Solar radiation
Sun, climate and the greenhouse effect
• Need to understand the physics before we can
understand the debate
• How can the sun influence climate?
• What role does the sun play in the energy balance
of the earth?
• What is light?
• What is energy balance?
• And what is the greenhouse effect?
Learning goals
• The Sun in the primary source of energy for the climate system
– What is light?
– What is radiation?
– What is flux?
• Energy flux decreases with distance from the sun
• Energy flux depends on area, therefore decreases with latitude
• Days, seasons, and latitude depend on geometry
• Units of energy flux: energy per unit area per second.
Watts per meter squared: W/m2
The climate system
Insolation
(INcoming SOLar radiATION)
Atmosphere
Solid earth
Energy coming in and
going out via radiation.
Biota
Ein = Eout
Hydrosphere
(ocean, etc)
Light
Can describe light as a wave (mostly in this class)
or as a collection of particles called photons
(a littlie bit in this class)
• Wave length
• Wave speed
Radiation and light.
• Electromagnetic radiation
1 billion nanometers is 1 meter!
Corona
Temperature of suns surface is ~6000K
Today
UV
171A
2 million K
(iron)
195A
1.5 million K
(iron)
284A
1 million K
(iron)
304A
70000 K
(helium)
The solar constant
• Amount of insolation at the top of the atmosphere.
S0 = 1361 W/m2
• (notice old books says 1370 W/m2!)
• About 13 100W incandescent lights
• About 52 equivalent brightness (26 W)
compact fluorescent bulbs
• About 100 equivalent LED lights!
U. Colorado, research in LASP
measures the solar constant!
FLUX: definition
• Notice we say the solar constant is amount of energy per unit area
(i.e., 1361 W/m2)
• While we have the energy given in Watts
1 Watt is 1 Joule per second J/s, and the “Joule” is a measure of energy)
• Consider a snickers bar. It contains 1133 Joules of energy
• In Duane GD120 there is about 1 person each meter square.
So the solar constant is about the same as
everyone eating a snickers bar every second!
• Flux is amount of energy per unit area…. So the area matters!
How does solar flux vary on Earth?
Scientific experiment to test
Let’s use the scientific method:
1. Hypothesis
2. Experiment
3. Observation
4. Conclusions
5. Theory, and leads to “Law”
How does flux change with distance
from emitter?
Planet two times further from sun, how much energy flux?
Sun
Earth
“Fars”
1 AU
2 AU
(Often refer to this sun-Earth distance as an astronomical unit, AU)
For a planet twice the distance from the sun
how much solar radiation would there be:
what is the solar constant on “Fars”
Solar constant for earth is 1361 W/m2
A.
B.
C.
D.
E.
340.25 W/m2
680.5 W/m2
1361 W/m2
2722 W/m2
5444 W/m2
Experiment 1
Inverse square law
The flux DECREASES as the
distance squared!
S1 = 1361 W/m2 (solar constant)
 d1 
S 2 = S1  
 d2 
2
S1=1
2
1
1
S 2 = S1   = 1361  = 340.25 W/m 2
4
2
S2=1/4
S3=1/9
Scientific experiment to test
Let’s use the scientific method:
1. Hypothesis
2. Experiment
3. Observation
4. Conclusions
5. Theory, and leads to “Law”
Other ways to change area?
How to change the area?
Consider the flux of energy from a light bulb
1)
2)
Light shines on some
area of a board
When tilted, light
shines on a larger area
of a board
Same amount of energy (same light bulb) …
but more area, how does this change the flux?
(Light appears dimmer on the board)
What is the energy flux on at the top of the
atmosphere at 45N?
Solar constant for earth is 1361 W/m2
A.
B.
C.
D.
E.
0 W/m2 (totally dark!)
681 W/m2 (half)
953 W/m2 (more than half)
1361 W/m2 (same as at the equator)
1909 W/m2 (less than double)
Experiment 2
Cosine dependence
Consider the flux of energy from a light bulb
Cosine
90
latitude
When tilted, light
shines on a larger area
of a board
Same amount of energy (same light bulb) …
but more area, means flux decreases
(Light appears dimmer on the board)
NASA SOHO mission
Solar and Heliospheric Observatory
http://sohowww.nascom.nasa.gov/
SINCE 1980, the effects of CO2 and other greenhouse
gases have dominated.
Different frequency of radio stations
KBCO
Could ask what is the wavelength of each radio station?
KBCO is 97.3 megahertz.
i.e., frequency is υ =97.3 million waves per second
λ=
c
ν
300000000m / s
=
97300000 / s
= 3.1 meters!
Get an antenna this size
for best reception!
Speed of light
• The speed of light is 300000000 m/s
Einstein tells us this is constant!
• Around the world in 0.14 seconds!
Consider an email going to the other side of the
world – 0.07 seconds!
• Compared to transmission from earth to a
communication satellite (about 64 thousand km
above the ground), bounced to another satellite,
and back down. Time = 0.5 seconds!
• This is why the internet (and phone calls) do
not use satellites any more!
Inverse square law
Intensity depends on distance from
energy source
Since area increase as the distance
squared.
The flux DECREASES as the
distance squared!
 d1 
S 2 = S1  
 d2 
2
S1=1
S2=1/4
S3=1/9
S1 = 1361 W/m2 (solar constant)
Final answer?
For a planet twice the distance from the sun
how much solar radiation would there be:
what is the solar constant on “Fars”
Solar constant for earth is 1361 W/m2
A.
B.
C.
D.
E.
340.25 W/m2
680.5 W/m2
1361 W/m2
2722 W/m2
5444 W/m2
Difference in solar energy flux with latitude
• Solar energy flux at the is 1363 W/m2
1370 W/m2
685 W/m2
North pole
cosine (90) = 0
cosine (60) = 0.5
cosine (45) = 0.7
1370 W/m2
equator
1m
cosine (0) = 1
1370 W/m2
South pole
Earth surface area
intercepting light
depends on latitude.
COSINE of latitude
Final answer?
What is the energy flux on at the top of the
atmosphere at 45N?
Solar constant for earth is 1361 W/m2
A.
B.
C.
D.
E.
0 W/m2 (totally dark!)
681 W/m2 (half)
953 W/m2 (more than half)
1361 W/m2 (same as at the equator)
1909 W/m2 (less than double)
Hint: cosine (45) = 0.7
Key points
• Solar radiation provides incoming energy to the climate system
• (Radiation going out from earth is a loss of energy)
• Energy flux is a measure of how much energy for each meter square of
area for each second.
• This allows us to know how much energy is coming into the climate
system
• Energy flux depends on the AREA over which the energy is spread.
– Decreases in the area is tilted
– Decreases with the distance from the emitter squared
• What happens to this energy normally, and what happened with
humid modification of climate?
Homework!