Lecture 7 - Surface weather maps and the station model notation

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 periodically 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; you can choose from close to 100 different weather symbols. 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.

We'll work through this material one step, one weather variable, at a time.

The center circle is filled in to indicate the portion of the sky covered with clouds (estimated to the nearest 1/8th of the sky) using the code at the top of the figure. 3/8ths of the sky is covered with clouds in the example above.

We'll learn how to name and identify clouds later in the course. In the meantime here are some sketches of common clouds together with their weather symbols. Here's a more complete chart of cloud photographs and also the symbols that can be used to note the cloud type on a surface weather map.

The air temperature in this example was 64^o F (this is plotted above and to the left of the center circle). The dew point temperature was 39^o F and is plotted below and to the left of the center circle. The box at lower left reminds you that dew points range from the mid 20s to the mid 40s during 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). Dew points are in the 20s, 10s, and may even drop below 0 during dry periods in Tucson.

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

Here are some additional wind examples

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.

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.

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. 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. 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: University of Arizona Atmospheric Sciences Department Weasther 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

Here are some examples of coding and decoding the pressure data.

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 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