On Sat, 2010-08-28 at 21:16 +1000, Luke O'Donnell wrote: > I'm not sure exactly the phenomenon you are referring to. Do you mean > that at cloud base (or within 100ft) that you find the strongest lift > significantly upwind of the cloud? As i'm sure most have, i've > experienced thermals being upwind of the cloud within a 1000 feet or > so - due to wind shear > The latter. I've never explored top of an offset thermal, mainly because the lift falls off - probably to do with horizontal flow at the top since, as you say, it must join up with the cloud somehow.
> > Shear wave (perhaps also known as thermal wave) where small scale > (sometimes strong) wave can form within about a km on the windward > side of a thermal, going as much as a few thousand feet over the top > of convection > Don't recall noticing that at the time. > > Another thermal forming as part of a street (or not) just upwind of > the cloud in questions, in which case a new cloud should form shortly > after > Its always been in mild streeting conditions with fairly widely spaced component clouds but the offset thermal has always been the the only one associated with the cloud, which AFAICR was fairly small. I don't think they've been new blue thermals or associated with an upwind cloud either. > I also recall hearing of the use of thermistors. In theory it should > work, there should be significant temperature differences between the > tips of wings (especially on larger gliders) while thermalling. > I was talking with an older club member today. He was quite definite that it was tried back in the wooden era and didn't work. > > Airspeed, as the inboard wing while circling has lower velocity, a > thermistor in the airstream would likley heat up as a result of less > wind-chill, giving false readings > If they do that its a sign that the wrong thermistors are being used. I learnt early on that you never use less than a 100K thermistor on a 9v system and that 500K or 2M thermistors are much better because high resistance thermistors don't heat up nearly as much as low resistance ones. You're right about thermal inertia too - for ground based thermal detectors anything with a thermal constant bigger than 3 seconds is totally useless, though hanging it out in a glider's slipstream may make that less important. I found the best thermistors for my system were all around 0.5-0.6mm in diameter and naked, i.e. no glass coating. > > Out of line of sight of the sun, could lead to uneven heating when > turning etc. > An effective solar shield is essential. It must be both reflective and insulated - kitchen foil over 6mm thick white foam works really well provided there's no way that direct or reflected sunlight can reach the thermistor bead. I used a foam/foil box with an open bottom with the thermistor just under its top surface and placed a piece of thin ply with chrome tape on its underside under the bead to prevent reflections from illuminating the bead. BTW, I heard back from WeatherJack. See my next post. Martin ------------------------------------------------------------------------------ Sell apps to millions through the Intel(R) Atom(Tm) Developer Program Be part of this innovative community and reach millions of netbook users worldwide. Take advantage of special opportunities to increase revenue and speed time-to-market. Join now, and jumpstart your future. http://p.sf.net/sfu/intel-atom-d2d _______________________________________________ Xcsoar-user mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/xcsoar-user
