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# Lecture7 sep25-bb (1)

## by Peter Shiv on Dec 18, 2013

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## Lecture7 sep25-bb (1)Presentation Transcript

• 1
• Electromagnetic Spectrum Sun 0.01 0.1 Earth 1  (m) 10 100 1000
• Incoming shortwave radiation Sun 0.01 0.1  E in Earth 1 10 Outgoing longtwave radiation longwave radiation 100  E out 1000
• Net Radiation = incoming radiation - outgoing radiation R net  E in  E out
• Net radiation is positive R net  E in  E out  0 E in E out Is earth’s temperature increasing or decreasing? 5
• Net radiation is negative R net  E in  E out  0 E in E out Is earth’s temperature increasing or decreasing? 6
• Net radiation is zero Radiative Equilibrium R net  E in  E out  0 E in =240 W/m2  E out =240 W/m2 The earth’s temperature is constant! 7
• E in Solar energy sun  Solar Luminosity (L) Constant flux of energy put out by the Sun L = 3.9 x 1026 W  Solar Flux Density (Sd) the amount of solar energy per unit area on a sphere centered at the Sun with a distance d Sd = L / (4 p d2) W/m2 8 d
• E in Incoming Short-Wave Radiation Solar Constant (S ) The solar energy density at the mean distance of Earth from the sun (d =1.5 x 1011 m) d S = L / (4 π d2) = (3.9 x 1026 W) / [4 x 3.14 x (1.5 x 1011 m)2] = 1370 W/m2 Earth 9
• E in Solar energy reaching the earth So = 1370 W/m2 Ein= (solar constant) x (shadow area) =So p re2
• E in How much solar energy reaches the Earth? Ein = So p re2 BUT THIS IS NOT QUITE CORRECT! **Some energy is reflected away** Ein re
• E out How much solar energy reaches the Earth? Albedo (A) = % energy reflected away Ein = So p re2 (1-A) A= 0.3 today Eout Ein Calculation is not required on the test re
• Energy Balance: Ein = Eout Ein = So p re2 (1-A) Eout =  T4(4 p re2) Eout Ein Calculation is not required on the test
• E in  E out Radiative Equilibrium Energy emitted by Earth = Energy absorbed by Earth  σTe4 x (4π R2Earth ) = S π R2Earth x (1-A)  σTe4 = S (1-A)/4 = 1370/4 W/m2 (1-0.3) =342.5 W/m2 (1-0.3) =240W/m2 Te  4 240 /   255K (-18 C ) o 14
• Is the Earth’s surface really -18 oC? oC 15
• Greenhouse Effect Average Te With atmosphere +15 °C 33 °C Average Te Without atmosphere -18 °C Greenhouse effect
• The property of Greenhouse gases is its ability to absorb and emit infrared radiation • Main Greenhouse Gases – Water vapor (H2O) – Carbon Dioxide (CO2) – Methane (CH4) – Nitrous Oxide (N2O) – Ozone (O3) • Bonded more loosely, vibrate and stretch and absorb heat • Release IR radiation • Other Gases in Atmosphere – N2 = 78% – O2 = 21% • Tight bonds, do not vibrate or stretch • IR radiation passes through http://www.ucar.edu/learn/1_3_1.htm
• Absorption of radiation by greenhouse gases Relative radiation Ultraviolet Visible Infrared Sun 6400oC Earth -18oC Atmospheric window Wavelength (m) Absorption Absorption by Absorption by carbon by water dioxide ozone vapor
• Others <1 °C CO2 2 °C Total Greenhouse Warming 33 °C H2O 31 °C
• Q: If H2O is a more important greenhouse gas than CO2, why do we worry about CO2 rather than H2O? Excess H2O in the atmosphere causes rain in a few days Excess CO2 in the atmosphere  takes a few hundred years to remove
• Earth energy balance Ri Ro Heat transfer  At a given latitude Ri – Ro = D  Ri = Incoming SW Radiation  Ro = Outgoing LW Radiation  D = Heat Transfer
• Review Questions 1. 2. 3. 4. Wien’s law tells us what? Stefan-Boltzman law tells us what? What is the earth’s radiative equilibrium temperature without atmosphere? How do you calculate the earth’s energy budget number 240 W/m2 (received by the earth’s surface from short-wave solar radiation or long-wave radiation emitted by the earth to the space) without the atmosphere? 5. What is a selective absorber? Why is the atmosphere a selective absorber? 6. Is the atmosphere a blackbody? Why? 7. What is the earth’s radiative equilibrium temperature with atmosphere? 8. What is the difference in the equilibrium temperature between with and without the atmosphere? How does H2O and CO2 contribute this temperature difference, respectively? 9. Why do we worry CO2 more than H2O in the atmosphere? 10. Since earth’s energy budget is in equilibrium, why the global becomes warming? 11. What is solar constant? 12. What is latent heat? How is latent heat an important source of atmospheric energy? 13. How does the average speed of air molecules relate to the air temperature? 14. What are the three heat transfer mechanisms? What are examples of these? 15. What is a blackbody? 16. What is radiative equilibrium? 22
• 23
• How much heat is required for water temperature to raise 1°C?
• Specific Heat the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius An example: for water, it takes 1 calorie to raise the temperature of 1 gram of water by 1°C. So the specific heat for water is 1cal/gram/°C
• Specific heat of various substances Substance cal/g/oC Water 1.000 Ice 0.500 Soil 0.250 Air 0.250 Gold 0.031 Takes more heat, temperature rises a little Takes a little heat, temperature rises a lot Q: Which city during summer will exhibit the largest diurnal temperature variation? A. Los Angeles or Las Vegas? B. C. Denver, CO or NYC?
• 27
• Incoming SR Long-wave R Reflected SR Reflected SR Albedo = Incoming SR Albedo=0.9 Snow emit Absorbed by snow Snow is a poor absorber of solar radiation but a great absorber and therefore emitter of long-wave radiation
• Table 2-2, p. 41
• 30
• Q: When is the minimum temperature during a day? Q: When is the maximum temperature during a day? Outgoing LW radiation emitted by the earth should be similar to the daily temperature, why? because Stefan-Boltzmann law E=T4
• Q: Why there is a lag between maximum incoming SR and temperature? Q: What determines temperature variations? Net radiation  Net R Net R=Incoming SW- Outgoing LW if Net R >0, surface is warming if Net R <0, surface is cooling Why lag?