Tuesday Mar. 11, 2008
Quiz #2 is Thursday this week. The Quiz #2 Study Guide is still available
online.
The 3rd Optional Assignment was collected in class today. A sheet
with answers to the questions was distributed in class.
You'll find answers to previous "in-class" and "hidden" optional
assignments online (go back to the class home page).
The Expt. #2 reports have been graded (after a superhuman effort) and
were returned in class. You are allowed to revise your
reports. The
revised reports are due on or before Thu., Mar. 27 (the Thursday after
Spring Break).
The Experiment #4 materials were also
handed out in class. There are
still several sets of Expt. #4 materials (and some Expt. #3 materials)
available. They'll be back in class on Thursday. You should
already
have done an experiment or be working on an experiment (or book
report). If you haven't you should check out the necessary
materials
this week.
Don't forget that the 1S1P
Assignment #2
reports (and optional worksheets) are due this week. If you don't
turn
in your report in class you will need to bring it by my office (PAS
588) by the end of the day on Friday.
We'll use
our simplified representation of radiative equilibrium
to understand enhancement of the greenhouse effect and global warming.
The figure (p. 72c in the photocopied Class Notes) on the
left
shows
energy balance on the earth
without
an atmosphere (or with an atmosphere that doesn't contain greenhouse
gases). The ground achieves energy balance by emitting only 2
units of
energy to balance out what it is getting from the sun. The ground
wouldn't need to be
very warm to do this.
If you add an atmosphere and greenhouse gases, the atmosphere will
begin to absorb some of the outgoing IR radiation. The atmosphere
will
also begin to emit IR radiation, upward into space and downard toward
the ground. After a period of adjustment you end up with a new
energy
balance. The ground is warmer and is now emitting 3 units of
energy
even though it is only getting 2 units from the sun. It can do
this
because it gets a unit of energy from the atmosphere.
In the right figure the concentration of greenhouse gases has increased
even more (due to human activities). The earth would find a new
energy
balance. In this case the ground would be warmer and would be
emitting
4 units of energy, but still only getting 2 units from the sun.
With
more greenhouse gases, the atmosphere is now able to absorb 3
units of the IR emitted by the ground. The atmosphere sends 2
back to the ground and 1 up into space.
The next figure shows a common misconception about the cause of global
warming.
Many people know that sunlight contains UV light and that
the ozone
absorbs much of the dangerous type of high energy radiation.
People also know that release of chemicals such as CFCs are destroying
stratospheric ozone and letting some of this UV light reach the
ground. That is all
correct.
They then conclude that it is
this additional UV energy reaching the ground that is causing the globe
to warm. This
is not correct. There isn't much UV light in sunlight in
the
first place and the small amount of additional UV light reaching the
ground won't be enough to cause global warming. It will cause
cataracts and skin cancer and those kinds of problems but not global
warming.
Next up in
NATS 101 - Causes of the seasons
First some very basic information (that every college graduate should
know)
Many people would have missed the 3rd question. Many
people think
the moon orbits the earth in about a day. This is because they
see it in about the same position in the sky on successive
nights. We can see what actually happens in the next figure (not shown in class)
On the
first night in Fig. A the person looks up and sees the moon. One
day later on
night B, the earth has completed one rotation on its axis and the
person
is looking up at the same point in space. The person doesn't see
the moon in the same position as the night before; the moon has moved a
little bit in its orbit. In Fig. C, a little more than 24 hours
after Fig. A, the person again sees the moon overhead. If you
were to make a note of the time the moon rises you would notice it
rises a little later each successive night.
Many people know that the earth's orbit around the sun is
not circular and that the distance between the earth and sun changes
during the year. Many people think this is the main cause of the
seasons. The earth is closest to the sun on the perihelion,
furthest on the apehelion.
The earth is closer to the sun in January than in
July. If
this were the main cause of the seasons, summer in Tucson would be in
January and winter would be in July. Summer and winter would both
occur at the same times in both hemispheres. Neither of these is
true. The changing
distance
between the earth and the sun has an effect but is not the main cause
of seasonal changes.
The main cause of the seasons is the fact that the earth is tilted with
respect to its orbit around the sun. This is shown in the next
figure.
This figure shows the tilted earth at four locations in its
orbit around the sun. You should be able to start with a blank
sheet of paper and
draw a
picture like this. Note how the N. Pole tilts away
from the sun
on Dec. 21st, the winter solstice. The N. Pole is tilted toward
the sun on June 21. Those are good places for you to
start your sketch. You should also be able to name and attach a
date to each of the four locations.
Before going on, try to imagine what this picture would like if instead
of standing at Point A you
moved to the other side of the scene and looked back toward the
sun from Point B. This
possibility wasn't covered in class. Click here
for a
sketch.
Seasons on the earth are caused by the changing orientation of the
earth relative to the sun. The figure above doesn't really
explain why this is true.
In the summer when the sun reaches a high elevation angle
above the
horizon, an incoming beam of sunlight will shine on a small area of
ground. The ground will get hot. The two people sharing the
shaft of summer sunlight will get a sunburn.
In the winter the sun is
lower in the sky. The same beam of sunlight gets spread out over
a larger area. The energy is being used to try heat a larger
amount of ground. The result is the the ground won't get as
hot. 4 people are able to share the winter sunlight and won't get
burned as quickly.
As sunlight passes through the atmosphere it can be absorbed
or
reflected. On average (over the globe) only about 50% of the
sunlight arriving at
the top of the atmosphere actually makes it to the ground. A beam
of sunlight that travels through the atmosphere at a low angle (right
picture above) is less intense than beam that passes through the
atmosphere more directly (left picture).
