AOS 100: Weather and
Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell
Miscellaneous
• Homework Reminder
• Exam Reminder
Review of September 17th: Radiation
• Radiation is the one form of heat transfer
that does not require a medium (air, water,
etc.)
• The energy of radiation is carried in the
form of electromagnetic waves
• All objects emit/receive radiation
• As objects emit radiation, they lose energy
and cool (unless they are gaining more
radiation from absorption or some other
process)
Review Continued
• Wien’s Law: the wavelength of radiation an
object emits depends on its temperature
• The peak wavelength (λ) of emission is
approximately ~ 2897/ T
• Stefan-Boltzmann Law: warmer objects emit
more energetic radiation than do colder objects,
by E=σ*T4
• Shorter wavelengths carry more energy
• The albedo of an object is a measure of the
amount of sunlight it reflects
• The Earth’s average albedo is .3, meaning it
reflects 30% of incoming solar radiation on
average
A Comparison of Emittances
From:www.csulb.edu/~rodrigue/geog140/
sunwavelength.gif
• The atmosphere
absorbs some
radiation
• However, this
doesn’t happen
uniformly for all
wavelengths
• Since the Sun’s
radiation and the
Earth’s radiation are
almost completely
separate
wavelengths, these
variations are
important!
• Much more of the
Earth’s radiation is
absorbed by the
atmosphere than the
Sun’s
Kirchoff’s Law
• Kirchoff’s Law: The efficiency of absorption at a certain
wavelength of energy is exactly equal to the efficiency of
emission in the same wavelength
• For example, snow is very good at absorbing IR
radiation, which means it is also very good at emitting IR
radiation
• If something is perfect at absorbing all wavelengths of
radiation (and thus perfect at emitting all wavelengths),
then we call it a blackbody
• The Earth is a close approximation to a blackbody (for a
given temperature, it will emit as much radiation as
possible for that temperature)
• This means Wien’s Law and Stefan-Boltzmann’s Law
can be used with good accuracy for the Earth
Stefan-Boltzmann Law Revisited
• Energy=σT4
• Let’s compare the Earth and Sun:
The Earth’s temperature is ~288 K
E=5.67*10-8*(288)4
E=390 Watts/m2
The Sun’s temperature is ~6000 K
E=5.67*10-8*(6000)4
E=73,483,200 Watts/m2
This means the Sun’s
surface emits roughly
190,000 times more
energy from every
square meter than the
Earth does
The Earth’s Energy Balance
• Of the radiation emitted from the Sun, the
amount that makes it to the Earth is
approximately 342 W/m2 at any given spot at the
top of the atmosphere
• But remember we need to factor in the albedo,
which is .3, so 30% of this is reflected to space
342*.7 = 239.2 W/m2
Using E=σ*T4, we get
239.2 = 5.67x10-8 * T4
Solving for T gets about 255 K
Energy Balance Continued
• This calculation gives us an average surface
temperature of 255 K
• 255 K is equal to about -18 ºC, or about 0 ºF
• But we know our Earth’s temperature is much
warmer!
• (more than 50 ºF warmer!)
• What’s going on then?
We have an atmosphere!
• Our atmosphere absorbs more longwave radiation
(the kind that the Earth emits) than it does
shortwave radiation (the kind the Sun emits)
Energy Balance Continued
• Our atmosphere emits the absorbed radiation in
all directions
• Therefore, half of it goes back towards the
surface and half goes out to space
• So the extra bit that the atmosphere is
continuously sending the surface keeps it
warmer
• The actual average temperature is about 288 K
(about 15 ºC or 59 ºF)
• Radiative equilibrium: the state where the rate
an object emits radiation is equal to the rate the
object absorbs radiation
• Due to the
Earth’s tilt,
different
locations
receive different
intensities of
sunlight
• This leads to
both differing
daytime
temperatures
and the different
seasons
The Seasons (Intro)
Diurnal Temperature Change
• Obviously, the Earth is only receiving
(shortwave) solar radiation when the Sun is
above
• However, the Earth is always transmitting
(longwave) radiation
• This means that the surface is often losing
energy at nighttime, and gaining it during the
daytime
• This is largely what causes temperature
changes from day to night, in addition to other
things such as warm or cold air advection
What can modify this?