The Optional Assignment and the Experiment #2 reports were collected
today. You'll find the answers to the Optional Assignment here. Answers to
the In-class Optional Assignment from last Friday are here.
The Expt. #3 materials should be distributed in class on Friday.
A small number of Topic 1 1S1P reports were returned.
There has been a big change in the weather in the past few
days. What happened? A strong cold front moved through the
region early in the afternoon last Saturday. We are now in the
cold air mass behind that front.
We spent a little time at
the beginning of class looking at weather conditions ahead of and
behind a cold front.
The air ahead of an approaching cold front is warmer and often
moist (not always true in the desert southwest). Saturday began
with warm temperatures and with dew points in the low 60s. Winds
generally blow from the southwest and pressure falls as the front
approaches.
Winds often get gusty as the front is actually passing. Also the
clouds associated with a cold front (remember the front will lift the
warm low density air ahead of the front) are often found in a narrow
band along the front. This is when you get rain or snow showers
(as in the video shown in class).
Once the front passes through, winds shift to the northwest, pressure
begins to rise, and temperatures drop. The air behind a cold
front is often much drier (in Tucson dew points dropped into the single
digits at one point). Note that the coldest air arrives a day or
two after the frontal passage. Also with dry air and few clouds,
nighttime temperatures can really drop. Tucson broke a low
temperature record on Monday morning with a low of 38 F (this beat the
old record by 5 F, so this really was an unusually strong and cold
front for this time of year).
We watched a video of a dramatic cold front passage recorded in Tucson
on Easter Sunday morning in 1999. You can watch the video
yourself by clicking on the following link (it may take some time for
the video to load): cold
front video
Next we reviewed the following two figures that were stuck onto
the end of last Friday's notes. They weren't discussed in class
last Friday.
The
greenhouse effect makes the earth's surface warmer than it would be
otherwise.
Here's one explanation of why that is true. At
left (energy balance on the earth without an atmosphere or at least
without greenhouse gases) the ground
is getting 2 units of energy. At right it is getting three, the
extra one is coming from the atmosphere. Doesn't it make sense
that ground that absorbs 3 units of energy will be warmer than ground
that is only absorbing 2.
Here's another line of reasoning. At left the
ground is emitting 2 units of energy, at
right the ground is emitting 3 units. Remember that the amount of
energy emitted by something depends on temperature. The ground
must be warmer to be able to emit 3 arrows of energy rather than 2
arrows.
Next we had a more realistic look at energy balance (Figs. 3-7, 3-8,
and 3-10 in the textbook).
First what happens to sunlight.
The top figure shows what we assumed in our simplified version of
the greenhouse effect. We assumed that 100% of the sunlight
arriving at the earth passed through the atmosphere and got absorbed by
the ground.
In reality about half (45%) of the incoming sunlight is transmitted and
gets absorbed by the ground. About 30% is reflected (colored blue
above) by clouds, by the ground, or scattered by air molecules.
About 25% is absorbed (UV is absorbed by ozone & oxygen, certain
wavelengths of IR are absorbed by greenhouse gases).
The complete picture of energy balance is more complicated. The
top figure below is our simplified version of the greenhouse
effect.
Green is incoming sunlight. Orange and red is IR emitted by the
ground
(red passes through the atmosphere, orange is absorbed by greenhouse
gases in the atmosphere). Pink is IR radiation emitted by the
atmosphere upward and downward.
At the bottom is a more realistic version.
First in green is the incoming sunlight. 25 units are absorbed in
the atmosphere, 45 units is absorbed by the ground (these are the same
numbers that were in the previous picture, p. 71 in the photocopied
ClassNotes).
The ground emits a total of 104 units (orange and red). Only 4 of
these pass through the atmosphere, the remaining 100 get
absorbed.
The atmosphere emits 66 units upward into space & 88 units downward
toward the ground. It may be that the bottom of the atmosphere is
warmer than the upper parts of the atmosphere (warmer objects emit more
energy than colder objects). Or it may just be that there is more
air in the lower atmosphere that is able to emit more radiation.
The 8 and 21 units at left are energy transported from the ground up
into the atmosphere by conduction & convection, and by latent heat,
respectively.
You can check for energy balance at three different points in the
figure.
First just above the top of the atmosphere. There are 25+45 = 70
units of incoming sunlight (we have left out the 30 units of reflected
sunlight). This is balanced by 4+66 = 70 units being emitted by
the ground and atmosphere upward into space.
Next the atmosphere itself. 8 (conduction & convection) + 21
(latent heat) + 100 (IR from the ground) + 25 (sunlight) = 154 units
are being absorbed. The atmosphere emits 66+88 = 154 units.
The atmosphere is in energy balance.
At the ground. The ground emits 104 units and transports 8 + 21
units to the atmosphere (conduction & convection). That's a
total of 133 units. The ground absorbs 45 (sunlight) + 88
(atmosphere) = 133 units. So the ground is in balance also.
A couple more things to note.
1. The atmosphere actually gets more energy from the
atmosphere than it does from the sun. Part of the reason is that
the atmosphere is emitting energy 24 hours a day while the sun is only
in the sky part of the day. Some sunlight is absorbed by the
atmosphere, then the atmosphere radiates energy upward and
downward. So some of the energy arriving at the ground from the
atmosphere was originally sunlight.
2. The ground emits more energy than it gets from the
sun. How can the ground do that and still be in energy
balance? The answer is because the ground also gets energy from
the atmosphere.
You can
use the simplified picture of radiative equilibrium to understand the
effects of clouds on nighttime low and
daytime high temperatures. You'll find this discussed on pps 72a
and
72b in the Classnotes.
Here's the simplified picture of radiative equilibrium (something
you're probably getting pretty tired of seeing). By now you
should be able to identify each of the colored arrows in the figure
above and explain what they represent.
The two pictures below show what happens at night when you remove
the
two green rays of incoming sunlight.
The picture on the left shows a clear night. The ground is losing
3
arrows of energy and getting one back from the atmosphere. That's
a
net loss of 2 arrows. The ground cools rapidly and gets cold
during
the night.
A cloudy night is shown at right. Notice the effect of the
clouds.
Clouds are good absorbers
of infrared
radiation. If we could see IR light,
clouds would appear black, very different from what we are used
to (because clouds also emit IR light, the clouds might also
glow). Now none of
the IR radiation emitted by the ground passes through the atmosphere
into space. It is all absorbed either by greenhouse gases or by
the
clouds. Because the clouds and atmosphere are now absorbing 3
units of
radiation they must emit 3 units: 1 goes upward into space, the other 2
downward to the ground. There is now a net loss at the ground of
only
1 arrow.
The ground won't cool as quickly and won't get as cold on a cloudy
night as it does on a clear night.
The next two figures compare clear and cloudy days.
Clouds are good reflectors
of visible
light. The effect of this is to
reduce the amount of sunlight energy reaching the ground in the right
picture. With less sunlight being absorbed at the ground, the
ground
doesn't need to get as warm to be in energy balance.
It is generally cooler during the day on a cloudy day than on a clear
day.
Clouds raise the nighttime minimum temperature and lower the daytime
maximum temperature.
Typical daytime highs and nighttime
lows in Tucson for this
time of year. Note how the clouds reduce the daily range of
temperature.
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 part
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.
