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Bergwinds
Coastal Lows
Cold Fronts
El Niño
Inversion Layers
Synoptic Charts
Station Models
Sea Surface Temps
The South Easter
Tephigrams
Winter Rainfall

BERGWINDS

Bergwind is the South African name for fohn wind or mountain wind. It is a hot, dry wind that blows from the interior of South Africa to the coast and usually is blustery. Most people find it rather unpleasant. It can be mild at times blowing at about 10km/h, but sometimes it can be really strong and may gust up to 100km/h causing some structural damage to buildings and uprooting trees.

FORMATION
Bergwinds usually occur when a strong high pressure exists south or south-east of the country and when a high pressure is also situated over the country. These conditions usually only occur in winter, but sometimes in summer too, hence bergwinds are mainly a autumn-winter-spring phenomenon.

Since air rotates anti-clockwise around a high pressure in the southern hemisphere the wind direction to the north of the high pressure will be easterly or north-easterly, especially along the west coast of southern Africa. The bergwinds will usually start blowing along the Namibian coast and the first indication of bergwinds is a rise in temperature, sometimes this rise can be rapid. In the winter of 1985 in Cape Town, the temperature rose from 3°C at 07:00 in the morning to 27°C by 07:35 that SAME morning. This is not a common occurrence, but rather an extreme case.

Since air in a high pressure descends and warms up as it descends it stands to reason that the off-shore winds will be warm to hot and the temperature will usually rise about 10 degrees Centigrade from the interior of SA to the coast. So the temperature at Upington may be 20°C, while at Alexander Bay it may be over 30°C.

At the same time a coastal low will develop along the coast. Off-shore flow ahead of the coastal low is usually easterly to north-easterly in direction along the west coast and north-westerly along the east coast. Humidities are usually very low, sometimes as low as 5%, usually between 10-40%.

The temperatures vary from 25°C to 35°C in winter and over 40°C in summer. Behind the coastal low the wind is on-shore and usually north-westerly to south-westerly in direction. It is cool and moist and usually associated with fog.

The coastal low moves down the west coast and around the Cape Point and then up the east coast of South Africa until it fills up near Maputo in Mozambique. The bergwinds obviously follow the same pattern. The highest official temperature ever recorded in South Africa (51.5°C) was recorded one summer during a bergwind occurring along the Eastern Cape coastline.

The bergwinds are usually followed by a cold front in winter.

COASTAL LOWS

A coastal low is a shallow low pressure system limited to the lower layers of the atmosphere and is formed when the wind blows from the land TO the sea, usually during bergwinds.

These coastal low pressure systems usually form off the Namibian or West Coast of SA. They then slowly move down the coast and round the SW Cape and then move up the East Coast of SA where they finally fill up along the Mozambique coast where the plateau ceases to exist. Click on the minimized image below to view a larger diagram showing how the coastal low is formed.

Air moves off the coast at a height of approximately 1500m. This creates a relative "vacuum" at the surface as air rises to replace the air that is moving away from the coast. In a low pressure system, air rises clockwise in the southern hemisphere and the resultant circulation around the low pressure is the same. Hence, prior to the presence of a coastal low, the winds are usually offshore and in Cape Town they are usually in the form of hot, dry NE bergwinds. Once the coastal low moves through the area, the hot, dry winds are replaced by cool, moist NW-SW winds and usually fog. Coastal lows usually precede cold fronts in winter.

COLD FRONT INFORMATION

Cold fronts are mid-latitude phenomena - meaning they are not tropical in nature and occur mostly south of 25°S or north of 25°N....find out more of these rain-bearers....

Cold fronts are mid-latitude phenomena - meaning they are not tropical in nature and occur mostly south of 25°S or north of 25°N. The word "front" means "boundary" and a cold front is a boundary between cold and warm air, where cold air is under-cutting (moving under) warmer air ahead of it. Fronts usually form far south where the westerly wind-belt meets the polar easterlies - somewhere near 60°S. A disturbance may cause the cold easterly air to invade the warmer westerly air and since cold air is denser, will, move under the warm air. Warm air could also invade the cooler air and ride up over it since, as you should know, warm air is less dense and it is force to rise by the colder air beneath it, it will want to rise further and as it does, air that rises, cools and as it cools, any water vapor in the atmosphere will condense and form clouds. It is suggested that you familiarize yourself with synoptic chart terminology before reading this article unless you know basic meteorological terms.

FIG.1 Formation of a cold front in the southern hemisphere. Below is a cross-section through a cold (blue) and warm (red) front - clouds are Ns (nimbostratus), Ac (altocumulus), As (altostratus), Cs (Cirrostratus), Ci (Cirrus).

HOWEVER, here is the BIG catch. Most times these cold or warm air invasions into the warmer westerlies die out and no low pressure cell forms etc...WHY ? Well, the mid-latitudes are controlled by what is know as a "top-down" process...a "what ?" I hear you ask....

