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Wed, 10 Mar 2010

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SOUTH EASTER

The South Easter, or better known as the Cape Doctor, blows mainly during the summer months and makes its first appearance during early spring during the months of August and September and usually lasts until late Summer and early autumn during the months of March and April.

The South-Easter is a fairweather or trade wind. It originates from the South Atlantic High (SAH) pressure system. The SAH moves further southwards in summer as the westerlies retreat polewards. The SAH then ridges south of the country and joins up with the South Indian High pressure system often forming a band of high pressure to the south of the country during summer. The SE wind varies in speed from 10 knots to about 70 knots (20-125 km/h) at times during gale-force wind events.

The South Easter's speed is directly proportional to the pressure gradient that exists between the SAH and the low pressure trough over the interior of South Africa. The diagram below shows the general summer circulation affecting the SW Cape.

Click to view a larger image.

Air flows anti-clockwise around a high pressure and clockwise around a low pressure in the southern hemisphere so the resultant wind over CT in summer is mainly from the south east. When the interior trough (low pressure) moves westwards and is situated closer to CT and the SAH is stronger than usual then the pressure gradient will increase between the two systems and the windspeeds will increase dramatically over CT. The opposite is also true.

During extreme SE conditions windspeeds of up to 160km/h (100mph) have been measured in Table Bay ! Double-decker busses have been blown over, not mentioning the number of pedestrians hanging on to poles for dear life ! The SE wind is generally not a blustery wind such as the north wester and generally is fairly uniform in speed. It is most active in late spring during the month of November, when GALE-FORCE SE winds have been known to last for up to a week on end.

Before we continue, we need to understand what an inversion layer is.

DEEP SOUTH EASTER CONDITITONS

The weather of Cape Town varies on a basic 7-day cycle. During summer the SAH starts to ridge south of Cape Town and the SE wind will be fairly strong over the entire area, strongest along the coast. At this stage we have what is known as a DEEP SOUTH EASTER. The inversion layer occurs at a height greater than the height of Table Mountain, i.e. >1000m above the ground.

Click to view a larger image.

You can see from the above diagram that the inversion is higher than Table Mountain and the wind is able to flow over the mountain. Under these deep SE conditions the wind will be fairly strong over the entire area with very few sheltered places. The well known "Table Cloth" (cloud) is usually present over the mountain during DEEP SE conditions. The SE wind is usually strongest over False Bay.

The map below shows that the strongest SE wind occurs along the SW coast and blows over the entire Peninsula. These cnditions usually last for about 1 to 4 days when a transition occurs to SHALLOW SE CONDITIONS described below.

Click to view a larger image.

SHALLOW SOUTH EASTER CONDITIONS

After 1 to 4 days of DEEP SE conditions the SAH ridges further south of the country and as it moves closer and closer to CT, the inversion layer also descends closer to the ground. The inversion layer is lowest in the centre of a high pressure, so it makes sense that as the ridging SAH moves closer to CT, the inversion layer drops lower and usually will drop lower than 1000m above sea-level. This means that the inversion layer is lower than the height of Table Mountain as seen in the diagram below.

Click to view a larger image.

The inversion layer is also known as a "capping" layer. What this means is that it acts as a "ceiling" and will not allow the SE wind to blow OVER Table Mountain. In effect it is "squeezed" towards the surface by the inversion layer that is descending. Seeing that air is a fluid it behaves just like water in many respects and just as a river flows faster in a narrow channel and slower in a wider one, so too, the SE wind. Since the the inversion is now lower, the SE windspeeds up and this effect is most evident at Cape Point. Topographical forcing such as the "cornering effect" and the thermal capping (inversion layer) cause the SE wind to blast Cape Point and certain well-known areas of the Peninsula such as Hospital bend. Seeing that the wind can't go over the mountain, it has to go around it. This is why places such as Clifton along the Atlantic coastline can endure dead calm conditions, whereas on the otherside of the mountain, at the airport, winds in excess of 35knots, or GALE-FORCE are found.

Towards the end of the ridging process when the SAH has ridged well south of the country and the inversion layer is at it's lowest altitude, the various effects on the wind cause extremely strong winds at Cape Point and mostly calm conditions elsewhere in the Peninsula and SW Cape. A shallow NW return flow often is found from Clifton northwards to Table Bay under these conditions. It is usually also very hot as the wind has died down. Shallow SE conditions usually last from 1 to 3 days. Some cloud, usually stratus, may develop along the eastern flank of Table mountain as shown in the diagram above, but NO Table Cloth is present.

Click to view a larger image.

The diagram above shows where the SE is the strongest during SHALLOW SE conditions. The return NW flow into Tabel Bay is evident as is the very strong SE wind at Cape Point which often reaches speeds of up to 100km/h at times in summer.

THE BLACK SOUTH EASTER

The Black South-Easter is a much misinterpreted phenomenon. It is refered to as a "BLACK" SE event when the SE wind is blowing, usually rather strongly, and it is raining at the same time. A Black SE usually is caused by a deep low pressure system over the SW Cape both in the upper air and on the surface and a very strong/intense SAH south of the country. The Laingsburg Floods and the Easter of 1994 are prime examples of Black South-easters in Cape Town. The tight pressure gradient between the systems cause the wind and the low pressure systems produce the rain. These Black SE events usually occur during spring and autumn, the months when the incidence of cut-off low pressure systems are high.

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