Tuesday Feb. 9, 2010
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A couple of songs ("Resistance" and "Undisclosed Desires") from Muse before class today.

The Practice Quiz and the Optional Assignment were returned in class today.  The average score on the quiz, 61%, is low but fairly typical for a Practice Quiz.  Average scores from several recent classes are summarized below.
Semester
 MWF class
T Th class
S10
 62%
 61%
F09
 66%
 68%
S09
 60 %
 ---
F08
 64%
 65%
S08
 64%
 66%

Be sure to keep any papers that are returned to you until grades are submitted at the end of the semester.  This is just in case there is any question about the computation of your grade.

The Bonus 1S1P and the Experiment #1 reports were both due today.  If you haven't yet returned your Expt. #1 materials please do so ASAP so that I can distribute the Expt. #2 materials in class on Thursday.

You might find a muddy smudge near the top of your quiz paper.  Here's the culprit:

This is Rajah, one of my two cats.


Rajah likes to help grade papers (looks like lab reports in this case).  The rain on Sunday meant he had muddy feet.


This is my other cat, Fox.  She's defending a bush of catnip that grows like a weed in my vegetable garden.
Before the quiz last week we saw how 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 way of trying to understand this topic.


A gallon of water weighs about 8 pounds (lbs).

If you submerge a 1 gallon jug of water in a swimming pool, an amazing thing happens -  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.
The upward bouyant force is really just another name for the pressure difference force covered earlier today (higher pressure pushing up on the bottle and low pressure at the top pushing down, resulting in a net upward force).  A 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 on the jug due to gravity.  The two forces are equal and opposite.

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 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 will weigh 12 pounds.




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 how small differences in density can determine whether an object floats or sinks.

A can of regular Coca Cola (which now has the exclusive franchise on the U. of A. campus, the figure above was drawn when Pepsi had the franchise)  was placed in a beaker of water.  The can sank.  A can of Diet Coke on the other hand floated. 

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.

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). 

Many people can fill their lungs with air and make themselves float, or they can empty their lungs and make themselves sink.

People must have a density that is about the same as water.
We spent the remainder of the day 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.

The figure above wasn't shown in class.
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 symbols can be found on the handout to be distributed in class).

The air temperature in this example was 38o F (this is plotted above and to the left of the center circle).  The dew point temperature was 32o F and is plotted below and to the left of the center circle.  The box at lower left reminds you that dew points range from the mid 20s to the mid 40s during much of the year in Tucson.  Dew points rise into the upper 50s and 60s during the summer thunderstorm season (dew points are in the 70s in many parts of the country in the summer).  Dew points are in the 20s, 10s, and may even drop below 0 during dry periods in Tucson.

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 SE toward the NW at a speed of 10 knots (SE winds in the top part of the figure, the wind examples in the bottom part are coming from the south).  A meteorologist would call these southeasterly 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 (these weather symbols were on the handout distributed in class)


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 and not covered in class) 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.

An In-class Optional Assignment was given during class and collected at the end of the period.  If you are reading the online notes and weren't in class, you can answer the two questions and turn in the assignment before class on Thursday for credit.  Here are the two questions.

 Question #1




Decode the station model data plotted and fill in the blanks below

Temperature __________          present weather conditions__________

wind direction __________          dew point temperature __________

cloud cover __________          wind speed __________

cloud type(s) __________           pressure__________      


Question #2
How would the following conditions be plotted on a surface weather map using the station model notation

 temperature = 75 F     wind from the SE at 15 knots
1/4 of the sky is covered with Fc and/or Fs clouds of bad weather (scud)
intermittent rain     dew point temperature = 49 F

We haven't learned how to decode the pressure data yet.


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 (which was not shown in class)


Here are some examples of coding and decoding the pressure data.  The numbers below differ somewhat from those used in class.


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
118 could be either 911.8 or 1011.8 mb. You pick the value that falls between 950.0 mb and 1050.0 mb (so 1011.8 mb would be the correct value, 911.8 mb would be too low).

Another important piece of information that is included on a surface weather 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 (that may differ somewhat from those used 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).


I told you I would finish coloring the map when I got back to my office (actually this is from a previous semester)

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.  On Thursday we will see what you can learn about the weather in the vicinity of surface highs and lows.