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STATION MODEL

The "station" what ? Since we started offering you the synoptic charts from the South African Weather Service there have been many queries regarding what all those lines and black circles with lines sticking into them and those numbers mean. Take a look at one synoptic chart offered by the SAWB :

A synoptic chart means "weather chart" and meteorologists need to plot weather data collected from hundreds of weather "stations" on a chart - why they are called stations, I don't know. Some kind of convention had to be adopted so that any meteorologist anywhere in the world can read such a chart and know what it means. Hence, each weather station on a synoptic chart is depicted by a circle and data collected at that station is plotted as shown in the simple example below.

The example above tells us the following :

dry bulb temperature : 16C
dewpoint temperature : 14C
atmospheric pressure : 1008.9 hPa
pressure tendency in last 3 hrs : steady fall of 2.6 hPa
wind direction : NW
windspeed : 30 knots
cloud cover : 8/8 (or 100%)
current weather : rain
cloud base is at 100m
clouds : cumulus and stratocumulus
rainfall past 3 hours : 15,0 mm
visibility : 5km

Wow - all that from a simple little diagram - yes, it is that powerful. Usually, not all the data is shown, and only temperature, current weather and pressure, cloud and wind data is shown. Let me explain how I deciphered the station model so that you can become an expert at interpreting synoptic charts.

Firstly let's pretend the circle is like a clock, so that I can use "the time" as the locator for data....

Let's start at 11 o'clock - the dry bulb temperature is the actual temperature of the air and is given in centigrade in South Africa and Fahrenheit in other countries like the USA.

At 7 o'clock we have the dewpoint temperature, which is the temperature at which water vapor will condense (into cloud/fog) if the air is cooled to that temperature. It is a useful indicator as to how much moisture is in the air - the closer the dewpoint is to the actual temperature, the more humid the air is.

weather symbols At 9 o'clock, we have the "current weather symbol", and there are many symbols representing many types of weather, and in this case, the single dot means light rain. The table to the left shows some of the more common types of weather and their symbols (if you go to meteorology school you have to learn all of them).

Also at 9 o'clock we have the visibility shown in kilometers.

At 2 o'clock we find the air pressure given in hecto Pascals (used to be millibars, but the metric system calls for it to be expressed in metric units - 1mb = 1 hPa). Air pressure is expressed using one decimal place, i.e. 1008.9 hPa and the last three digits are used, so for example:

1008.9 = 089, 1033.5 = 335, 995.3 = 953

BUT, what happens if the pressure is 1095.3 hPa you ask would that not mean 953 also ?....don't worry - the highest pressure ever recorded is about 1080mb and the lowest about 880mb, and neither were recorded in SA, so we never have to worry about not reading the correct figure from the pressure data on the station model....to complicate things a little, over the land (non-coastal stations) the figure at 2 o'clock actually does not represent a pressure reading, but rather the height above sea-level that a a certain pressure level is found - for example, over the land a number like 546 on the 850 hPa chart represents a height of 1546m where the 850 hPa pressure (surface) is found in the atmosphere.

windspeed symbols cloud cover symbols

Wind data is represented by the "stick" that points out of the station model. Wind direction is always given where the winds blow FROM, i.e. as though the winds blow INTO the station, so in the example above, the wind direction is NORTH WEST (i.e. the wind is coming FROM the NW or 315). The speed is represented by "feathers" or "barbs" that are usually perpendicular to the stick. Each barb is 10 knots and a knot is one nautical mile per hour which is equal to 1.8 kilometers per hour. So in the example above the wind speed is 20 knots or 36 km/h. Half a barb is 5 knots. At 50 knots, windspeed is indicated by a solid triangle as shown below.

Finally, the cloud cover is shown by how much of the station model is filled. See the examples above.

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