lecture7 sep25-bb (1)
TRANSCRIPT
1
Lecture 7Net radiation
Greenhouse effect Specific heat
AlbedoDaily temperature
Electromagnetic Spectrum
(m)
10001001010.10.01
Sun Earth
10001001010.10.01
Sun Earth
shortwave radiationIncoming shortwave radiation inE
longtwave radiationOutgoing longwave radiation outE
Net Radiation= incoming radiation - outgoing radiation
net in outR E E
5
Net radiation is positive
0net in outR E E
=360 W/m2=240 W/m2
inE outE
Is earth’s temperature increasing or decreasing?
6
Net radiation is negative
0net in outR E E
=240 W/m2
=360 W/m2
inEoutE
Is earth’s temperature increasing or decreasing?
7
Net radiation is zeroRadiative Equilibrium
0net in outR E E
The earth’s temperature is constant!
=240 W/m2 =240 W/m2
inE outE
8
Solar energy
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 asphere centered at the Sun with a distance d
Sd = L / (4 p d2) W/m2
sun
d
inE
9
Incoming Short-Wave Radiation
d
Earth
Solar Constant (S )The solar energy density at the mean distance of Earth from the sun (d =1.5 x 1011 m)
S = L / (4 π d2)= (3.9 x 1026 W) / [4 x 3.14 x (1.5 x 1011 m)2]= 1370 W/m2
Solar constant
inE
Solar energy reaching the earth
So = 1370 W/m2
pre2
Ein= (solar constant) x (shadow area)=So p re
2
re=the radius of the earthr e
inE
How much solar energy reaches the Earth?
Ein = So re2
BUT THIS IS NOT QUITE CORRECT!
**Some energy is reflected away**
Einre
inE
How much solar energy reaches the Earth?
Albedo (A) = % energy reflected away
Ein = So re2 (1-A)
A= 0.3 today
reEin
Eout
Calculation is not required on the test
outE
Energy Balance:
Ein = Eout
Ein = So re2 (1-A)
Eout = T4(4 re2)
Eout
Ein
Calculation is not required on the test
14
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
4 240 / 255K (-18 C)oeT
in outE E
Is the Earth’s surface really -18 oC?
15 oC
Average TeWith atmosphere +15 °C
-18 °CAverage Te
Without atmosphere
33 °C Greenhouseeffect
Greenhouse Effect
What causes the 33oC difference?
• 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
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
Ultraviolet Visible Infrared
Absorption of radiation by greenhouse gases
Rela
tive r
ad
iati
on
Wavelength (mm)
Sun6400oC
Earth-18oC
Absorption by water vapor
Absorption by carbon dioxide
Absorption by ozone
Atmospheric window
Total Greenhouse
Warming 33 °C
Others <1 °C
CO2 2 °C
H2O 31 °C
Q: If H2O is a more important greenhouse gas than CO2, why do we worry about CO2 rather than H2O emissions?
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
Heat transfer
iR
oR
At a given latitude
Ri – Ro = D
Ri = Incoming SW Radiation Ro = Outgoing LW Radiation D = Heat Transfer
22
Review Questions1. Wien’s law tells us what?2. Stefan-Boltzman law tells us what? 3. What is the earth’s radiative equilibrium temperature without atmosphere? 4. 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?
23
Specific Heat
How much heat is required for water temperature to
raise 1°C?
Specific Heatthe 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
Substance cal/g/oC
Water 1.000
Ice 0.500
Soil 0.250
Air 0.250
Gold 0.031
Specific heat of various substances
Q: Which city during summer will exhibit the largest diurnal temperature variation?
A.Los Angeles or Las Vegas?B.C. Denver, CO or NYC?
Takes more heat, temperature rises a little
Takes a little heat, temperature rises a lot
27
Albedo
Albedo =Incoming SR
Reflected SR
Albedothe fraction of solar energy reflected from
the Earth back into space Incoming SR
Reflected SR
Absorbed by snow
Snow
Albedo=0.9
emit
Lon
g-w
ave
R
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
Daily temperature
Daily Temperature VariationsQ: When is the minimum
temperature during a day?
Q: When is the maximum temperature during a day?
E=sT4
because Stefan-Boltzmann law
Outgoing LW radiation emitted by the earth should be similar to the daily temperature, why?
Net radiation
Whylag?
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 LWif Net R >0,
surface is warmingif Net R <0,
surface is cooling
+Net R
-Net R
-Net R
Similar reason for why the warmest period of a year is in July/August and not on June 21 (summer solstice)