Friday, Sept. 15, 2006

The optional assignments were collected in class.  Answers to the questions are available online.

The Experiment #1 reports are due next Monday.  You should already have returned the materials.  If you haven't returned the materials you should do so on Monday when you turn in your report.  The materials are needed for Experiment #2. 
We'll start some new material today.  Today and Monday 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 next Monday.

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. 

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.  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.

Now we'll look at the example 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 is the skies are clear.  You fill in the circle completely if the skies are overcast.  The symbols for 1/4, 1/2, and 3/4 are pretty straightforward.

The air temperature in this example was 85o F and the dew point temperature was 43o F.  Some of the common weather symbols are shown.

You can 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 SW toward the NE.  A meteorologist would call these southeasterly 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.

The pressure data is a little "tricky," we'll look at what is done there on the next page.

With the pressure change data you must remember to insert a decimal point.



Meteorologist 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..

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 circle).

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 mb or 1011.6 mb.  You pick the value that falls between 950.0 mb and 1050.0 mb, the usual range of sea level pressure values.  Thus the correct pressure in this case would be 1011.6 mb.

Time on a surface weather map is usually given in Universal Time.  There is a 7 hour time zone difference between Tucson (Mountain Standard Time year round) and Universal Time.

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 60o F isotherm highlighted in yellow above passes through one city reporting a temperature of exactly 60o. Mostly it goes between pairs of cities: one with a temperature warmer than 60o and the other colder than 60o.  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 yellow) passes through a city 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 1006.8 mb and 1009.7 mb highlighted in yellow.  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. 

The winds blowing around a surface center of low pressure are shown above.  In the top view at upper left the winds are shown with arrows.  The winds are shown using the station model notation in the top view at upper right.

Surface winds converge into surface low pressure centers.  The air in the center of the low rises.  Rising air cools.  Cooling is what you need to make clouds.  Thus cloudy stormy weather is found with surface lows.



The winds associated with a surface center of high pressure.  There wasn't time in class to show this figure.

This figure wasn't shown in class either.  The horizontal and vertical air motions around high pressure are pretty much just the opposite of what you find around low pressure.  Surface winds spin clockwise and spiral outward away from surface centers of high pressure.  Air from higher in the atmosphere sinks in the center of the high to replace the diverging air at the surface.  Sinking air is compressed and warms.  This keeps clouds from forming and you generally find clear skies with surface high pressure.