Monday Feb. 8, 2010
click here to download today's notes in a more printer friendly format

A couple of songs ("Resistance" and "Undisclosed Desires") from Muse before class today.

The In-class Optional Assignment from last Friday was returned today.  Here are answers to the two questions.

The 1S1P Bonus Assignment and Experiment #1 reports were collected in class today.  It will take about a week to grade the experiment reports, a little bit longer for the 1S1P reports perhaps.

Experiment #2 materials will be distributed on Wednesday or Friday this week.  If you haven't yet returned Expt. #1 materials please do so as soon as you can, the graduated cylinders are needed for Expt. #2.

Because we rushed through it at the end of class last Friday, we reviewed how pressure data is coded and decoded on surface weather maps.  You'll find that near the end of the Fri., Feb. 05 online notes.

Another important piece of information that we didn't cover last Friday and 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 spend most of the period learning about some of the analyses of weather data that are done on surface weather maps. 

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. 

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.  You often see cloudy skies and stormy weather associated with surface low pressure.


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).