Any sailplane pilots?

Most people can't see a minute hand moving, but it moves nonetheless.
It's a matter of patience. The problem with observing cloud rotation
is that the cloud is constantly changing shape, therefore you cannot
time lapse the same way you can when observing vertical development
(or determining that a minute hand moves). As noted in my impolite
post, you need to spend a half hour on your back. Then we can move on
to discussing whether there's any real advantage to be had.

(Kirk Stant) wrote in message . com...
"K.P. Termaat" wrote in message ...
My experience is that it works, especially on days with very low humidity,
but no boomers and only low.


"Mike Rapoport" schreef in bericht
ink.net...

You will find less lift over water of any kind, even if it is contained in
vegetation. The best lift is always over the highest, dryest, darkest
surface around. The water vapor idea is...well...it is hard to find a
place
to start...but it won't work

Mike
MU-2


Have to disagree with you, Mike - out here in Arizona, in the desert
areas that are not irrigated, we often find good lift directly over
small cattle "tanks" - small shallow ponds that are scattered around.
A lot of us have noticed this and compared notes, and it works; if too
low to get to high, dark ground, I'll head for the nearest pond and it
will usually turn up a nice thermal. We think it may be due to the
fact that the ponds are in a natural low spot, and coupled with the
little bit of moisture, could be the necessary trigger for a thermal.

Now obviously, large irrigated farm fields or river basins are death
to thermals - but a local lake (reservoir) seems to have little effect
on thermal activity - could it be all the drunk boaters?

What's the old saying about never saying never?

Kirk
LS6-b


The adiabatic rate of moist air is about 1.5C/1000ft. For dry air it
is 3C/1000ft. Therefore, assuming that the water temperature is the
same as the surrounding ground (which would be true if the water is
shallow), I could see how one will experience greater lift above
water.

"Andrew Sarangan" wrote in message
om...
(Kirk Stant) wrote in message
. com...
"K.P. Termaat" wrote in message
...
My experience is that it works, especially on days with very low
humidity,
but no boomers and only low.


"Mike Rapoport" schreef in bericht
ink.net...

You will find less lift over water of any kind, even if it is
contained in
vegetation. The best lift is always over the highest, dryest,
darkest
surface around. The water vapor idea is...well...it is hard to find
a
place
to start...but it won't work

Mike
MU-2


Have to disagree with you, Mike - out here in Arizona, in the desert
areas that are not irrigated, we often find good lift directly over
small cattle "tanks" - small shallow ponds that are scattered around.
A lot of us have noticed this and compared notes, and it works; if too
low to get to high, dark ground, I'll head for the nearest pond and it
will usually turn up a nice thermal. We think it may be due to the
fact that the ponds are in a natural low spot, and coupled with the
little bit of moisture, could be the necessary trigger for a thermal.

Now obviously, large irrigated farm fields or river basins are death
to thermals - but a local lake (reservoir) seems to have little effect
on thermal activity - could it be all the drunk boaters?

What's the old saying about never saying never?

Kirk
LS6-b


The adiabatic rate of moist air is about 1.5C/1000ft. For dry air it
is 3C/1000ft. Therefore, assuming that the water temperature is the
same as the surrounding ground (which would be true if the water is
shallow), I could see how one will experience greater lift above
water.

Not for unsaturated air it isn't. The moist rate only applies to saturated
air (ie in clouds)

Mike
MU-2

Chris OCallaghan wrote:

Granted, our attention spans are short, but spending a half hour on
your back watching the development of a cumulus cloud can be very
instructive.

We've been talking about dust devils, which rotate as fast as 20 times
minute, but you seem to be talking about rotations slower than 10 times
per hour. No wonder I don't notice rotations like that while flying.

But you needn't even do this if you are observant during
your climbs. You will note that around the edges of the cloud you'll
occasionally see vortices forming when the light hits them right.

I do see these generally small clumps of rotations in the vertical
plane, near cloud edges. This isn't what I would call "clouds rotating"
in this context (which direction to circle), by which I mean a
substantial amount of the cloud going around horizontally like a dust
devil does. I have seen roll clouds rotating rapidly, but that isn't
relevant to dust devils.

Or
if there is clag below cloud base, it will often have a discernable
rotation. When observed, they often herald localized cores that can be
exploited.

I have never observed this, though I always head for "tendrils" or
clumps of cloud forming below the nominal cloud base, as the lift is
usually much better. The movement, if present, has been invariably
upwards. What speed are the cores rotating at? I'm sure I'd notice if it
was anything like dust devil speed.

