Monday Feb. 27, 2012
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A couple of Roy Orbison songs before class today. The first
was "Mean Woman
Blues" and was followed by "Pretty Woman".
I
should probably have showed the two videos in class.
An optional assignment from early February, the upper-level charts
Optional Assignment, and the in-class Optional Assignment from last
Friday have all been graded and were returned today. We're still
working on the Surface Weather Map Analysis 1S1P assignment.
The Experiment #2 reports were collected today. If everything
goes according to plan, the Experiment #3 materials will be distributed
in class on Friday.
Class started with a demonstration of the two rules governing the
amount and kind of EM radiation emitted by an object. The rules
are
There was a question about why a lower case greek letter is used
in the
Stefan-Boltzmann law (σ = sigma) while an upper case lambda
is
used
in Wien's Law (λ = lambda). While walking back to my office
after class
I
got to thinking about greek characters and realized that of the greek
letters above were lower
case. The upper cases for both are shown below.
You'll find a little more information about Greek characters at
the end of today's notes.
OK back to the demonstration. It consisted of an
ordinary 200 W tungsten bulb is connected to a dimmer switch (see p. 66
in the photocopied
ClassNotes). We'll be looking at the EM radiation emitted by the
bulb
filament.
The graph at the bottom of p. 66 has been split up into 3 parts
and
redrawn for improved clarity.
We start with the bulb turned off (Setting 0).
The
filament will be at room temperature which we will assume is around 300
K (remember that is a reasonable and easy to remember value for the
average temperature of the earth's surface). The bulb will be
emitting radiation, it's shown on the top graph above. The
radiation is very weak so we
can't
feel it. The wavelength of peak emission is 10
micrometers which is long wavelength, far IR, radiation so
we
can't see it.
Next we use the dimmer switch to just barely turn the bulb on (the
temperature of the filament is now about 900 K).
The bulb wasn't very bright at all and had an orange color. This
is curve 1, the middle figure. Note the far left end of the
emission curve has
moved left of the 0.7 micrometer mark - into the visible portion of the
spectrum. That is what you were able to see, just the small
fraction of
the radiation emitted by the bulb that is visible light (but just
long wavelength red and orange light). Most of the radiation
emitted by the bulb is to the right of the 0.7 micrometer mark and is
invisible IR radiation (it is strong enough now that you could feel it
if you put your hand next to the bulb).
Finally we turn on the bulb completely (it was a 200 Watt bulb so
it
got
pretty bright). The filament temperature
is now about 3000K. The bulb is emitting a lot more visible
light, all the colors, though not all in equal amounts. The
mixture of the colors produces a "warm
white" light. It is warm because it is a mixture that contains a
lot more red, orange, and yellow than blue, green, and violet
light. It is interesting that most of the radiation emitted by
the bulb is still in the IR portion of the spectrum (lambda max is 1
micrometer). This is
invisible light. A tungsten bulb like this is not especially
efficient, at least not as a source of visible light.
You were able to use one of the diffraction gratings
handed out in class to
separate the
white light produced by the bulb into its separate colors.
When you looked at the bright white bulb filament through one of
the
diffraction gratings the colors were smeared out to the right and left
as shown below.
Some of the gratings handed out in
class behaved a little
differently
and spread out the colors horizontally, vertically, and diagonally.
I'm going
to change the order in which the material was presented a little
bit.
Let's look at the light emitted by
the sun and the earth.
The curve on the left is for the sun. We have used Wien's
law and a temperature of 6000 K to calculate λmax
and got
0.5 micrometers. This is green light; the sun emits more green
light than any other kind of
light. The sun doesn't appear green because it is also emitting
lesser amounts of violet, blue, yellow, orange, and red - together this
mix of
colors appears white. 44% of the radiation emitted by the sun is
visible light, Very nearly half of sunlight (49%) is IR light
(37% near IR + 12% far IR). 7% of sunlight is ultraviolet
light. More than half of the light emitted by the
sun (the IR and UV light) is invisible.
100% of the light emitted by the earth (temperature = 300 K) is
invisible IR light. The
wavelength of peak emission for the earth is 10 micrometers.
Because the sun (surface of the
sun) is 20 times hotter than the earth the sun's
surface emits energy at a much higher rate than the earth. Note
the
vertical
scale
on
the
earth
curve
is different than on the sun
graph. If both the earth and sun were plotted with the same
vertical scale, the earth curve would be too small to be seen.
In the demonstration at the start of class we also learned that
ordinary tungsten bulbs (incandescent
bulbs) produce a lot of
wasted energy. This is because they emit a lot of invisible
infrared light that doesn't light up a room (it will warm up a room but
there are better ways of doing that). The light that they do
produce is a warm white color (tungsten bulbs emit lots of orange, red,
and yellow light and not much blue, green or violet).
Energy
efficient
compact
fluorescent
lamps
(CFLs)
are
being
touted
as
an
ecological
alternative
to
tungsten bulbs because
they use substantially less electricity, don't emit a lot of
wasted infrared light, and also last
longer. CFLs come with
different color temperature ratings.
The bulb with the hottest
temperature rating (5500 K ) in the figure
above is meant to mimic or simulate sunlight (daylight). The
temperature of
the sun is 6000 K and lambda max is 0.5 micrometers. The spectrum
of the 5500 K bulb is similar.
The tungsten bulb (3000 K) and the CFLs with temperature ratings
of
3500 K and 2700 K produce a warmer white.
Three CFLs with the temperature ratings above were set up in class
so
that you could see the difference between warm and cool white
light. Personally I find the 2700 K bulb "too warm," it makes a
room
seem gloomy and depressing (a student in class once said the
light resembles Tucson at night). The 5500 K bulb is "too cool"
and
creates
a stark sterile atmosphere like you might see in a
hospital corridor. I prefer the 3500 K bulb in the
middle.
