Monday Sep. 21, 2009
click here to download today's notes in a more printer friendly format

3 or 4 songs from Pink Martini to start class today ("Let's Never Stop Falling in Love," "Sympathique," "Hey Eugene," and "Lilly") if I remember correctly.  Pink Martini will be at the Rialto Theatre this coming Friday.

The Expt. #1 reports and Optional Assignment #1 were collected today.  Here are the answers to the Optional Assignment.  Materials for Expt. #2 will be distributed this week.

Quiz #1 is on Wednesday this week.  The final version of the Quiz #1 Study Guide is available online.  Quiz #1 will cover material on the Quiz #1 Study Guide and the Practice Quiz Study Guide.
We quickly reviewed how cloud, temperature, dew point temperature, and wind observations were plotted on surface weather maps using the station model notation.  You'll find that in the Friday Sep. 18 online notes.

A symbol representing the weather that is currently occurring is plotted to the left of the center circle.  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 a 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. 

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.
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




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

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


We had a little time, before the end of class, to get started on some of the analyses of weather data that are done on surface weather maps.  The following information won't be on this week's quiz.
A bunch of weather data has been plotted (using the station model notation) on a surface weather map in the figure below. 
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.

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 Friday after the quiz we will see what the weather is like in the vicinity of low and high pressure centers.

The following topic won't be on this week's quiz either
Tomorrow, Tue., Sep. 22, is the fall equinox!

On the equinoxes, the sun rises exactly in the east and sets exactly in the west.  The picture below shows the position of the sun at sunrise (around 6:30 am on the spring and fall equinox in Tucson).


At noon you need to look about 60 degrees above the southern horizon to see the sun


The sun sets exactly in the west at around 6:30 pm on the equinoxes in Tucson


This is a 2 pm class
Most of you are more likely to see the sun set (perhaps) than see the sun rise.  The figure below shows you about what you would see if you looked west on Speedway (from Treat Ave.) at sunset.  In the winter the sun will set south of west, in the summer north of west (probably further south and north than shown here).  On the equinoxes the sun sets exactly in the west.

If you aren't careful, you can get yourself seriously injured, even killed, on or around the equinoxes.  Can you figure out how that might happen?



  June 21, the summer solstice, is the longest day of the year (about 14 hours of daylight in Tucson).  The days have slowly been getting shorter all semester. This will continue up until Dec. 21, the winter solstice, when there will be about 10 hours of daylight.  After that the days will start to shorten as we make our way back to the summer solstice.

The length of the day changes most rapidly on the equinoxes.  The fall equinox is on Sep. 22 this year.