Wednesday Sep. 15, 2010
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Some mariachi music from Mariachi Luz de Luna (performing with
Calexico at the Barbican Theater in London) to celebrate Mexican
Independence Day.
The due date for the 1st Optional Assignment
has been extended until next Monday (Sep. 20).
The Quiz #1 Study Guide is now
available. Quiz #1 (next Wednesday, Sep. 22) will cover material
on the Quiz #1 Study Guide and the Practice
Quiz Study Guide.
We spent the first portion of the period looking at how
temperature
changes with increasing altitude in the atmosphere. Temperature
can increase, decrease, even remain constant with increasing
altitude. The figures below are more clearly drawn versions of
what was done in class.
The atmosphere can be split
into layers
depending on whether
temperature is increasing or decreasing with increasing altitude.
The two lowest layers are shown in the figure above. There are
additional layers (the mesosphere and the thermosphere) above 50 km but
we won't worry about them.
1. We live in
the troposphere. The troposphere is found, on average, between 0
and about 10 km altitude, and is where temperature usually decreases
with
increasing altitude. [the troposphere is usually a little higher
in the tropics and lower at polar latitudes]
The troposphere contains most of the water vapor
in the atmosphere (the water vapor comes from evaporation of ocean
water and then gets mixed throughout the troposphere by up and down air
motions) and is
where most of the clouds and weather occurs. The
troposphere can be stable or unstable (tropo means to turn over and
refers to the fact that air can move up and down in the
troposphere).
2a. The thunderstorm shown in
the figure with its strong updrafts and downdrafts indicates unstable
conditions. When the thunderstorm reaches the
top of the troposphere, it runs into the bottom edge of the
stratosphere which is a very stable layer. The
air can't continue to rise into the stratosphere so the cloud
flattens out and forms an anvil (anvil is the name given to the flat
top of the thunderstorm). The
flat anvil top is something
that you can go outside and see and often marks the top of the
troposphere.
2b. The summit of Mt. Everest is a little over 29,000
ft. tall and is
close to the average height of the top of the troposphere.
2c. Cruising altitude in a passenger jet is usually between
30,000 and 40,000, near or just above the top of the troposphere, and
at the bottom of the stratosphere.
3. Temperature remains constant between 10 and 20 km
and then
increases with increasing altitude between 20 and 50 km. These
two sections form the stratosphere. The stratosphere is a
very stable air layer. Increasing temperature with increasing
altitude is called an
inversion. This is what makes the stratosphere so stable.
4. A kilometer is one
thousand meters. Since 1 meter is about 3 feet, 10 km is about
30,000 feet. There are 5280 feet in a mile so this is about 6
miles (about
is usually close enough in this class).
5. Sunlight is a mixture of ultraviolet (7%),
visible (44%), and
infrared light (49%). We can see the visible light.
5a. On average about 50% of the sunlight
arriving at the top of
the atmosphere passes through the atmosphere and is absorbed at the
ground (20% is absorbed by gases in the air, 30% is reflected back into
space). This warms the ground. The air in contact with the
ground is warmer than air just above. As you get further and
further from the warm ground,
the
air
is
colder
and
colder.
This
explains
why air temperature decreases with increasing altitude in the
troposphere.
5b. How do you explain increasing temperature with
increasing
altitude in the stratosphere.
The ozone layer is found in the stratosphere
(peak concentrations are found near 25 km altitude). Absorption
of
ultraviolet light by ozone warms the air in the stratosphere and
explains why the air can warm. The air in the stratosphere is
much less dense (thinner) than in the troposphere. So even though
there is not very much UV light in sunlight, it doesn't
take as much energy to warm this thin air as it would to warm denser
air closer to the ground.
6. I didn't mention
this point in class. That's a manned
balloon;
Auguste Piccard and Paul Kipfer are
inside. They were the first men to travel into the
stratosphere (see pps 31 & 32 in
the photocopied Class Notes) We'll have a look at a short segment
of video at some point that describes their voyage. It really was
quite a daring trip at the time at the
time,
and they very
nearly didn't survive it.
We spent the 1/3rd of class
on a new topic -
Surface Weather Maps. We began by learning how
weather data are
entered onto surface weather maps. We'll finish this up in class
next Friday.
Much of our weather is produced by relatively large
(synoptic scale)
weather systems. To be able to identify and characterize these
weather systems you must first collect weather data (temperature,
pressure, wind direction and speed, dew point, cloud cover, etc) from
stations across the country and plot the data on a map. The large
amount of data requires that the information be plotted in a clear and
compact way. The station model notation is what meterologists
use.
A small circle is plotted on the map at the location where
the
weather
measurements were made. The circle can be filled in to indicate
the amount of cloud cover. Positions are reserved above and below
the center circle for special symbols that represent different types of
high, middle,
and low altitude clouds. The air temperature and dew point
temperature are entered
to the upper left and lower left of the circle respectively. A
symbol indicating the current weather (if any) is plotted to the left
of the circle in between the temperature and the dew point; you can
choose from close to 100 different weather
symbols (on a handout distributed in class). The
pressure is plotted to the upper right of the circle and the pressure
change (that has occurred in the past 3 hours) is plotted to the right
of the circle.