More than once I've noted vorticity on the edege of a large
cloud, shifted my circle to it, and been rewarded with much improved
lift.

Me too, but it's not been in the horizontal plane.

Spending some time on your back, you will observe that not only are
there localized vortices, but that the entire system slowly rotates.
Sometimes it's easier to see this than at others. However, patience is
required since the rotation is very slow, but nontheless discernable.

I haven't looked for rotation this slow, and while it's interesting, it
doesn't seem relevant to the choice of circling direction.

Best to start with a wisp and watch its full development. Choose one
as close to directly overhead as possible. The closer your view to
directly beneath, the more obvious the cloud rotation becomes.

I'll try this next time I'm stuck on the ground under cumulus clouds.

"Brian Case" wrote in message
om...
On the other hand I can't tell you how many times(numerous) I
intercepted a dust devil at 1000-1500 AGL and climbed out at less then
1kt or even did not climb. Most times however I get 3-6 kts out of
them. I have see dust devils go to 7-8000 feet up. Hate to tell the
one gentleman this that wouldn't fly into a dust devil, but if he flys
using thermals he is just flying into dustless dust devils, As far as
I can tell the only difference is if it is lifting air over an area
were it can pick dust up or not.

We are apparently sharing different definitions of "dust devils." The
ones that rage across the part of the country I fly mostly do not fit the
structure of a thermal.
Thermals, at least where I am, are rising volumes of air created by
differential temperatures on the surface. One of our best local thermal
engines is the black paved surface of Runway 10/28 at IYK. The equipment
parking lot for the highway maintenance yard is another. The location of
these is pretty constant and reasonably predictable and reasonably benign.
Dust devils on the other hand, while they may begin at convective
sources, are cyclonic whirlwinds that travel laterally across the ground,
sometimes for miles. DDs in our area are typically less than five meters in
diameter. One monster dust devil that went across a portable weather station
at the Naval Air Warfare Center spun the anemometer over 80 knots before it
ripped the mast apart. We watched that one travel about ten miles. On
another occasion, a monster went across a mobile home park and took the roof
off a home and dissassembled tool sheds like card houses.

Casey Wilson wrote:
Dust devils on the other hand, while they may begin at convective
sources, are cyclonic whirlwinds that travel laterally across the ground,
sometimes for miles. DDs in our area are typically less than five meters in
diameter. One monster dust devil that went across a portable weather station
at the Naval Air Warfare Center spun the anemometer over 80 knots before it
ripped the mast apart. We watched that one travel about ten miles. On
another occasion, a monster went across a mobile home park and took the roof
off a home and dissassembled tool sheds like card houses.

Yeah, those are the kind of dust devils I look for 8^)

I remember one I saw in a valley north of Tonopah, when I was down low
looking for lift. It had one huge central column and six smaller ones
twisting around it. I could see huge pieces of sagebrush literally
getting blown out of its path. I pulled into it at roughly 1500 feet
AGL, and centered a 14 knot climb with dust and twigs flying all around
me. I was at 18000 feet in what seemed like a moment. Even at that
altitude, there was plenty of dust, and looking down into was like
looking into the maw of a huge snake that stretched all the way back
down to the ground.

You don't know what you're missing 8^)

Marc

I've seen this effect many times in Australia. Kept finding
lift over small irrigation ponds (altitude several hundred
feet). Decided to do an experiment whilst awaiting a buddy
who was interminable slow to get airborne. There was an
irrigation pond about 1km from the airfield. After losing
sufficient altitude doing aero, spoilered down to several
hundred feet over this pond, then climbed out and repeated
the procedure. Four or five times (like I said, he's slow).

The interesting features here (and in Arizona) a
- very dry air, and
- shallow irrigation tank/pond subject to good heating (warm water)
Don't know that I understand the physics, but extremely
consistent.

Beaver Pond Lift is however a different phenom...

Best Wishes for 2004 to all, Dave "YO"


Peter Creswick wrote in message ...
Mike Rapoport wrote:

The moisture doesn't really help lift until the air is fuly saturated and
starts condensing releasing heat. Also, the air above the water is cooled
by evaporation and is cooler than the surrounding air. I will never say
never and I don't dispute your or others experience, but the explanation
doesn't make sense to me.

Mike
MU-2

"Kirk Stant" wrote in message
om...
"K.P. Termaat" wrote in message
...
My experience is that it works, especially on days with very low
humidity,
but no boomers and only low.


"Mike Rapoport" schreef in bericht
ink.net...

