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The weathermen at Cape Town International assure us that the summer south-easter is predominantly a low level intense form of sea-breeze which usually only goes up to about 2500 feet (Table Mountain is just on 3500 at it's highest point, Maclear's Beacon). On stronger "macro south- easterly" days, it can pump up to just over 4000 feet - that's when the palm trees at Camps Bay "face off-shore" (an observation I've come to note in the peninsula is that wind in the lee of a mountain - that is blowing in the same direction as the prevailing wind - always tends to be significantly stronger than on the windward side of the mountain).
There are other significant factors that contribute to the wind shadow phenomena which are the exact angle from
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which the wind is prevailing, the topography of the peninsula (which splits or diverges the wind), the humidity on the day and the actual temperature in the city bowl.
The ideal "recipe" for a wind shadow, regardless of how strong it is elsewhere (False Bay etc) goes something like this: - A prevailing south-easter over the general peninsula area.
- Open, clear blue skies.
- Little or no cloud about over Table Mountain top, front face and the cableway.
- 4.. A minimum city bowl temperature of 25 degrees (and climbing).
Then of course there are the indicators or classic signs that give a clue as to the quality, degree and extent of the shadow.
The signs that indicate a shadow possibly developing are:- A calmer morning than the preceding "pomping" overnightsouth-easter.
- Dissipating orographic cloud over Table Mountain andDevil's Peak.
- A dark and obvious (straight) wind line extending from the Cape Town docks breakwater towards Robben Island.
- Flags in the area of the Woodstock off-ramp from the N1 change from off-shore "survival status" to relaxed on-shore flapping (litter on the railway yard fences - just before the N1 incoming splits to the elevated freeway and Oswald Pirow Street - is a good substitute for no flags).
- On the Bakoven/Camps Bay side a clear, well-defined wind line extending ideally perpendicularly off-shore (to the west).
- Wind at La Med on the field should light and from the north-west (not prevailing - that is the south-easters job in a wind shadow).
A confirmed wind shadow will exhibit all of the above, but it is critical to observe all these signs and check for any even slight changes in the pattern on the day. Most importantly, if the clear wind line patterns on the water showing the wind/no wind areas deteriorate to erratic or not well-defined divisions, then the wind is about to come through.
Some of the other clues come from our feathered friends, the Jonathan's of the air, who love playing in the turbulence between the approaching south-easter (never seen then playing on the edge of an entrenched wind shadow -says something about the vertical shear dividing these areas). Simply put, if you see the gulls playing around in a vertical separation as opposed to a horizontal one over Camps Bay or La Med, expect the wind to change.
Flying in the wind shadow offers some rewarding stuff if you read it correctly - it can also be just as
dangerous. We have one of the very few coastal sites in the world where when it is howling 30 knots
plus elsewhere nearby, we still get to fly. Make sure you understand what you are letting yourself in for
and equip yourself for any eventuality. If you are unsure of the conditions then contact an experienced
pilot to verify what is happening on the day. Don't just rock up, see no wind where you don't want to
see any and throw yourself off.
Wind shadow flying has the potential to provide the best thermal activity for Lion's Head and the best
altitude gains - figures like 7 m/s(sustained) and altitude gains of 1100m ATO have happened more
than once and to more than one person - trick is to spot the day. Enjoy the wind shadow.
Let me begin by explaining how the SE is supposed to work according to the weatherman. The SE is primarily an intense low-level sea breeze which sets up over the peninsula and usually (in summer) only blows up to about 2000 to 2500 feet (Table Mountain is 3500 feet high [1083m at Maclears Beacon]).
What happens is the wind diverges (splits) around the Table Mountain chain, causing areas like Bakoven (and North) and Green Point (andSouth) to fall into an area of dead air (or wind shadow). The upper level air, directly above these areas, has no prevailing wind influencing it and does not usually have the SE blowing over the top of it (as we have discovered on occasion flying the East side of Table Mountain). One can often commute to town via De Waal Drive/Hospital Bend and see the gum trees blowing in a strong SE, only to get down into town and see the Kentucky flags in the gardens facing North in a gentle breeze. If you think you've struck it lucky as far as the wind shadow goes, take the trouble to drive up to the corner of Signal Hill first and view the entire area of Table Bay to confirm the extent and degree of the wind shadow. Again, check it out from Kloof Nek and make your call.
The other major influence on the degree and extent of the wind shadow is of course cloud cover and temperature, but let me get to those in the list of things to look at when assessing the conditions.
Here are some handy pointers that I have found work well for me in seeing whether or not a SE wind shadow is A: Going to set up and B: In place:- There is a SE forecast up to as much as 40 kph (for this time of year) and up to as much as 50 kph (in mid summer), combined with clear skies & a temperature of 25 degrees or higher in the city bowl.
- The barometric pressure at sea level needs to be higher than 1018 hPa (lower pressures tend to allow the SE to blow at higher altitudes, i.e.. over the mountain).
- Little or no cumulous cloud should be present above the back table or Apostles.
- During the early morning one can already see areas of wind and no wind on the western seaboard (what you are looking for is no wind between Bakoven and the harbor).
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- There is a strong SE in place over the southern suburbs and Milnerton area, but town is calm to light northerly with a temperature of higher than 25 degrees C.
- If entering town from De Waal Drive, the Kentucky flags are showing a light northerly.
- If entering town along the N1, the wind stops at the boat shop near the old power station and the litter on the Culemborg fence is showing light northerly.
- If you came in over Eastern Boulevard, the Holiday Inn flags are usually confused about the wind direction, because they are in the transition area.Confirm yoursuspicions by checking the Waterfront entrance flags and the Quickfit flags in Buitengragt Street.
- Look back as you drive up Kloofnek Road and ascertain exactly where the wind shadow lies.
- Check for the same from Kloofnek towards Llandudno. 7.What you are looking for is a clearly discernible line, dividing the wind/no wind areas. If they are undefined or varied then it may not be right.
- Make sure there is no cumulous cloud spilling down the Table Mountain front face or the Apostles.
- The ideal wind shadow runs perpendicularly off shore from the new Bakoven Hotel and perpendicularly off shore from the container basin in the harbor. If it is anywhere inside of those parameters, best check with a pilot of high experience of this phenomena.
- Once you've decided to fly, if when you commence your walk up Lion's Head, you feel wind chasing you from the car park, abort (remember to differentiate between a thermal and a prevailing breeze).
Many amazing flights have been done by flying in the wind shadow. Some pilots feel it's not safe, but I
believe it's because some of us have tested the parameters of the wind shadow too hard and flown when it's been well inside of what I described above.
It is a unique phenomena and on occasion has produced exceptional flying. The best ever recorded height gains and lift strength of 7.2 m/s where recorded on a day when False Bay and Table Bay were white capped all the way to the horizon. To confirm this, on the same day two other pilots took off 30 mins later and got to the same height in 6.5 m/s lift. The conditions were so unusual, that we all came back to land safely at La Med with BIG EYES.
For those permitted to fly Table Mountain, the front face take off frequently works in these conditions too
(which are co-incidentally the best f/face conditions).
-fin-
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