The sun shines for more time in the summer than in the
winter. In
Tucson the days (daylight hours) are around 14 hours long near the time
of the summer
solstice. In the winter the sun only shines for 10 hours on the
winter solstice. Days are 12 hours long on the equinoxes.
Scattering
of light makes the sky appear blue, makes clouds white,
and
turns the sun red at sunset.
A simple demonstration (also one of the prettier demonstrations of the
semester) will, hopefully, give you a pretty good idea
what scattering
is (you'll find a picture like this on p. 107 in the photocopied class
notes).
A thin beam of bright red laser light was shined across the
front of
the classroom. No one in the class could see this beam of
light. To see the beam you would need to stand over where the
beam struck the wall and look back toward the laser. The laser
light is very intense and could damage your eyes, so this wouldn't be a
very good thing to do.
Students in the class could see a red spot on the wall because the
light hitting the wall was scattered or splattered and sent off in a
multitude of directions. A individual ray of laser light was sent
to everyone
in the class (and because the intense light is split up into so many
rays,
the individual rays are weaker and safe to look at).
Next we clapped a couple of chalkboard erasers together. When
particles of chalk dust fell into the laser beam they intercepted some
of the laser light and scattered it. Again everyone in the room
got their own personal ray of light coming from each of the particles
of chalk. We use chalk because it is white, it scatters rather
than absorbs light. What would you have seen if black particles
of soot had been dropped into the laser beam?
In the 3rd part of the demonstration we made a cloud by pouring some
liquid nitrogen into a cup of water. The numerous little water
droplets made very good scatterers. So much light was scattered
that the spot on the wall fluctuated in intensity (the spot dimmed when
lots of
light was being scattered, and brightened when not as much light was
scattered).
Just about everything in the atmosphere can scatter light (some
particles and gases might absorb light). These scatterers fall
into two categories: (1) those that have sizes equal to or greater than
the wavelength of the light being scattered and (2) those that are much
smaller. Air molecules fall into the 2nd group. Members of
this group scatter short wavelengths of light much more readily than
long wavelengths. We will see what effects this has.
In this first figure we imagine going outside at midday,
turning toward
the south and looking up at the sun when it is high in the sky (you
shouldn't do this of course, sunlight is too intense and will blind
you). We assume that the sunlight arriving at the top of the
atmosphere is made up of equal amounts of all the colors. This
isn't true, but that's what we'll assume. As the sunlight passes
through the atmosphere some of the shorter wavelengths will be
scattered by air molecules. The unscattered light that makes it
to the ground will be most of the original red, orange, and yellow with
some of the green, blue, and violet light removed. The resulting
mixture will still be very bright but will have become a warmer white
color than it was originally.
If you were to turn toward the north, away from the sun and look up at
a clear sky you would see blue light.
First of all you see light coming from the sky because some
of the
sunlight has been intercepted by air molecules and redirected (just
like the chalk dust and cloud droplets made the laser beam visible in
the demonstration). The sky would appear black if it weren't for
the fact that air molecules scattered light.
Two things to notice about this scattered light: first it is much
weaker than the unscattered sunlight and is safe to look at (imagine if
that weren't the case and it was dangerous to look at the sky), second
the light is blue because it is mainly the shorter wavelengths that are
being scattered.
Why is the sky blue and not green or violet? The sky isn't violet
because there isn't as much violet light in sunlight as there is green
and blue. Also our eyes might not be as sensitive to violet as
they are to blue and green. The sky probably isn't green because
that color isn't scattered as readily as the blue and violet
light. Blue is a sort of compromise.
You might have noticed looking west late in the day that the
setting
sun is not as bright and is redder than it is at midday (it is still
not safe to look at the setting sun, a lot of sunlight is invisible and
we can't judge how bright it really is). The rays of sunlight
travel a
much
longer path through the atmosphere at this time of day and much more of
the sunlight is scattered. Essentially all of the shorter
wavelengths are removed from the unscattered beam of light. You
are left with a mixture of yellow, orange, and red. Sometimes
just the orange and red light are left.
This next figure wasn't discussed
in class.
As we saw with the laser demonstration, the water droplets
in clouds
are very good scatterers of light. The cloud droplets (typically
around 10 or 20 micrometers in diameter) are larger than the wavelength
of visible light (0.4 to 0.7 micrometers). Cloud droplets scatter
all of the colors equally. When white light strikes a cloud, the
scattered light is also white (and not as bright).
Here are a
couple of more common phenomena produced by the scattering of light (these weren't mentioned in class)
The person in this figure would see a crepuscular ray, a
shaft of
sunlight that passes through a hole in a cloud layer. The
sunlight is scattered by particles in the air. Rays of sunlight
that would ordinarily pass through the adjacent parts of the sky are
reflected by the clouds. These parts of the sky appear
darker. You'll find a nice photograph of crepuscular rays on in
Fig. 15.6 on p. 407 in the textbook.
Scattering of sunlight by air molecules turns distant
mountains blue
and eventually makes them fade from view
(there is eventually much more sunlight being scattered by air than
there is sunlight being reflected by the mountains; there is a limit to
how far you can see even when the air is very clean).
A nearby mountain might appear dark green or brown. You are
mainly seeing light reflected off the mountain. As the mountain
gets further away you start seeing appreciable amounts of blue light
(sunlight scattered by air molecules). As the mountain gets even
further the amount of this blue light from the sky increases.
Eventually the mountain gets so far away that you only see blue sky
light and none of the light reflected by the mountain itself.
You'll find a nice photograph of the changing colors of distant
mountains in Fig. 15.5 on p. 406 in the text.