You will find less lift over water of any kind, even if it is
contained in
vegetation. The best lift is always over the highest, dryest, darkest
surface around. The water vapor idea is...well...it is hard to find a
place
to start...but it won't work

Mike
MU-2


Have to disagree with you, Mike - out here in Arizona, in the desert
areas that are not irrigated, we often find good lift directly over
small cattle "tanks" - small shallow ponds that are scattered around.
A lot of us have noticed this and compared notes, and it works; if too
low to get to high, dark ground, I'll head for the nearest pond and it
will usually turn up a nice thermal. We think it may be due to the
fact that the ponds are in a natural low spot, and coupled with the
little bit of moisture, could be the necessary trigger for a thermal.

Now obviously, large irrigated farm fields or river basins are death
to thermals - but a local lake (reservoir) seems to have little effect
on thermal activity - could it be all the drunk boaters?

What's the old saying about never saying never?

Kirk
LS6-b

Have seen similar effects over the small dams on farms here too. My idea is that the air over the
water cools by evaporating water out of the pond. In so doing it looses more heat and hence
contracts more (gets denser) than it gains buoyancy by water vapour increase, ie, it gets both
colder and denser overall than the surrounding surface air. As the dense pool of air becomes
greater, it spreads out, ie, sort of collapses on itself, and pushes out over the edges of the pond
/ dam, particularly down slope over the dam wall, creating a miniature equivalent of a valley wind
in the creek or down the slope, thus acting as a wedge trigger to lift the warm dry air off the
ground.

I can claim no specialist meteorological knowledge, but I believe the
following to be undeniable:

1. When a thermal forms, there is to some extent at least an inflow of
air, to prevent a vacuum forming under the rising airmass.

2. It is extremely unlikely that the inflow from every direction is
equal and symmetrical. If Coriolis effects are likely to be swamped by
local initial conditions for the inflow, the resultant rotation may be
in either direction.

3. It follows that there will be an element of rotation imparted to the
resulting thermal bubble/plume/call-it-what-you-will.

4. In some cases at least, the resultant rotation may be enough to be
noticeable.

5. During the inflow phase, any resultant rotation will speed up, by
conservation of angular momentum, like a dancer speeding up a spin by
pulling in the arms.

6. This rotation is certainly visible in dust devils, tornados, and
other smaller local eddies. Even in the UK's usually mild thermals, one
occasionally sees bits of straw, grass cuttings, or dead leaves picked
up and whirling round. In the days of stubble fires (farmers buring off
fields, after harvest - now banned) bits of burning straw could be seen
in the rotating thermals.

[I have used "rotation" in the layman's sense that the air and its
contents are going round. There is a technical use of the word, if I
recall my fluid dynamics correctly, that the water in a whirlpool has
"zero rotation" which may also apply in thermals, for the same reason -
it is a mechanism where things can go round quite fast without a
significant input of angular kinetic energy. It is a feature of a
rotating fluid mass where the middle is going round faster than the
outside, which happens in whirlpools and when you pull the plug out of a
circular basin full of water. I propose to leave it there. ]

Chris N.

On Mon, 29 Dec 2003 10:24:40 GMT, "K.P. Termaat" wrote:

My experience is that it works, especially on days with very low humidity,
but no boomers and only low.
I'am talking about small shallow ponds in dry area's especially when the
ponds are surrounded by sandy grounds with higher vegetation like trees. The
buoyancy impuls from the evaporated water is apparently just good enough to
start the thermal which then sucks air from its heated up vicinity.
Has saved me many times when I was still flying my Pik20D or more recently
my DG800S.

Karel, NL
V-2cxT
Water vapour has a molecular weight of a bit over 18 and dry air a bit
more than 28. Water vapour at the same pressure as the air around it
is considerably less dense than dry air. More water vapour= more
bouyancy.

Then again this may have more to do with low spots in the ground. I've
always found quarries (holes in the ground)to be excellent lift
sources when low.

Mike Borgelt

I'll buy that one, JJ.

--
Bert Willing

ASW20 "TW"


"JJ Sinclair" a écrit dans le message de
...

I am not disputing the facts, I am disputing the explanation.


OK, Its time for my Minden pond theory. We have a small pond about 5 miles
east
of the airport that consistently produces thermals. JJ's explanation;
Heated
air that is slowely moving over the ground by the wind, suddenly comes to
the
cooler pond edge. This cool edge of the pond acts as a trigger that forces
the
heated air to break loose and start rising and that's why thermals can be
found
at the edge of small ponds.
JJ Sinclair