Begin Quiz #2 Material

Monday, September 16

We'll be using page 37a, page 37b, page 38a, and page 38b from the ClassNotes package

Surface weather maps

We will begin by learning how weather data are entered onto surface weather maps.

Much of our weather is produced by relatively large scale (synoptic scale) weather systems - systems that might cover several states or a significant fraction of the continental US.  To be able to identify and locate 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 meteorologists use.

Station model notation

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; there are close to 100 different weather symbols that you can choose from.  The pressure is plotted to the upper right of the circle and the pressure change (that has occurred over the past 3 hours I believe) is plotted to the right of the circle. 

The center circle is filled in to indicate the portion of the sky covered with clouds (to the nearest 1/8th of the sky) using the code at the top of the figure (which I think you can mostly figure out).  5/8ths of the sky is covered with clouds in the example shown.

In addition to the amount of cloud coverage, the actual types of clouds present (if any) can be important.  Cloud types can tell you something about the state of the atmosphere (thunderstorms indicate unstable conditions, for example).  We'll learn to identify and name clouds later in the semester and will just say that clouds are classified according to altitude and appearance.

Positions are reserved above and below the center circle for high, middle, and low altitude cloud symbols.  Six cloud types and their symbols are sketched above.   Purple represents high altitude in this picture.  Clouds found at high altitude are composed entirely of ice crystals.  Low altitude clouds are green in the figure.  They're warmer than freezing and are composed of just water droplets.  The middle altitude clouds in blue are surprising.  They're composed of both ice crystals and water droplets that have been cooled below freezing but haven't frozen.
There are many more cloud symbols than shown here(click here for a more complete list of symbols together with photographs of the different cloud types.  You can click on any of the cloud images to get a larger picture and additional examples of each cloud type)

Air temperature and dew point temperature
The air temperature and dew point temperature are found to the upper left and lower left of the center circle, respectively.  These are probably the easiest items to read.

Dew Point Temperatures (F)
70s	common in many parts of the US in the summer
50s & 60s	summer T-storm season in Arizona (summer monsoon)
20s, 30s, 40s	most of the year in Arizona
10s or below	very dry conditions

Wind direction and wind speed
We'll consider winds next.  Wind direction and wind speed are plotted(page 37b in the ClassNotes)

A straight line extending out from the center circle shows the wind direction.  Meteorologists always give the direction the wind is coming from.  In the example above the winds (the finely drawn arrows) are blowing from the NW toward the SE at a speed of 5 knots.  A meteorologist would call these northwesterly 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.  The wind speed in this case is 5 knots.  If there's just a short barb it's positioned in from the end of the longer line (so that it wouldn't be mistaken for a 10 knot barb).

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, we will just consider one knot to be the same as one mile per hour.

Winds blowing from the east at 20 knots.

A few more examples of wind directions (provided the wind is blowing) and wind speeds.  Note how calm winds are indicated.  Note also how 50 knot winds are indicated.

Here are four more examples to practice with.  Determine the wind direction and wind speed in each case.  Click here for the answers.

Weather (that may be occurring when the observations were made)
And maybe the most interesting part.

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. 

Pressure
The pressure data is usually the most confusing and most difficult data to decode (page 38a in the ClassNotes)




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.  That is done to save room on the surface map.   We'll look at this in more detail momentarily.

Pressure change data (how the pressure has changed during the preceding 3 hours) is shown to the right of the center circle.  Don't worry much about this now, but it may come up next week.

The figures below show the pressure tendency, the symbol following the pressure change value.  This is a visual record of how pressure has been changing during the past 3 hours. 

 
Again this is something we might use when trying to locate warm and cold fronts on a surface weather map.  Don't worry too much about it now. Sea level pressure
Before being plotted on a surface map, pressure data must be corrected for altitude.


Meteorologists hope to map out small horizontal pressure differences on a surface map.  It is the small horizontal differences in pressure that cause the wind to blow and create storms.  If corrections for altitude were not made, the large vertical changes in pressure caused by altitude would dominate and would completely hide the horizontal pressure variations.

Here's an example of what would be done with a station pressure measurement made in Tucson.


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 sea level pressure estimate is the number that gets plotted on the surface weather map.  And actually there is one additional complication:


To save space only a portion of the full sea level pressure value is plotted on the map.  When reading a weather map you need to remember to replace the missing 9 or 10 and the decimal point.

Do you need to remember all the details above and be able to calculate the exact correction needed?  No.  You should remember that a correction for altitude is needed.  And the correction needs to be added to the station pressure.  I.e. the sea-level pressure is higher than the station pressure.

Coding and decoding pressure

When reading pressure values off a map you must remember to add a 9 or 10 and a decimal point.  For example 131 could be either 913.1 or 1013.1 mb. You pick the value that falls closest to 1000 mb average sea level pressure. (so 1013.1 mb would be the correct value, 913.1 mb would be too low).  A couple more examples are shown below.


Here are a few more examples to try on your own (answers are given below): 422, 700, 990.  Caution: It is values like 990 where you are likely to make a mistake.  The 990 value looks reasonable, 990 mb.  But you do still have to add a leading 9 or 10.

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Answers

Coding pressures (you must remove the leading 9 or 10 and the decimal point.

1035.6 mb ---> 356
990.1 mb ---> 901
1000 mb = 1000.0 mb ---> 000

Decoding pressures (you must add a 9 or a 10 and a decimal point) and pick the value closest to 1000 mb.

422 ---> 942.2 mb or 1042.2 mb ---> 1042.2 mb
700 ---> 970.0 mb or 1070.0 mb ---> 970.0 mb
990 ---> 999.0 mb or 1099.0 mb ---> 999.0 mb 

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Time
Another important piece of information on a surface 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, the Prime Meridian. There is a 7 hour time zone difference between Tucson and Universal Time (this never changes because Tucson stays on Mountain Standard Time year round).  You must add 7 hours to the time in Tucson to obtain Universal Time.