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The incoming solar energy
Review of last lecture
• Ice age: a long time (millions to tens of millions of years)
of cold temperatures and extended ice cover. >5 in earth’s
history (including now). Develop slowly, but end more
• Glacial cycle: has a 100,000 year oscillation period
• Past abrupt climate change
• Tipping points
Energy basics
• Energy: the ability to do work
• Many forms: electrical, mechanical, thermal,
chemical, nuclear, …
• Joule (J): standard unit of energy (1 J=
0.239 calories)
• Watt (W): rate of energy flow (W = 1 J/s)
Methods of
Energy Transfer
• Conduction
– Molecule to molecule transfer
– Heat flow: warm to cold
– e.g. leather seats in a car
• Convection
– transferred by vertical movement
– physical mixing
– e.g. boiling water
• Radiation
– propagated without medium (i.e. vacuum)
– solar radiation provides nearly all energy
– The rest of this chapter deals with radiation
• Everything continually
emits radiation
• Transfers energy in waves
• Waves are both electrical
and magnetic, hence
electromagnetic radiation
• Travels at the speed of light
(300,000 km/s). It takes 8
minutes for light from the
Sun to reach Earth, and
4.3 years for light from the
next nearest star, Proxima
Centauri to reach us.
Radiation Quantity and Quality
• Quantity: how much? 
Wave height (Intensity).
• Quality: what kind? 
Wavelength: distance
between crest and crest (or
trough and trough).
generally reported in μm
(microns)- one millionth of
a meter.
The Electromagnetic Spectrum
The limitations of the
human eye!
A man detected by different
Physical Representation of Radiation
• Blackbodies: purely hypothetical bodies that
absorb and emit the maximum radiation at all
• The Earth and the sun are close to blackbodies.
What determines the quantity (intensity)?
The Stefan-Boltzmann Law
• States that the intensity of radiation emitted from a
blackbody is a function ONLY of temperature
where I is the intensity of the radiation, T is the
temperature in K, and  is the Stefan-Boltzmann
constant, 5.67 x 10-8 W m-2 K-4)
• So, hotter bodies emit more energy than colder
bodies (double T, 16x more radiation)
Earth (290K) I = 401 Wm-2
Sun (6000K) I = 7.3 x 106 Wm-2.
So ISun >> Iearth
What determines the quality (wavelength)?
The Wein’s Law
• States that the peak wavelength of radiation
emitted from a blackbody is a function ONLY of
max = b/T
where max is the peak wavelength of the radiation,
T is the temperature in K, and b is the Wein’s
displacement constant (2.90 x 10-3 m K)
• So, hotter bodies emit at shorter wavelengths
Double T, half wavelength
Earth (290K) max = 10 μm
Sun (6000K) max = 0.5 μm
Wavelength of Sun and Earth Radiation
Sun =
Peak 0.5 μm
Earth =
(4-100 μm)
Peak 10 μm
Composition of sunlight
The rainbows
The Sun also emits high energy particles:
Northern light (Aurora)
• Caused by electrons
and protons
• They can only
penetrate into the
atmosphere from
the magnetic north
pole and south pole
• They collide with
the atmospheric
molecules to
generate the aurora
Video: The solar cycle
The sunspot number A good index for solar activity
• Sunspots are temporary
phenomena on the
surface of the sun that
appear visibly as dark
• Caused by intense
magnetic activity
• The number of sunspots
changes significantly
between solar maximum
and solar minimum
The solar cycle
• is the periodic change in the sun's activity (including
changes in the levels of solar radiation and ejection of solar
material) and appearance (visible in changes in the number
of sunspots, flares, and other visible manifestations).
• has a duration of about 11 years.
• has been observed (by changes in the sun's appearance
and by changes seen on Earth, such as auroras) for
hundreds of years.
The solar cycle and earth’s climate
Change of precipitation associated with solar maximum
Red-Increase, Blue-decrease (from Meehl et al. 1999, Science)
Solar cycle and US air temperature
Solar cycle and grape production
in France
What is energy? 3 methods of energy
The names of the 6 wavelength categories in
the electromagnetic radiation spectrum
Intensity of radiation (Stefan-Boltzman law):
Wavelength of radiation (Wein’s law):
The wavelength range of Sun (shortwave)
and Earth (longwave) radition
The 11-year solar cycle
max = b/T
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