The figure below is from an article
on compact fluorescent lamps in Wikipedia for those of you that weren't
in class and didn't see the bulb display. You can
see a clear difference between the cool white bulb on the left
in the figure below and the warm white light produced by a tungsten
bulb (2nd from the left) and 2 CFCs with low temperature ratings (the 2
bulbs at right).
There is one downside to these energy efficient CFLs. The
bulbs
shouldn't just be discarded in your ordinary household trash because
they contain mercury. They should be disposed of properly (at a
hazardous materials collection site or perhaps at the store where they
were purchased).
It probably won't be long before LED bulbs begin to
replace tungsten and CFL bulbs. At the present time the LED bulbs
are pretty expensive.
We now
have most of the tools we will need to begin to study energy balance on
the earth. It will be a balance between incoming sunlight
energy and outgoing energy emitted by the earth. This will
ultimately lead us to an explanation of the atmospheric greenhouse
effect.
We will first look at the simplest kind of situation, the earth without
an atmosphere (or at least
an atmosphere without greenhouse gases). The next figure is on p.
68 in the
photocopied Classnotes. Radiative equilibrium is really just
balance between incoming and outgoing radiant energy. Pages 68
and 69 were somehow left out of the photocopied ClassNotes.
Copies of these two pages were handed out in class.
You might first wonder how it is possible for the earth (with a
temperature
of around 300 K) to be in energy
balance with the sun (6000 K). At the top right of the figure you
can see that because the earth is located about 90
million miles
from the sun and it only absorbs a very small fraction of the
total energy emitted by the sun.
To understand how energy balance occurs we start, in Step #1, by
imagining that the earth starts out very cold (0 K) and is
not emitting
any EM radiation at all. It is absorbing sunlight however (4
of
the
6
arrows
of
incoming
sunlight in the first picture are absorbed, 2 of the
6 are being reflected) so it
will
begin to warm This is like opening a bank account, the balance
will be zero at first. But then you start making deposits and the
balance
starts to grow.
Once the earth starts to warm it will also begin to emit EM
radiation, though not as much as it is getting from the sun (the
slightly warmer earth in the middle picture is now colored blue).
Only the four arrows of incoming sunlight that are absorbed are shown
in the
middle figure. The two arrows of reflected sunlight have been
left off because they don't really play a role in energy balance (reflected
sunlight
is
like
a check
that bounces - it really doesn't affect your bank account
balance). The earth is emitting 3 arrows of IR light
(in red). Because the earth is still gaining more
energy than it is losing the
earth will warm some more. Once you
find money in your bank account you start to spend it. But as
long as deposits are greater than the withdrawals the balance will grow.
Eventually it will warm enough that the earth (now shaded brown
& blue)
will
emit the same amount
of energy as it absorbs from
the sun. This is radiative equilibrium, energy balance (4 arrows
of absorbed energy are balanced by 4 arrows of emitted energy).
The
temperature at
which this occurs is about 0 F.
That is called the temperature of radiative equilibrium (it's about 0 F
for the earth).
Note that it is the amounts of energy not the kinds of energy that
are important. Emitted radiation may have a different wavelength
than
the absorbed energy. That doesn't matter. As long as the
amounts are
energy the earth will be in energy balance.
Before we
start to look at radiant energy balance on the earth with an atmosphere
we
need to learn about filters. The atmosphere will filter sunlight
as it
passes through the atmosphere toward the ground. The atmosphere
will
also filter IR radiation emitted by the earth as it trys to travel into
space.
We will first look at the effects simple blue, green, and red glass
filters have on visible light. This is just to be able
to interpret a filter absorption curve or graph.
If you try to
shine white light (a
mixture of all the colors) through a
blue filter, only the blue light passes through. The filter
absorption curve shows 100% absorption at all but a narrow range of
wavelengths that correspond to blue light. The location of the
slot or gap in the absorption curve shifts a little bit with the green
and red filters.
The following figure is a simplified, easier to
remember,
representation of the
filtering effect of
the atmosphere on UV, VIS, and IR light (found on p. 69 in the
photocopied notes). The figure was redrawn after class.
You can use your own eyes to tell
you what the filtering
effect of the
atmosphere is on visible light. Air is clear, it is
transparent. The atmosphere transmits visible light.
In our simplified representation oxygen and ozone make the
atmosphere pretty nearly completely opaque to UV light (opaque is the
opposite of transparent and means
that light is blocked or absorbed; light can't pass through an opaque
material). We
assume that the
atmosphere absorbs all incoming UV light, none of it makes it to the
ground. This is of course not entirely realistic.
Greenhouse gases make the
atmosphere a
selective absorber of IR light - the air absorbs certain IR wavelengths
and
transmits others. It is the atmosphere's ability to absorb (and
also emit) certain wavelengths of infrared light that produces the
greenhouse effect and warms the surface of the earth.
Note "The atmospheric window"
centered at 10 micrometers. Light emitted by the earth
at this
wavelength (and remember 10 um is the wavelength of peak emission for
the earth) will pass through the atmosphere. Another transparent
region, another window, is found in the visible part of the spectrum.
Neither of these
windows was mentioned in class on Monday.
You never know where what you learn in ATMO 170A1 (and in other
math and science classes) might turn up.
If you're ever traveling around Greece by bus (as I did some time ago
with a friend) it will be helpful to be able to figure out the names of
Greek cities so that you can be sure you're getting on the right bus.
For example, see if you can figure out what the following 4 Greek city
names are.
When you think you got them figured
out, click here
