Hi Nic,

I can't give you specifics, partly because I know very little about it, and partly because, as yo well know, we have very varied topography round here. Where I live we hardly know there is a southeaster, but just over the hill you had better have your hair glued on. I know ofa  house near the crest of teh hill, where a silk-floss tree grows happily up to perhaps four metres, but at that point it peters out into dead twigs within about 15 cm, where the wind catches it. Go for a walk up the mountains on the peninsula, and you will find bushes of steenbokboegoe (Coleonema) hiding behind each other and behind rocks, even filling clefts with dense growth whose surface exactly extrapolates the contours of the rock. I actually knew one lightly built man who got picked up by the SE at an Adderly Street corner and dashed against a lamppost (I think; I wasn't there when it happened) and died in hospital a few days later without regaining consciousness).

All the signs of wind being affected by ground contours!

But as a general rule, you can assume that where there is a lot of "roughness", rocks, buildings and so on, ignoring channelling and blocking of the wind, the windspeed might be slowed down by forty or fifty percent, depending on who is measuring what, or arguing what. Open ground  slows it by perhaps half as much. say 20%.  But what happens as you go up is a fairly smooth increase in speed to a maximum perhaps 100m to 200m above the "roughness".

Remember, there we are talking about a mass of air blowing more or less as a unit. A few km above ground there can be different layers of air not only at different speeds, but blowing in different directions. Many of our most attractive cloud patterns are the results of the shear and ripples between such layers. Any time you see rippled or barred clouds far away from the mountains, that is likely to be the cause, much as the flow of a smoothly running stream, or intersecting streams, causes the ripples on its surface.

So in Cape Town, at a thumbsuck, you might guess that a 30kph wind at ground level, would be something like 60kph at about twice the height of the local skyscrapers, and also about the top of the mountain.

Mind you, some of us went to the top of Hangklip some moths ago, and the wind at the top was seriously dangerous; you had to hang onto the rocks if you didn't want to be blown off the crag. It felt liquid!

Cheers,

Jon