Thursday Sept. 16, 2010
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Today, Sept. 16, is the 200 anniversary of Mexican
Independence. A little Mariachi music from Luz de
Luna (appearing with Calexico at the Barbican Theater in London) before
class seemed appropriate. You heard Aires Del Mayab
featuring Lulu Olivares, Cancion Del Mariachi
with Ruben Moreno, and Corona (I wasn't able to find a video of the
performance at the Barbican Theater). Calexico and Mariachi Luz
de Luna are both local groups; there's a good chance I'll play some
more of their music at some point during the semester.
The Quiz #1 Study Guide is now available
online. Quiz #1 (next Thursday, Sep. 23) will also cover material
on the Practice Quiz Study Guide.
There was an In-class
Optional
Assignment today. If you weren't
in class today and discreetly turn in answers to the questions at the
beginning of class next Tuesday you can earn at least partial
credit. Optional
Assignment #1 (a take home assignment) is due at the start of class
next Tuesday.
We spent most of the class today
on a new topic -
Surface Weather Maps. We began by learning how
weather data are
entered onto surface weather maps.
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.
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
are
used
to
identify the actual types of high, middle, and low altitude clouds (the
symbols can be found on the handout to be distributed in class, click here if you didn't
pick up a copy of the handout).
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:
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 (these weather symbols were on the handout distributed in class,
click here if
you didn't get a copy of the handout)
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. We'll look at this in more detail
momentarily.
Pressure change data (how the pressure has changed during
the preceding
3 hours)
is shown to the right of the center circle. You must
remember to add a decimal point. Pressure changes are usually
pretty small.
Here are
some links to surface weather maps with data plotted using the
station model notation: UA Atmos. Sci.
Dept. Wx page, National
Weather Service Hydrometeorological Prediction Center, American
Meteorological
Society.
Here's how
you can decode the pressure data.
Meteorologists hope to map out small horizontal pressure
changes on
surface weather maps (that produce wind and storms). Pressure
changes much more quickly when
moving in a vertical direction. The pressure measurements are all
corrected to sea level altitude to remove the effects of
altitude. If this were not done large differences in pressure at
different cities at different altitudes would completely hide the
smaller horizontal changes.
In the example above, a station
pressure value of 927.3 mb was measured in Tucson. Since Tucson
is about 750 meters above sea level, a 75 mb correction is added to the
station pressure (1 mb for every 10 meters of altitude). The sea
level pressure estimate for Tucson is 927.3 + 75 = 1002.3 mb.
This is also shown on the figure below
Here are some examples of coding
and decoding the pressure data. We tried to cover this in about
the last 5 minutes of class which meant rushing things a little bit ( a
lot actually ). So we'll review this at the start of class next
Monday.
To save room, the leading 9 or 10 on the sea level pressure
value and
the decimal
point are removed before plotting the data on the map. For
example the 10 and the . in
1002.3 mb would
be removed; 023
would be plotted on the weather map (to the upper right of the center
circle). Some additional examples are shown above.
When reading pressure values off a
map you must remember to
add a 9 or
10 and a decimal point. For example
116 could be either 911.6 or 1011.6 mb. You pick the value that
falls between 950.0 mb and 1050.0 mb (so 1011.6 mb would be the correct
value, 911.6 mb would be too low).
Another
important piece of information on a surface map is the time the
observations were collected. Time on a
surface map is converted to a universally agreed upon time zone called
Universal Time (or Greenwich Mean Time, or Zulu time).
That is the time at 0 degrees longitude. There is a 7 hour time
zone difference between Tucson (Tucson stays on Mountain
Standard Time year round) and Universal Time. You must add 7
hours to the time in Tucson to obtain Universal Time.
Here are some examples (not done in class)
2:45 pm MST:
first convert 2:45 pm to the 24
hour clock format 2:45 + 12:00 = 14:45 MST
then add the 7 hour time zone correction ---> 14:45
+ 7:00 = 21:45 UT (9:45 pm in Greenwich)
9:05 am MST:
add the 7 hour time zone
correction ---> 9:05 + 7:00 = 16:05 UT (4:05 pm in England)
18Z:
subtract the 7 hour time zone
correction ---> 18:00 - 7:00 = 11:00 am MST
02Z:
if we subtract the 7 hour time
zone correction we will get a negative
number.
