Tuesday Jan. 30, 2007

The first optional assignment was collected in class today.  Answers to the questions were handed out in class that you can use to study for the Practice Quiz this coming Thursday.  A copy of the Practice Quiz Study Guide was handed out in class.

The Monday (and Tuesday) afternoon reviews for the Practice Quiz were cancelled.


We'll cover the ideal gas law in class on Thursday before the Practice Quiz.   Some reference to the first of the ideal gas law equations should be made in the Experiment #1 reports.
The ideal gas law is another way of thinking about and understanding pressure.
We'll start some new material today.  This week we'll learn how weather data is entered onto surface weather maps and learn about some of the analyses of the data that are done.  We'll also have a brief look at upper level 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 is used.

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 (a handout with many of these symbols was distributed in class).  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 (weather symbols were included on the class handout).  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.

Here is the example we studied in class.


Starting at the top of the page you can see the symbols used to indicate the cloud cover.  You leave the circle blank if the skies are clear.  You fill in the circle completely if the skies are overcast (that was the case Tuesday morning).  The symbols for 1/4, 1/2, and 3/4 are pretty straightforward.  You try to estimate to the nearest eighth how much of the sky is covered with clouds.

The air temperature in this example was 48o F (this is plotted above and to the right of the center circle).  The dew point temperature was 42o F and is plotted below and to the left of the center circle.  The box at lower left reminds you that dew points in the 30s and 40s occur 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).  The 42 F dew point Tuesday morning was up considerably from a 20 F dew point on Monday afternoon when this material was covered in the MWF section of the class.

Some of the common weather symbols are shown.  A symbol representing the current weather is plotted to the left of the center circle.  There are about 100 different weather symbols that you can choose from.

You can see and hopefully start to understand how the wind speed and direction are plotted.  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 NE toward the SW. A meteorologist would call these northeasterly winds.  Small barbs at the end of the straight line give the wind speed in knots.  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.

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 pretent that one knot is the same as one mile per hour.

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.   The pressure tendency symbols are explained below:

In (a) the pressure rose then started to fall, the overall change is a drop in pressure.  In (b) the pressure has been falling steadily.  In (c) the pressure fell then started to rise, the overall change is an increase in pressure.  Steadily rising pressure is shown in (d).

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.  Decoding the pressure is explained on p. 37 in the photocopied notes.



Meteorologists hope to map out small horizontal pressure changes on surface weather maps.  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 for Tucson is 927.3 + 75 = 1002.3 mb.

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
203 could be either 920.3 or 1020.3 mb. You pick the value that falls between 950.0 mb and 1050.0 mb (so 1020.3 mb would be the correct value, 920.3 mb would be too low).  185 could be either 918.5 mb or 1018.5 mb.  The correct pressure in this case would be 1018.5 mb.  995 could be either 999.5 mb or 1099.5 mb, the correct value is 999.5 mb.

We didn't have time to cover the last section on p. 27.  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 (Mountain Standard Time year round) and Universal Time.  You must add 7 hours to the time in Tucson to obtain Universal Time. 

To convert 1 pm MST to Universal Time, you first convert the MST to the 24 hour clock format.  1 pm MST is 13:00 MST.  Then you add 7 hours. 13:00 + 7:00 = 20:00 UT.

To convert 15Z to MST (the example shown above) you subtract 7 hours.  15:00 - 7:00 = 8:00 am MST.

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.

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 and drizzle in the NE portion of the map above or the rain shower at the location 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.


Isotherms, temperature contour lines, are 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 brown above passes through one city reporting a temperature of exactly 40o (highlighted in yellow).  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.


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 brown) passes through a city where the pressure is exactly 1008.0 mb (highlighted in yellow).  Most of the time the isobar will pass between two cities.  The 1008 mb isobar passes between cities with pressures of 1006.8 mb and 1009.7 mb.  You would expect to find 1008 mb about halfway between those two cites, that is where the 1008 mb isobar goes.

Next we'll look at what you can expect to see in the vicinity of centers of Low and High pressure.


Winds spin in a counterclockwise 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]

The convergence causes the air to rise at the center of the low.  Rising air expands and cools.  If the air is sufficiently moist clouds can form and begin to rain or snow.  Thus you often see cloudy skies and stormy weather associated with surface low pressure.


Everything is pretty much the opposite with surface high pressure centers.  Winds spin clockwise and spiral outward (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 you associated clear skies with high pressure.