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Thu, 11 Mar 2010 | |
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You are in: Weather > SA regional > Ecucation  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 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.
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.
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. [ back ]
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ALEXANDER BAY 28/18 ALLDAYS 34/20 BEAUFORT WEST 33/19 BELA-BELA 33/19 BETHLEHEM 29/12 BISHO 29/19 BLOEMFONTEIN 34/18 CALVINIA 30/16 CAPE ST LUCIA 29/23 CAPE TOWN 23/18 CLANWILLIAM 29/17 DE AAR 34/20 DURBAN 29/22 EAST LONDON 29/23 ERMELO 28/10 FAURESMITH 34/19 FICKSBURG 29/12 GEORGE 27/19 GRAAF-REINETT 30/18 GRAHAMSTOWN 28/20 HERMANUS 25/18 JOHANNESBURG 30/14 JOUBERTINA 26/19 KIMBERLEY 34/21 KLERKSDORP 32/17 KOMATIPOORT 30/17 KROONSTAD 32/14 LADYSMITH 28/11 LANGEBAAN 25/19 LEPHALALE 34/19 LYDENBURG 28/13 MAFIKENG 33/21 MUSINA 35/21 NELSPRUIT 28/14 NEWCASTLE 28/11 OUDTSHOORN 29/19 PAARL 28/16 PHALABORWA 35/21 PIETERMARITZBURG 30/16 PLETTENBERG BAY 25/21 POLOKWANE 31/16 PORT ALFRED 27/21 PORT ELIZABETH 28/23 PORT ST JOHNS 28/19 POTCHEFSTROOM 33/14 PRETORIA 32/16 QUEENSTOWN 29/15 RICHARDS BAY 28/23 ROBERTSON 28/17 RUSTENBURG 32/17 SASOLBURG 32/14 SKUKUZA 31/17 SOWETO 30/14 SPRINGBOK 33/16 STILBAAI 24/20 SUTHERLAND 29/15 TZANEEN 31/17 ULUNDI 30/17 UMTATA 30/16 UPINGTON 36/23 VEREENIGING 32/14 VREDENDAL 28/17 VRYBURG 35/21 WELKOM 34/17 WITBANK 29/15 WORCESTER 28/16 |
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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).
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