfaces. The visible light waves are then re-radiated as heat waves. Half of the energy goes into causing water to evaporate, photosynthesis, and causing winds, ocean waves, and water currents.
Main Greenhouse Gases Water Vapor Not a harmful greenhouse gas Carbon Dioxide (CO2) burning fossil fuels, waste, trees/ wood, other chemical reactions (such as when cement is mixed) Can be absorbed from the atmosphere be plants during photosynthesis
Main Greenhouse Gases (continued) Methane (Ch4) during the production/ transportation of coal, natural gases, and oil also from the decay of organic waste (much of the time from sitting in landfills) Nitrous Oxide (N2O) combustion of fossil fuels/ other solid wastes through agricultural and industrial activities and production
Main Greenhouse Gases (continued) Fluorinated Gases Hydrofluorocarbons, perfluorocarbons, hexafluoride these are the most common/ everyday things that we use that contribute to the greenhouse effect things such as the emissions from refrigerators, air conditioning units, aerosols, foams, etc... these are all very powerful, but emitted in such small amounts that they only make up a fraction of the emissions
Analysis: In total, the heat from sunlight of wavelengths 2.8, 3, 4, 8,9,10, and 15-30 microns will be trapped in the Atmosphere by harmful greenhouse gasses. Water vapor also traps a lot of radiation but also increases reflectivity of the Earth, and so is beneficial where global warming is concerned. O2 and O3 absorb a little of the visible spectrum (.38-.72ish microns) and also, importantly, all ultraviolet radiation(<.38 microns) More than half of the infrared radiation (>.72 microns) is absorbed by the other GHGs.
Albedo In simple terms, it is how much light a surface reflects Based on a scale of zero to one, but can also be stated as a percentage Albedo = scattered power/incident power a planet's albedo is really an average figure determined by what the planet is made up of depends on... the frequency of the incoming radiation the distribution of the incoming radiation the surfaces/ terrains that a planet is made up of
Stefan-Boltzmann Law "Total Power/Radiation Incident per unit Surface Area of the radiator is directly proportional to the Temperature (ºK) of the radiator to the fourth power." P = e A (T 4 -T c 4 ) e = The emissivity of the radiator. is always between 0 (radiates no heat) and 1 (Blackbody). = constant of proportionality/stefan's constant = 6.78*10-8 Wm -2 K -4. A = surface area T = temp of radiator T c = temp of surrounding system The lovely Ludwig Boltzmann and Joseph Stefan, respectively. How Radiators interact with surfaces Good emitters = good absorbers => same constant e used in both equations Emission depends on heat difference and works towards equilibrium, once T=T c, there is no emission. Darker objects, higher emissivity Larger objects, higher emissivity
Intensity of Sun's Radiation Incident on the Earth Temp. of sun = 5773 K Surface A of Sun = 4*3.14*696,000 2 = 6.08e12 m Star = Blackbody (or very close to it) Avg. Temp. of Earth = 291.6 K ZAP!! so: e A(T 4 -T c 4 ) = 1*6.78e-8*6.08e12(5773 4-291.6 4 ) = 4.58e20 Watts
Emissivity The emissivity of an object is the ratio of the amount of heat radiated by the object at a certain temperature to the amount of heat radiated by a black body at the same temperature. The range of emissivity values is from 0 to 1. A black body has an emissivity of 1.
Emissivity (continued) Incident Energy=Reflected Energy+Absorbed Energy+Transmitted Energy *if you set Incident Energy=100%... 100% = % Reflected Energy + %Absorbed Energy + %Transmitted Energy *since Absorbed Energy=Emitted Energy, and %Emitted=Emissivity... 1 = Emissivity + Reflectivity + Transmissivity *most objects have a very low transmissivity; thus Transmissivity=0 1 = Emissivity + Reflectivity
Emissivity Comparisons Infrared vs. Visible light Metallic surfaces have a low emissivity and high reflectivity for both infrared and visible light Some substances may have very different emission rates in infrared and visible light Metallic vs. Nonmetallic Metals have a low emissivity due to their high reflectivity, and nonmetals have a high emissivity as they have a low reflectivity. See the table on the next slide and remember: "Incident Energy = 1 = Emissivity + Reflectivity"
Emissivity of Different Substances
Surface Heat Capacity Heat capacity, or specific heat ("C"), is the amount of energy it takes to raise a certain amount of a substance by a certain amount of temperature. In Q=mc T; the "c" is the specific heat of the object.
Black Body A black Body is an ideal body that absorbs all types of electromagnetic wave at any frequency at any angle A black body at thermodynamic equilibrium is an ideal emitter A black body at thermodynamic equilibrium emits a specific kind of electromagnetic radiation called black body radiation, The radiation is emitted according to Plank's law, meaning that it has a spectrum that is determined by the temperature alone
Black Body Radiation Black-body radiation is the type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body held at constant, uniform temperature. The radiation has a specific spectrum and intensity that depends only on the temperature of the body
Blackbody Radiation at Different Temperatures Every blackbody absorbs and emits all wavelengths of radiation. The intensity with which it emits different wavelengths varies based only on the temperature of the blackbody. As the temperature increases, the "peak wavelength," the wavelength emitted with the most intensity, gets shorter. So what? By measuring the intensity of the wavelengths emitting from a blackbody, scientists can calculate the temperature. This is especially helpful for astronomers, who treat stars and planets as blackbodies (even Earth!). When the peak wavelength is in the visible spectrum, the blackbody glows different colors. For example, if a star is glowing bright red, astronomers know it is near 5000 K. If it is closer to yellow, it is closer to 6000 K. The intensity curve, or emission spectrum, is determined using Planck's law, which is a function whose only variable is temperature.
Wien's Law and Peak Wavelength of Blackbody Radiation Wien's Law states that the product of a blackbody's temperature and its peak wavelength are a constant. (Peak Wavelength)x(Temperature in Kelvin) = 2.898x10^-3 meters = 2898 microns. At room temperature, the peak wavelength is 9.66 microns. It is way out of the visible spectrum, which is why room temperature blackbodies don't glow. Around 798 K: The Draper Point. Blackbodies begin to glow dull red. 5800 K: Approximate temperature of the sun. Its peak wavelength is about 0.5 microns.
Infrared Infrared light lies between the visible and microwave portions of the electromagnetic spectrum. It can not be seen by humans but can be sensed in the form of heat.
Infrared Radiation and Greenhouse Gasses Radiation is absorbed from the sun by the earth in the form of visible light. Eventually the heat is re-emitted by the earth in the form of infrared radiation. Certain gases in the atmosphere have the property of absorbing infrared radiation. Oxygen and nitrogen the major gases in the atmosphere do not have this property. The infrared radiation strikes a molecule such as carbon dioxide and causes the bonds to bend and vibrate. The molecule gains kinetic energy by this absorption of IR radiation. This extra kinetic energy may then be transmitted to other molecules such as oxygen and nitrogen and causes a general heating of the atmosphere.
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