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Thermal Detection on the ground?

Date: 19th March 2014

I receive a fair number of questions about ice climbing and flying, and often write back to people. I think I’ll start putting more of the answers up on here for others to comment on as well. Here’s one I found interesting:


Hi Will,
I saw your part 3 about thermals from the internet. My hobby is free flight model airplane contest completion and try to pick good thermals. I  know the sun heats the earth and creates thermals. I’m trying to invent a thermal detector to pick when flying my planes. I’m using a strain gage sensitive load device that measures lift on a 32 inch disk about 10 or 20 feet in the air on a pole.
    My question is how much lift would there be on the disk with a normal thermal? I would like to know what to expect in PSI  or grams per square foot etc. I’m trying to set the sensitivity of a analog readout
     Please help me if you can
Hi Jack, interesting.

That low to the ground I think thermals would have a distinct horizontal component, maybe more than a vertical component. Thermals “pull” air in from the sides as they release upward. If you watch the grass as a thermal releases it “swirls” most of the time.
So maybe measuring changes in wind direction and strength on a hot , relatively calm day would give you more of a clue than trying to measure “lift” on your disc?
When searching for thermals low on a paraglider one of the best indications of a nearby thermal is rapid changes in ground speed.  If your speed accelerates and your sink rate decreases while your wing pitches slightly forward you’re often flying toward a thermal.
If your speed decreases and your sink rate decreases but your wing still pitches forward a bit then you’re flying away from a thermal. If your speed decreases but your real sink rate (not a pendulum swing that your vario measures) remains constant or increases then you’ve just hit a “gust” front on or are in some mechanical turbulence off a ridge or other feature. Novices often try to turn in increased windwind without a thermal component, confusing the decrease in speed and their wing falling back behind them (instead of pressuring and pitching slightly forward or at least not falling back dramatically) with a thermal…
If I were you I would put up a bunch of poles with wind streamers as far you can reasonably can. You can watch the thermals coming as the wind streamers start flapping in sequence. If you watch two flags on the opposite sides of a big field they often will point toward each other when a thermal is releasing out of the field. Airports have this happen a lot on light wind days–the wind socks at either end of the runway will be pointing toward the middle of the runway as that is where the thermal is releasing. That can be frustrating for towing hang gliders and paragliders into the air too.
The simplest “analog” readout would be one pole with some streamers on it. Watch them change direction, get your model into the air.
Curious what you find, please let me know!

Posted in: Blog


  1. Jack Marsh   March 19, 2014 9:20 am

    Thanks for the reply. We have been using mylar streamers for years.
    I have seen them going streight up at times. That’s a strong thermal
    and the best time to fly. I have had some success in the past with a
    disk in a pole. Ready to try my new one. I’m also trying not to launch
    my plane in down air. Planes come down fast then.
    Thanks, I will let you know this summer if my detector works

  2. Ian Wallace   March 23, 2014 12:25 pm

    Hi Will,

    Kudos to Jack for experimenting in developing a Thermal Finder. Good luck in your efforts mate. From my experience: reading, studying, and practicing (over & over again) Wills tip’s on locating thermals will lead you to eventually be able to KNOW where thermals are or will be, and to develop the senses to visualize and/or feel them is the way to go.

    Will, this is a bit off topic, but I meant to contact you years ago to thank you very much for your 3 part Thermals (& the XC) articles at http://gravsports.com/Paragliding%20Pages/paragliding_pages.htm They’ve helped me immensely in paragliding by learning to locate & track thermals, thereby attaining greater altitudes & much longer airtime!

    And please thank your Dad too, for his excellent advice and tips for safe & efficient hiking in his book, ‘The Canadian Hiker’s & Backpacker’s Handbook’. I’ve combined your paragliding tips with your Dad’s hiking tips for my Hike & Fly adventures :)

    The two of you are helping people minimze risks & maximize fun, in so many ways. Thanks!!!

    Happy travels!


    Adventure is out there!

  3. Bernard Guest   April 16, 2014 6:34 am

    I am on the Canadian free flight FAI team. We fly high performance free flight (not RC) aeroplanes with the main object of getting them into lift and consistently flying for more than three minutes. The airplanes fall into three classes: tow line gliders, rubber powered, and internal combustion powered (google F1A, F1B, and F1C if you are curious). For us picking lift on the ground is critical and we are pretty good at it. As noted above, mylar streamers are a very useful tool for tracking local drift directions and interpreting the near ground air mass. Other tools commonly used by our crowd are precision temperatures gauges and anemometers. All of the good free flight teams have a “thermistor pole” with a temp gauge at 3 to 4 m above the ground and an anemometer. We watch the temp and wind speed; in general, when we see the wind drop and the temp rise then we know that we are sitting in a patch of lift. When the wind abruptly picks up (we call this “the fill”) we interpret this as meaning the the bubble of lift has broken loose, sucking air in behind it. A recent innovation by the Canuck team (from the mind of our team aerodynamiscist Brian Eggleston of Low Drag Airfoil (LDA) fame) is to use a helium filled balloon tethered to a precision scale to directly measure lift on the ground. We use a largish balloon (2′ to 3′ diameter) and about 15 to 20 m of light line. What we see is that the balloon pulls up on the scale when it is in a thermal generating as much as 10 to 15g of lift over and above its normal lift (we zero the scale when the system is set up). So, from this we can say that there is a strong vertical components to the thermal even close to the ground. We have successfully detected lift using this contraption even in light wind. The balloon leans over of course but the vertical component of its lift is still measured by the scale (which does not see the horizontal component) and so we can still detect thermals.

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