We will add 24:00 to 02:00 UT then subtract 7 hours
02:00 + 24:00 = 26:00
26:00 - 7:00 = 19:00 MST on the previous day
2 hours past midnight in Greenwich is 7 pm the previous day in
Tucson
A bunch of weather data has been
plotted (using the station model notation) on a surface weather map in
the figure
below (p. 38 in the ClassNotes).
Plotting the surface weather
data
on a map is
just the
beginning.
For example you really can't tell what is causing the cloudy weather
with rain (the dot symbols are rain) and drizzle (the comma symbols) in
the NE portion of the map above or the rain
shower along the Gulf Coast. Some additional
analysis is needed. A meteorologist would usually begin by
drawing some contour lines of pressure to map out the large scale
pressure pattern. We will look first at contour lines of
temperature, they are a little easier to understand (easier to decode
the plotted data and temperature varies across the country in a fairly
predictable way).
Isotherms, temperature
contour lines, are usually drawn at 10 F
intervals.
They do two things: (1) connect points on the map that all
have the same temperature, and (2) separate regions that are warmer
than a particular temperature from regions that are colder. The
40o F isotherm highlighted in yellow above passes through
a city which is reporting a temperature of exactly 40o.
Mostly
it
goes
between
pairs
of
cities:
one
with a temperature warmer than 40o and the other
colder
than 40o. Temperatures
generally decrease with
increasing
latitude: warmest temperatures are usually in the south, colder
temperatures in the north.
Now the same data with isobars
drawn in. Again they
separate
regions with pressure higher than a particular value from regions with
pressures lower than that value.
Isobars are generally drawn at 4 mb intervals. Isobars also connect points on the map
with the same pressure. The 1008 mb isobar (highlighted in
yellow) passes through a city at Point
A where the pressure is exactly
1008.0 mb. Most of the time the isobar
will pass between two
cities. The 1008 mb isobar passes between cities with
pressures
of 1009.7 mb at Point B and
1006.8 mb at Point C.
You would
expect to find 1008 mb somewhere in between
those two cites, that is where the 1008 mb isobar goes.
The pattern on this map is very different from the
pattern
of
isotherms. On this map the main features are the circular low and
high pressure centers.
Just locating closed centers of high and low pressure will already
tell you a lot about the weather that is occurring in their vicinity.
1.
We'll start with the large nearly circular centers of High and Low
pressure. Low pressure is drawn below. These figures are
more neatly drawn versions of what we did in class.
Air will start moving
toward low
pressure (like a rock sitting on a hillside that starts to roll
downhill), then something called the Coriolis force will cause
the
wind to start to spin (we'll learn more about the Coriolis force later
in the semester). In the northern hemisphere winds spin in a
counterclockwise (CCW) direction
around surface
low pressure
centers. The winds also spiral inward toward the center of the
low, this is called convergence. [winds spin clockwise around low
pressure centers in the southern hemisphere but still spiral inward,
don't worry about the southern hemisphere until later in the semester]
When the converging air reaches the
center of the low it starts to rise.
Rising air expands (because it is moving into lower pressure
surroundings at higher altitude), the expansion causes it to
cool. If the air is moist
and it is cooled enough (to or below the dew point temperature) clouds
will form and may then begin to rain or snow. Convergence is 1 of 4 ways of causing air
to rise (I didn't mention this in
class but I will next week). You often
see
cloudy skies and stormy weather associated with surface low pressure.
We didn't have time to discuss the following figure. It shows
winds blowing around surface centers of high pressure (in the northern
hemisphere).
Surface high pressure
centers are pretty much just the opposite situation. Winds
spin
clockwise
(counterclockwise
in
the
southern
hemisphere)
and spiral outward.
The
outward motion is called divergence.
Air sinks in the center of
surface high pressure to
replace the diverging air. The sinking air is compressed and
warms. This keeps clouds from forming so clear
skies are normally found with high pressure (clear skies but not
necessarily warm weather, strong surface high pressure often forms when
the air is very cold).
This was about all the time we could spend on surface maps.
We'll probably spend all of next Tuesday on this same topic :)
We also covered a short section on Archimedes Law during class
today. On Tuesday 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).
