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 fair weather 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. 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 wind speeds will increase dramatically over CT. The opposite is also true.

During extreme SE conditions wind speeds 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 CONDITIONS
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. 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 conditions usually last for about 1 to 4 days when a transition occurs to SHALLOW SE CONDITIONS described below.

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. 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 wind speeds 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 other side 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.

The diagram above shows where the SE is the strongest during SHALLOW SE conditions. The return NW flow into Table 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 referred 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.

A "tephi" what ? If you are a pilot or a meteorologist, you are about the only people on this planet able to decipher this very useful chart. Click on the link below fo a typical tephigram issued by the South African Weather Service (SAWS) :

Tephigram for Cape Town, South Africa.
On the tephigram you will see myriads of criss-crossing lines - they will be explained, but all this diagram really is telling us what the conditions are like from the surface to the upper atmosphere. It is essentially an energy diagram. It tells us which layers are moist and which are dry, and what the winds are doing at certain altitudes. Data collected by a weather balloon and radiosonde is plotted on a tephigram.

Let's look at individual features of the tephigram :
• dry Bulb temperature sounding line
• dewpoint temperature sounding line
• significant levels
• flight Levels
• temperature lines (vertical)
• potential temperature lines (horizontal)
• pressure level lines (curving from bottom left to upper right)
The lines on the tephigram all represent a particular atmospheric variable, and so let's look at them all individually without the sounding data plotted on it.

This it the tephigram plot with all the lines on it, but without any data plotted on it. There are essentially 5 lines on the tephigram :


• horizontal lines = dry adiabats (lines of equal potential temp in Kelvin)
• vertical lines = isotherms (lines of equal temperature measured in °C)
• bottom left to upper right curving lines = isobars (lines of equal pressure in hPa)
• top left to bottom right = wet adiabats (lines of equal equivalent potential temp in Kelvin)
• dashed diagonal lines = saturated mixing ratio (lines of equal water vapour content in g/kg)
Wow, what does all that mean you wonder...let's look at all individually and explain what we mean.



Firstly, an adiabatic process is one where no transfer of heat takes place. For instance, if if you took a parcel of air and made it rise into the atmosphere, it will cool as it expands (since pressure decreases with height), but no heat is gained or lost, sure the temperature drops, but that is a result of the air molecules being further apart, and not owing to heat loss. Heat is only lost during diabatic processes such as evaporation and condensation.

Bergwinds are the best examples of adiabatic processes ! It is the reverse of the process of a parcel of air rising, expanding and cooling - a parcel of air brought to the surface from higher in the atmosphere, will compress and the temperature will increase at 10°C per 1000m as it descends. So how does this help me on the tephigram. Well, the process just described with bergwinds, is useful - air at a high altitude if brought to 1000m will have a certain temperature, called the potential temperature which is an indication as to the amount of energy in the atmosphere. How would you calculate potential temperature - well, we will explain at the end...
These lines represent the temperature of the atmosphere.



These lines represent the pressure of the atmosphere.

We have already described what an adiabatic process is, and the explanations above assumed a DRY atmosphere. A wet adiabatic process is a pseudo-adiabatic process in reality, since there is a slight heat transfer, but we will explain this now. What if a parcel of air that rises and cools at 10°C per 1000m has moisture in it (like in the real world). You should know that as air cools, its capacity to hold moisture decreases and at a certain temperature (called the dewpoint temperature) the water vapour will undergo a phase change, i.e. it will condense into the liquid form and form cloud or fog droplets. If this parcel of air continues to rise, it cools less quickly, since condensation has released heat into the environment, i.e. air around the parcel, so it only cools at about 6.5«C per 1000m until all the moisture is condensed out of the parcel. When that happens, it will be dry and continue cooling at the dry adiabatic rate, called a lapse rate of 10°C per 1000m if it is still rising.

However, let's say that we want to know how much energy there is in the atmosphere (which is always moist) and let's assume that we choose a parcel of air in a cloud and bring it to 1000m. Well, it will warm at 6.5°C per 1000m, and not at 10°C per 1000m since as it warms the water will evaporate, a process that needs heat to occur, so 3.5°C per 1000m goes into evaporating the water back into vapour. When that parcel finally reaches 1000m the temperature it now has is known as the equivalent potential temperature and is a better measure of the energy in the real atmosphere, than the potential temperature described before.

Finally, the last of the lines - the mixing ratio lines. Mixing ratio merely refers to the amount of water vapour (grams) in the atmosphere per kilogram of air and is a great measure of the direct amount of water vapour in the atmosphere.

So, we release a weather balloon with a radiosonde that collects the following data every 10 seconds or less as it rises :
• temperature
• pressure
• humidity
• wind direction and speed
So at almost every point in the atmosphere, we know the temperature, pressure, water vapor content and what the winds are doing.

We plot the temperature measured by the radiosonde against the pressure it was measured at, and soon we will have the temperature line you saw on the first figure. We also then plot the dewpoint line which is calculated from the humidity measurements. Finally the wind data is plotted at certain significant levels (i.e. 850 hPa, 700 hPa, 500 hPa, 400 hPa, 300 hPa, 200 hPa and 100 hPa) or at predetermined flight levels and these winds are plotted in the same manor that station model winds are plotted.

Many, many different atmospheric variables essential to forecasting can be calculated from the tephigram. These include stability indices, thunderstorm likelihoods, inversions, how quickly thunderstorms are likely to develop in the day, how strong the SE wind will blow in Cape Town and many other variables. The tephigram in the first figure, shows a moist layer and a dry layer - where the two dark lines are closer together, the more moist the atmosphere, the further apart, the drier the atmosphere. Needless to say, to try and explain exactly how these variables are calculated, would only be of interest to pilots and meteorologists, and are difficult to explain in the context of this article.