SOLAR ENERGY
► Solar energy is
transmitted to earth in the
form of short and long
wave (SW and LW)
radiation.
SW is light.
LW is heat.
Incoming Solar Radiation, or
INSOLATION is the energy
which drives the atmospheric
system.
Wien’s Law: lmax = C/T
Where l is the wavelength of most intense radiation,
C = 2897 mm/K and T is the temperature in Kelvins (K)
Radiation Basics
The sun emits all types of radiation. Anything above 700 nm is Long Wave
(LW). Below 700 nm is Short Wave (SW).
SW
LW
lmax = C/T
l = 2897mm K /288 K = 10mm
1 mm = 1000 nm
lmax = C/T
l = 2897mm/K /6100 K  0.5mm
Stefan-Boltzmann’s Law
E = sT4
where E is the energy emitted,
s is a constant ( ),
T is the temperature in Kelvins (K) = 5777 K
(http://solarsystem.nasa.gov/planets/sun/facts)
This works for perfect emitters (blackbodies) like the sun.
E = 5.67 x 10-8 W m-2 K-4 x (5777 K)4
= 6.315 x 107 W m-2
E is the area under the curve
Surface area = 6.08 x 1018 m2
Total Energy = 3.84 x 1026 Watts
This is what is emitted by
the sun. But at the surface
of the Earth that is not how
much energy is absorbed.
Solar radiation spreads out evenly in all directions
Not to scale, of course
So, the total radiation from E = sT4 gets spread out over a sphere with the radius of
the Earth’s orbit. What is the energy/unit area at the Earth’s mean orbital radius?
The orbit is not a circle but the mean radius is 1.495 x 1011 m
Area of a sphere = 4r2
Ans: Area = 3.84 x 1026 W/ 2.81 x 1023 m2 = 1367 W m-2 = 1.98 cal cm-2 min-1
What happens to the insolation when it gets to the Earth?
A given amount of
radiation covers a
smaller area when
overhead than when at
a low angle; it is more
concentrated
In addition to spreading out at low sun angles, the
insolation must travel through more atmosphere
The low sun (near sunset) shows more red
due to atmospheric dust and pollution.
More of the SW radiation is being absorbed
and scattered away.
Seasonal effects on sun angle and insolation
91,000,000 miles
Aphelion
94,000,000 miles
Perihelion
The time of year causes the sun’s angle in the sky and the amount of daylight
Depending on your latitude, the sun’s trajectories across the sky are
very different
Lengths of day and night
vary more between the
seasons at higher
latitudes. This makes
climate more seasonal at
the poles than the equator
DAY LENGTH
Six months daytime (March-Sept), six
months night (Sept - March)
North Pole 90°N
Arctic circle 66.5°N
More
seasonal
One day with 24 hours daylight
(June 21st); one day with 24 hours
darkness (Dec 21st)
Sun is overhead once a year
(June 21st). Day length always at
least 10 hours.
Tropic of Cancer
23.5°N
Constant day length - 12 hours day
and night all year round
Equator 0°
Tropic of Capricorn
23.5°N
More
seasonal
Sun is overhead once a year
(Dec 21st). Day length always at
least 10 hours.
Antarctic circle 66.5°S
South Pole 90°S
One day with 24 hours daylight
(March 21st); one day with 24
hours darkness (June 21st)
Six months daytime (Sept - March), six
months night (March-Sept)
Radiation Exercise (Practice)
You are marooned on a tropical island somewhere in the Pacific Ocean. You
don’t know where. You have built a boat to escape but you only have food and
fresh water for 2 days so you must get to land in that time or die. So you need to
know where you are now to decide the right direction to go.
Like the movie “Castaway”, you have a watch always set to Memphis time. On
the island, you have no tools, no compass, no telescope. You have the sun.
This is your island (it’s the one from Castaway)
Procedure
Split into groups of 3-4. Brainstorm ideas of what you could do. For now,
you don’t have to do any calculations. You must, however, come up with
a plan which may involve calculations. You want to know as precisely as
you can, where you are and what direction to go once you are in the boat
on the ocean.
After 15-20 minutes, each group presents their plan. The most workable
plan (as determined by your professor) will be the one the class uses.
Once you have the plan, an assignment will be made to actually put the
plan into action to determine your location and a suitable direction.
You have until Feb 2 to execute the plan, make your determinations, and
write it up scientifically, i.e., include a statement of the problem, your
method for solving it, your solution, and final conclusions. Keep this to 12 pages.
Energy must be transferred from Equator to Poles
Global Energy Flux
Polewards of 40°N and S, there is an annual heat deficit.
Equatorwards of 40°N and S, there is an annual heat
surplus.
Without movement of heat energy, the poles would
become steadily colder and colder, while the equator
would get progressively warmer. This clearly does not
happen.
This heat transfer (flux) occurs by:
Ocean currents; cold polar water flows towards the
equator while warm water flows from equator to pole.
Air mass movements and storms which blow warm air
towards the poles and cold air towards the equator.
Also, winds transport poleward water vapor which
releases latent heat upon condensing in the cooler air.
An excess of evaporation which takes up and stores
latent heat in water vapor, releasing it towards the pole
where it condenses.
OCEAN CURRENTS
Labrador current carries
cold water from equator
to pole down east coast of
N.America
Cold
Sea temperature
A similar counter-clockwise movement of
warm water polewards, and cold water
equatorwards can be seen in the Pacific.
Gulf Stream
carries warm
water from
equator towards
the pole, and NW
Europe
Warm
Surface currents generally do not cross the
Equator.
The surface currents connect with deep ocean currents to form the
thermohaline circulation.
From http://www.ncdc.noaa.gov/paleo/ctl/thc.html
Air Masses
Heat Transfer by Humidity
POLES - Precipitation exceeds evaporation
at high latitudes; condensation releases
latent heat stored in water vapor.
EQUATOR Evaporation exceeds
condensation (and
precipitation); this uses
heat energy and stores it
in the form of water
vapor. Red shows high
humidity from high
rates of evaporation
Cold, north winds blowing south.
Warm south winds blowing north.
A typical situation that helps correct the energy imbalance.
Winds
Winds
Winds
Jan 20-22, 2014
Energy Budget
What comes in
versus what goes out.
Having an atmosphere introduces clouds, reflection, and scattering.
Albedo is reflection.
Low albedo is not
very reflective.
Reflection is pretty
intuitive but
albedoes are not
always obvious (see
water)
As a whole, the Earthatmosphere-ocean has
an albedo of 30%
Scattering is molecular. Unlike reflection, it happens in all
directions, even forward.
Absorption is just what it sounds like. The radiation is absorbed. But
after that, the absorbing body’s temperature rises so it emits radiation in
accordance with the Stefan-Boltzmann and Wien’s laws.
The Earth’s Energy Budget is Balanced
(not always true for people)
Next:
How does radiation lead to
temperature climatology?