Scenic Cumulus: This view could be of a summer storm brewing on the Highveld in Gauteng, except this one is at Swifts Creek, Australia.
Photo: Fir0002/Flagstaffotos ( GFDL licence )

Weather info

Mainly for Johannesburg - Dunnottar - Vaal Dam area

The forecasts have been discontinued, but the page remains for info purposes

Laura's forecast

Apologies, the forecasts have been discontinued.

(The sms info is still available. Remember that you will not be getting the sms info unless you have subscribed or are a Fun Ventures student. If you have paid but are not receiving the sms info, please remind me where and when you have done so.)

Ulf's Forecast

Every day:

Sadly, we will never see another forecast from Ulf again. Ulf Arndt passed away on 20 November 2010 after a car accident.

We miss you so much, Ulf!

For the time being, Ulf's codes, info and methods of forecasting remain available on his Skygod website.

Code available from in the NOAA-READY-ANALYZE Folder
Instructions at

Factors that Influence Laura's Forecast
The factors that influence the sms and info that I put out:
  • Look outside - this can be deceiving as very often in the early morning the weather may be perfect with little or no wind (or the opposite) and change completely in a few hours time. But it is always interesting and regenerating to the soul to step out and see what is happening, hear the birds singing, and breath the cool fresh air, or feel the rain.

  • Ask Ulf what his prediction is. Really. But now I can only wonder what he would have said.

  • Look on the computer at the Dunnottar and other regional iWeathar weather stations. Also the Metars. Both Oliver Tambo (FAJS) and Germiston (FAGM) reflect most closely what is happening in Dunnottar. It is also quite relevent to the Vaal Dam, but the Vereeniging (FAVV) metar is also informative, even if sometimes a bit on the low side. In summer I also check the radar and radsat images to see if there are any storms brewing and where they are.

  • Call the Met Office (Weather Bureau) at Oliver Tambo airport to get
    • their prediction for the day
    • the latest aircraft sounding

From all of the above information, I make a decision on whether it will be flyable during the day, and what time to go out, if it is flyable. The aircraft data plays a big role.

The difficult one is when the wind is marginally strong, especially in the upper air - do we or don't we go out? That decision will likely be influenced by what the windspeed is doing from 7000'asl up to 10,000'asl, according to the aircraft sounding. There is a serious possibility that stronger winds will make their way towards the ground with time during the day. Similarly, lighter winds will usually eventually make their way downwards.

The other difficult decision is the prediction on when the rain will start, if it is going to rain? I hope the Met will be right, but often they are not. It may rain or not at all, or it may rain earlier or later than predicted! But one can very often fly before or after the rain at Dunnottar and other sites, so it is not necessarily a deterrent. One's mindset must remain positive!The radar is a big help in this situation.

Another awkward situation is when the aircraft data is not available, the clouds are scuttling across the sky with speed, and the Met has given a flyable forecast! It's a toss up - do I believe the Met or not?!

Long term forecasts hold no charm for me, especially not when we are going away for a weekend. Apart from the problem that one has to book and pay a deposit for accommodation that is often difficult to cancel, long term forecasts often turn out to be incorrect. We have proven that it could be flyable - though the window may be short - on most days. We should be flexible enough to enjoy our time away with doing other things as well - paradriving (looking at other sites), cycling, hiking, investigating the local attractions, talking with other pilots, and just generally having fun and relaxing. If we "lose" a day, so what? We are living in a too hectic world, and occasionally it is good to step back and really do very little, get in touch with nature and/or human nature. It helps us deal better with the "real" world - if what we are doing is really the "real" world - when we return.

Factors that used to Influence Ulf's Forecast
Ulf liked NOAA, and also looked at a number of other websites. He would often give a long term forecast, although he would usually not say much when a bad forecast was expected! He said:

    On the Skygod Website I put some links under the weather paragraph on how one can go about figuring out what the weather will be doing. These are the links:
    Weather page, or Sounding info, or NOAA Sounding.

    Thermal strength is not related to the cloud size. In Sept/Oct we have dry air, before the rains start, and we have very turbulent thermals, with no clouds. The lapse rate determines the thermal strength. And how long the air had a chance to warm up before it got triggered.

    My thermal strength calculations work on the US East Coast soaring formula that I found some while ago. As trigger temperature I use the sounding temperature on the ground. Comparing what I forecast and the max climbs I had, one can 2x to 3x the values that I post for your maximum climb in a thermal. But for an average climb the values are close.

    Wind wise I take the lowest and the next higher expected windspeed and average it. A good day forecast shows the same 10kt windspeed lower and higher up. For this weekend we got 10kt for the ground, and the next layers higher up are 20kt or more, what is not a good sign.

    The NOAA data is using GFS (Global Forecast System). Windguru does their forecast on GFS. XCSkies uses GFS,... so they all use the same data and if they all agree then one can assume it might really turn out this way.

    Windguru specializes on windspeed on the ground (what they tend to have far too low) and cloudcover or rain.

    My code focuses on expected climb rate, thermal height, wind on the ground (if we can takeoff and land without too much trouble), and the GUI shows cloudbase and OD possibility.

    None of the code can predict if you will experience a nice flight or have some rock'n roll. If you are a low airtime pilot or have not flown for some time, I would recommend that you plan to come out in the afternoons. Mornings have small, rough, punchy little thermals. 12-14 is small core thermal time, and you might not like it. From 14.00 onwards thermals get bigger, smoother and by 15.00 you should have the best thermic conditions. Also phone your buddies upfront before you come out in the afternoon. If you get hold of them, ask them why they are not flying. If they do not answer, assume they are having a good time and you might still get some smooth afternoon flight.

Storm cloud

Unflyable weather: A storm cloud forming over Centurion, Pretoria on 29 October 2009.
Photo: Dietmar Neumann

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Original published 7 November 2009 by Laura Nelson. Updated 27 December 2010.

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