On 8/28/2012 11:08 AM, John Cochrane wrote:

One point not reiterated yet here -- the atmosphere down low is very
different from what you're used to at 2000 feet and above.
Snip...

A short list of what's different down low: The atmosphere is much more
turbulent. Thermals, such as they are are much smaller. In this layer,
many small punchy thermals will start. Many will die. The ones we use
up higher consist of many little parcels of hot air that have
coalesced. Most thermals are either short lived, or basically
unworkable to a modern glider. You're in the boundary layer where wind
is being affected by the ground, so there is wind-induced turbulence.
Punches of strong lift/gust followed by sink when you make a half turn
will be the norm.

The ground picture will be totally different to the pilot. If you turn
downwind at altitude, you don't notice that much. If you turn downwind
at 300 feet, all of a sudden the ground will rush by and, this being a
high stress moment, you may pull back. Just as the gust you turned in
fades, or the thermal turns to sink. And when the canopy fills with
trees going by at 70 mph, the urge to pull back will be really strong.
You may push forward to recover at altitude, but it's really really
hard to do with the ground coming up fast.

So, just because you've never unintentionally spun at altitude does
not mean your chances at 300 feet are the same.
Snip...

John Cochrane

Science now can create movies & pictures of the accuracy/reality of what John
asserts above. Roughly 10 years ago I attended a presentation that included
LIDAR movies and pictures of thermals from ground to ~1500' agl. I expect
atmospheric imaging technology has significantly advanced since then.

Any "somewhat experienced" glider pilot would instantly recognize the 1,500'
images as being a thermal. However, in the absence of previous exposure to the
presentation, it required an explanation of what one was looking at, before
Joe Average Glider Pilot might recognize the rising air patterns from ground
level to the base of "a recognizable thermal" as being a coalescing thermal.
(For doubting-Thomas readers, there were multiple thermal examples, so we
weren't looking at the notorious "sample of one".)

Aficionados of tornado photographs might have a glimmer, because the closest
visual wavelength pictures I've seen that kinda-sorta mimic what the LIDAR
imagery showed, have been tornadoes with multitudes of thin, ropy,
mini-twisters feeding into the main funnel well above ground level.
Near-ground-level organization of some "multiple rope twister" photos I've
seen is scanty to non-existent. It doesn't take too much imagination to equate
"plenty of low-altitude garbage" I - and probably many RAS readers - have
tussled with striving for a low-altitude save, with what LIDAR and tornado
photos suggest (to me, anyway) isn't uncommon low-level thermal organization.
"Dynamic" is a pale descriptor of what goes on between ground level and the
agl level a modern sailplane can effectively use.

Unless your ship has the thermaling radius of an insect or a small bird, "what
John C. said" is likely to be in your low-level future. Is it worth betting
your life on?

For the record, the lowest I ever thermaled away from was 650' agl (Dalhart,
TX) on a day with 15-20 knot ground winds (not uncommon there). Yeah, right
above the launch airport. It took me 20 minutes and multiple low points, and
despite being both on vacation and on top of my game at the time, I was
sufficiently wrung out by the process that it also required some decompression
time once I'd established myself, before I could talk myself into heading out
on-course. The save was right after launching, when I was fresh.

Bob W.