1. 1

2. Electromagnetic Spectrum
Sun
0.01
0.1
Earth
1
 (m)
10
100
1000

3. Incoming shortwave radiation
Sun
0.01
0.1
 E in
Earth
1
10
Outgoing longtwave radiation
longwave radiation
100
 E out
1000

4. Net Radiation
= incoming radiation - outgoing radiation
R net  E in  E out

5. Net radiation is positive
R net  E in  E out  0
E in
E out
Is earth’s temperature increasing or decreasing?
5

6. Net radiation is negative
R net  E in  E out  0
E in
E out
Is earth’s temperature increasing or decreasing?
6

7. 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

8. 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

9. 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

10. E in
Solar energy reaching the earth
So = 1370 W/m2
Ein= (solar constant) x (shadow area)
=So p re2

11. 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

12. 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

13. Energy Balance:
Ein = Eout
Ein = So p re2 (1-A)
Eout =  T4(4 p re2)
Eout
Ein
Calculation is not required on the test

14. 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

15. Is the Earth’s surface really -18 oC?
oC
15

16. Greenhouse Effect
Average Te
With atmosphere
+15 °C
33 °C
Average Te
Without atmosphere
-18 °C
Greenhouse
effect

17. 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

18. 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

19. Others <1 °C
CO2 2 °C
Total Greenhouse
Warming 33
°C
H2O 31 °C

20. 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

21. 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

22. 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. 23

24. How much heat is required for water
temperature to raise 1°C?

25. 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

26. 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. 27

28. 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

29. Table 2-2, p. 41

30. 30

31. 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

32. 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?