FIG.2 Formation of a cold front in the southern hemisphere.

FIG.3 The "top-down" process.
Well, I think most of you have heard of the "jetstream" - this is a strong current of air (like a river) in the upper atmosphere (10-13km altitude) that snakes around the globe usually between above the surface westerly wind belt (remember, you name a wind FROM where it blows, so westerlies mean the winds blow FROM west TO east). Now, these upper winds/jetstream are FAR stronger than the surface winds...think of a river - the surface waters flow faster than the bottom waters - why, well, friction slows the water at the bottom of the river - the atmosphere is NO different....now the jetstream is not a constant speed airflow or "river", rather, think of it like traffic on a highway - in some places the traffic speeds along and at other places the traffic slows down to perhaps to a crawl.

The jetstream is no different - now where it really speeds along in the upper atmosphere, meteorologists refer to it as a "divergent" area, and where it slows down, a "convergent" area. Once again, think about the traffic analogy - when cars on a highway move away from a congested area, the cars start getting spaced further apart as they pick up speed - i.e. the cars are diverging from one another, and where they encounter a snarl up, they become closer together and "converge" on one another....

Where air in the upper atmosphere diverges it really means that air is leaving a certain area faster than it can be replaced, and somehow, it needs to be replaced ASAP - most of it is replaced from the upper atmosphere, but some of the air is replaced from the surface, almost like a giant vacuum cleaner in the upper atmosphere sucking up air from the surface. See figure 3 above.

Now, lets go back to the original scenario that I explained on the surface, i.e. the cold air from the easterlies undercutting the warm westerly air...the warm is forced upwards since it is less dense....NOW here is the connection - IF the upper atmosphere /jetstream is DIVERGENT, i.e. needing air to replace from the surface, it will AID the warm air at the surface (that has been forced upwards by the cold air) to RISE to the upper levels and the stronger the upper-level divergence the more air that is needed from the surface and the faster the air will rise and bingo - a LOW PRESSURE cell is formed on the surface along the boundary between the cold polar easterly air and the warmer westerly air. This connection between the upper and lower atmosphere is called the "top-down process" - since a low pressure will not develop sufficiently into a deep low pressure and cold front if the upper atmosphere does not aid it with strong divergence aloft.

FIG.4 The upper and low circulations super-imposed - dashed lines are surface isobars (equal pressure lines) and solid isobars are of the upper-level circulation.
Cold fronts with their low pressure cells move eastwards, since the air in the jetstream moves eastwards and the surface features are "dragged" in the same direction. However, since the air in the jetstream moves a lot faster than at the surface, the area of strong divergence aloft (and which is causing the low pressure at the surface to exist) sometimes OVERTAKES the surface low pressure and the connection between the upper level divergence and lower atmosphere low pressure is disrupted and the low pressure at the surface will weaken and eventually die out and the cold front weakens and dissipates.

Hence, sometimes the jetstream in autumn in the southern Atlantic is such that active cold fronts may be formed in the southern Pacific Ocean west of Chile and move eastwards and appear strong on satellite images/1STWX/old in the mid Atlantic but weaken and die out or move south of the country owing to the way the jetstream is aligned in the upper atmosphere.....So, to cut a long story short, at certain times of the year, the jetstream is further south (in summer) and most of the cold fronts move south of the country, and during winter, they occur further north and CT gets a lot more rain. However, sometimes for a few days or even weeks, the jetstream may align itself such that cold fronts will weaken as they approach CT frustrating forecasters to no end....

EL NIÑO / LA NIÑA INFORMATION

There are numerous websites out there with great information - a list of some excellent resources can be found below:

El Niño Theme Page

What is an El Niño?

What is an La Niña?

NOAA's El Niño Page

El Niño - and What is the Southern Oscillation Anyway?

Impacts of El Niño and benefits of El Niño prediction

Where can I find data on El Niño?

INVERSION LAYERS

Weather you live in Cape Town or Johannesburg, you will have seen how inversion layers trap smoke, fog and smog near the ground in winter, but did you know it also is one of the causes of the howling SE winds over the Peninsula and determines whether or not the famous "Table Cloth" will form on Table Mountain.

A surface inversion layer is a layer within the first 1500m of the atmosphere. It occurs when a air within a high pressure descends and as it descends, it warms the atmosphere, much like a bicycle pump warms up when you pump up a wheel. When air is compressed it will warm up. As the air descends from heights of about 12km it will warm and the warmest air will be found at this inversion layer which can be several meters thick. The air temperature then cools towards the surface.

So if one had to move from the surface of the earth when an inversion was present, you would notice that the air warms as you ascend and it will become warmer until you reach the inversion layer when it will start cooling again. Inversion conditions usually suppress any convective development and usually very stable conditions occur with inversion conditions. Inversion layers occur throughout summer over CT. They also occur in winter on fine cold mornings when most of the smog is trapped within the lowest 200m above the ground.

