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SA REGIONAL




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THE SYNOPTIC CHART

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 "H"s and "L"s mean. Take a look at two synoptic charts 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.

On the chart above, we see the following :

  • isobars, or lines joining areas of equal pressures, much like contours on a map indicating equal heights
  • pressure values in hecto Pascals, e.g. 1024 hPa indicated by the upper blue oval on the map above
  • high pressure cells, i.e. the South Atlantic High Pressure cell is seen SW of Cape Town
  • low pressure cells,i.e. the low pressure cell at the bottom of the map associated with a
  • cold front and
  • coastal low pressure cell along the Namibian coastline
  • station model as seen at Cape Town where the wind is NW 15 knots, and the temp is 15C

Air flows from a high pressure which is the center of descending or sinking air (which suppresses cloud development), and hence clear, skies, to areas of low pressure where air converges or rises and where clouds usually form. However, air cannot flow across isobars form high to low pressures, but instead blows parallel to isobars. Well, if air flows parallel to the isobars, how does air get from the high pressure to the low pressure, surely air would just go around these cells instead ?

The answer is that in reality, near the surface, air actually does flow across the isobars owing to friction between the air and the surface of the earth. Hence, flow is across the isobars towards centers of low pressures and away from centers of high pressures. In the southern hemisphere, the flows clockwise around low pressures and counter-clockwise around high pressure cells.

A pressure gradient exists between high pressure cells and low pressure cells. Think of a high pressure being a mountain and a low pressure being a valley - if you drew a line from the highest point of the mountain to the lowest point of the valley you would have one side of a triangle of a right-angle triangle, and little trigonometry would tell you that the ratio of the height to the horizontal distance is the gradient, the the higher the mountain and lower the valley, the steeper the gradient.

Well, the stronger a high pressure is (i.e. the higher the pressure) and the deeper a low pressure is, the stronger the gradient will be, or if the distance is shortened between a high pressure and low pressure, so too, would the pressure gradient be increased, all resulting in stronger winds . On a synoptic chart, isobars that are closely packed together indicate stronger winds, and vice versa. On the map above, the strongest pressure gradient exists between the South Atlantic High pressure and the deep low pressure (988 hPa) near the bottom of the map and strong SW winds wil be found in the region between them.

The red arrows indicate the direction of the wind, and remember, wind is named where it blows from.

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