So
we'll work through this material one step at a time (refer to p. 36 in
the photocopied ClassNotes). Some of the figures below were
borrowed from a previous semester or were redrawn and may differ
somewhat from what was drawn in class.
The center circle is filled in to indicate the portion
of
the sky
covered with clouds (estimated to the nearest 1/8th of the sky) using
the code at the top of the figure. 3/8ths of the sky is covered
with clouds in the example above. Then symbols (not drawn in class)
are
used
to
identify
the
actual types of high, middle, and low altitude clouds (the
cloud symbols can be found here if you didn't
get a copy of the handout distributed in class today). You do not, of
course, need to remember all of the cloud symbols.
A straight line extending out from the center circle
shows the wind direction. Meteorologists always give the
direction the wind is coming from.
In this example the winds are
blowing from the NW toward the SE at a speed of 5 knots. A
meteorologist would call
these northwesterly winds.
Small barbs at the end of the straight
line give the wind speed in knots. Each long barb is worth 10
knots, the short barb is 5 knots.
Knots are nautical miles per hour. One nautical mile per hour is
1.15 statute miles per hour. We won't worry about the distinction
in this class, you can just pretend that one knot is the same as one
mile per hour.
Here are some additional wind
examples that
weren't
shown
in
class:
In (a) the winds are from the NE at 5 knots, in
(b) from the
SW at 15
knots, in (c) from the NW at 20 knots, and in (d) the winds are from
the NE at 1 to 2 knots.
A symbol representing the weather that is currently
occurring is plotted to the left of the center circle (in between the
temperature and the dew point). Some of
the common weather
symbols are
shown. There are about 100 different
weather symbols that you can choose
from (click here
if you didn't get a copy of the handout distributed in class today).
There's no way I could expect you to remember all of these weather
symbols.
The sea level pressure is shown above and to the right
of
the center
circle. Decoding this data is a little "trickier" because some
information is missing that you need to put back in. We'll look
at this in more detail on Friday.
On Monday we saw that the relative strengths of the
downward graviational force and the upward pressure difference force
determine whether a parcel of air will rise or sink. Archimedes
Law is another, somewhat simpler, way of trying to understand this
topic.
A gallon of
water weighs about 8 pounds (lbs).
If you submerge the gallon jug of water in a swimming pool, the
jug
becomes, for all intents and purposes, weightless. Archimedes'
Law (see figure below, from p. 53a in the photocopied ClassNotes)
explains why this is true.
Archimedes first of all tells you
that the surrounding fluid will exert an upward pointing bouyant force
on the submerged water bottle. That's why the submerged jug can
become weightless. Archimedes law also tells you how to figure
out how strong the bouyant force will be. In this
case the 1 gallon bottle will displace 1 gallon of
pool water. One
gallon of pool
water weighs 8 pounds. The upward bouyant force will be 8 pounds,
the same as the downward force. The two
forces are equal and opposite.
Archimedes law doesn't really tell you what causes the upward bouyant
force. If you're really on top of this material you will
recognize that it is really
just another name for the
pressure difference force that we covered on Monday (higher pressure
pushing
up on the bottle and low pressure at the top pushing down, resulting in
a net upward force).
Now we imagine pouring out all the water and filling the 1 gallon
jug
with air. Air is about 1000 times less dense than water;compared
to water, the jug
will weigh practically nothing.
If you submerge the jug of air in a
pool
it will displace 1 gallon of
water
and experience an 8 pound upward bouyant force again. Since there
is no downward force the jug will float.
One gallon of sand (which is about 1.5 times denser than water)
jug weighs 12 pounds (being a detail kind of person I actually checked
this out).
The jug of sand will sink because
the downward force is greater
than
the upward force.
You can sum all of this up by saying anything that is less dense
than
water will float in water, anything that is more dense than water will
float in water.
The same reasoning applies to air in the atmosphere.
Air that is less dense (warmer)
than the air around it will
rise.
Air that is more dense (colder) than the air around it will sink.
Here's a little more
information about Archimedes.
There's a colorful demonstration that shows how small differences
in density
can determine whether an object floats or sinks.
Both cans are made of aluminum which has a density almost three times
higher than water. The drink itself is largely water. The
regular Pepsi also has a lot of high-fructose corn syrup, the diet
Pepsi
doesn't. The mixture of water and corn syrup has a density
greater than plain
water. There is also a little air (or perhaps carbon dioxide gas)
in each can.
The average density of the can of regular Pepsi (water & corn syrup
+
aluminum + air) ends up being slightly greater than the density of
water. The average density of the can of diet Pepsi (water +
aluminum + air) is slightly less than the density of water.
We repeated the demonstration with a can of Pabst Blue Ribbon
beer. That also floated, the beer doesn't contain any corn syrup
(I don't think).
In some respects people in swimming pools are like cans of regular and
diet soda. Some people float (they're a little less dense than
water), other people sink (slightly more dense than water).