Sunday, 3rd November 1985, taken in Constantia, Cape Town, looking east towards the Boland Mountains. © Kevin Levey
The photograph to the right was taken by Kevin Levey on Sunday 3 November 1985 in Constantia, Cape Town looking towards the NE at around 3:30pm. Large convective cumulonimbus clouds had built up during the day over the Boland mountains, but were severely restricted in their upwards growth by a very strong inversion, probably at about 2000m. This can be clearly seen from the very flat tops of the two massive clouds.

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 the synoptic charts offered by the South African Weather Services (SAWS) :

Ocean surface synoptic chart
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 to the right, 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 15°C

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 will 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.

THE 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 South African Weather Service (SAWS):

Ocean surface synoptic chart
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.

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 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 to the left.

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

SUMMER SEA TEMPERATURE DIFFERENCES AROUND CAPE TOWN

Ever wondered why the ocean temperatures are so cold at Clifton and the Atlantic coastline beaches in summer, while at the same time the waters in False Bay are warm in comparison?

Actually it is rather a simple explanation. During summer, the dominant wind direction is SE, i.e., the trade winds that blow from the SE towards the NW. The dominant South Easterly wind direction has two consequences :

However, let's first look at the action that the wind performs on the ocean surface. Without getting too theoretical, it is is sufficient to know that when wind blows over an ocean surface, a stress is induced on the ocean surface. In short, this means the wind "pushes" or "pulls" the surface layers of the water along with it. Hence, if we apply this to the Cape Town situation in summer the two consequences are :

FALSE BAY
The direct action of the SE wind is to pile up the warm surface waters along the Peninsula beaches such as Fish Hoek and Muizenberg. Hence water temperatures in summer approach the 22°C (72°F) mark.

ATLANTIC COASTLINE BEACHES
Just the opposite occurs here. The strong SE winds pull all the warm water AWAY from the coast. However, this water has to be replaced. In reality the warm surface waters are replaced by very cold water from great depths along the Cape Peninsula. This water is usually in the realms of 10-14°C (50-57°F) and is known as "upwelled" water, for it has been "upwelled" from greater depths. This cold water is nutrient rich and is usually very clear and blue. So, the next time you wander off down to Clifton and the water is crystal clear, you can bet your bottom Dollar that it is going to be freezing.....If you think that this is cold, try swimming in the great Lakes of the USA in summer......
Keep in mind that the cold water is the result of the SE wind that has blown over the surface of the ocean for sometime. So whenever the SouthEaster has been howling, you can be guaranteed that the Atlantic coastline beaches are going to have freezing waters....but then the hot bods make up for that shortfall !!!!!

SOUTH EASTER INFORMATION

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.

TEPHIGRAMS

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.

WINTER RAINFALL DIFFERENCES AROUND CAPE TOWN

Rainfall around the Cape Peninsula and the Boland varies greatly in space. Newlands is one of the wettest suburbs in the country with an annual rainfall of well over 2000mm (80 inches) while only 30km (18mi) away within view the Cape Flats (Airport) only receives about 550mm (22 inches). Jonkershoek near Stellenbosch also receives very high rainfalls and holds the South African annual rainfall record of over 4000mm (160inches). The diagram below shows how the rainfall is distributed around the CT area.

RED = 0-500mm
YELLOW = 501-1000mm
GREEN = 1001-1500mm
BLUE = 1501-2000mm
PURPLE = > 2000mm

So what causes these great differences in the rainfall ?

Topography, i.e. the mountains play a very significant role in the distribution of rain and also the wind direction.

Usual geographic theory tells us that when a moisture laden onshore wind encounters a mountain it will rise and moisture will condense and rain will occur on the windward side. The leeward side remains dry as the moisture is not able to rise up and over the mountain, which acts as a barrier. Hence the leeward side is usually very dry. This is typical of the George-Oudtshoorn area. This is depicted in the diagram below.

However, let us take the usual scenario in Cape Town in winter when a fresh to strong north westerly wind is blowing. Since Table Mountain is not too high, the airflow is able to go up and over the mountain. Table Mountain therefore acts as a trigger mechanism, i.e. it forces air to rise up higher than it would have if it had not encountered the mountain. As it is forced up, the moisture condenses and most of the rain falls over Newlands, where the uplift is strongest.

As the airmass moves away from the mountain the upward forcing is no longer there and the rainfall also decreases proportionally as one moves away from the mountain. This is why Newlands and most areas such as Wynberg, Constantia and Claremont get so much rain in winter compared to other regions in and around Cape Town. Also, the mountain causes great wind turbulence in it's wake, hence these same areas also get some of the strongest winds in the Peninsula during really bad north westerly gales.