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)