On Wednesday, 26 April 2000 03:00:00 UTC-4, Tom Seim wrote:
> I had a nice flight last Saturday in somewhat windy conditions. I have a new
> Garmin Pilot III that displays a fairly high resolution flight track. I
> noticed that I could estimate wind speed (and, of course, direction) by
> watching how much my thermal circles overlapped. Once set up in a thermal I
> fly a reasonably consistent bank angle and speed. This results in a circle
> diameter of 600-700 feet and a time period of 22 sec. This works out to a
> wind drift of 20 mph (17 kt) if the circles touch (a drift of about 700 feet
> per circle). The winds are 10 mph (9 kt) if the circles have a 50% overlap
> (ever other circle touches). This gives me a quick (and rough) estimate of
> the winds aloft.
>
> Obviously this will not work if your are continuously adjusting your circle
> position.
>
> You can calculate your circle diameter by:
>
> Diameter (feet) = 0.54 * Speed (kts) * Time Period (sec)
>
> The wind speed per circle is:
>
> Wind speed = 0.68 * Time Period (sec) / Diameter (feet)
>
>
> --
> Tom Seim, 2G DG-400
> Richland, WA

Question:

Is the speed at which a thermal drifts the same as the winds aloft?

On Wednesday, June 3, 2020 at 7:33:52 PM UTC-4, weemacgregor wrote:
> Is the speed at which a thermal drifts the same as the winds aloft?

No.

On Wednesday, June 3, 2020 at 8:39:12 PM UTC-4, Dave Nadler wrote:
> On Wednesday, June 3, 2020 at 7:33:52 PM UTC-4, weemacgregor wrote:
> > Is the speed at which a thermal drifts the same as the winds aloft?
>
> No.

So then if a pilot, or glide computer, estimates the wind from the changing ground speed while circling in a thermal, is the wind speed underestimated?

At 01:37 04 June 2020, wrote:
>On Wednesday, June 3, 2020 at 8:39:12 PM UTC-4, Dave Nadler wrote:
>> On Wednesday, June 3, 2020 at 7:33:52 PM UTC-4, weemacgregor wrote:
>> > Is the speed at which a thermal drifts the same as the winds aloft?

Yes
and
No... If the thermal is being produced by a power station chimney, then
20 feet higher than the chimney, the drift of the thermal is zero. It is
only when the thermal reaches a few thousand feet above the chimney that
the drift equals the wind aloft speed.

Also remember that for even higher altitudes your IAS is not your TAS, so
on a calm day, your GPS speed will differ from your IAS. ( about +2% per
1000ft.)

Dave Nadler wrote on 6/3/2020 5:39 PM:
> On Wednesday, June 3, 2020 at 7:33:52 PM UTC-4, weemacgregor wrote:
>> Is the speed at which a thermal drifts the same as the winds aloft?
>
> No.
>
Usually close enough, though, given all the other uncertainties in soaring
calculations. And, I think, they are the same if the vertical wind gradient is zero.

My flight computer (iGlide) displays two wind numbers:

- the drift while circling
- the "instant" wind from the Butterfly vario

My impression is the majority of the time, they agree within 2 knots and 10
degrees. The rest of the time they differ more than that, and sometimes
considerably more. Of course, the measurements are done very differently, and the
circling wind is not just once around, but averaged over several circles, which
are at different altitudes as the glider climbs in the thermal.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1