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Old Nov 10, 2012, 09:17 PM
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Daemon's Avatar
Lakewood, Colorado
Joined Aug 2002
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Repeating myself from an earlier discussion on the effects of the Coriolis effect.

Coriolis force is simply the earth turning under your feet while
large moving masses tend to follow a straight path due to inertia. Problem
is, we're only talking a maximum of 4 tenths of one percent of 1 degree
(that's 0.004 degree) per second of rotation at the highest latitudes, and less as you
approach the equator. We're about in the middle. It acts upon moving air
masses the size of states or countries as they're pulled into a low pressure
area, encouraging enough bend in their paths (as viewed from the
earth's rotating frame of reference) to start slow rotation which is only
accelerated enough to observe by decreasing rotational inertia as the system
contracts. But at the scale of the masses of wind that we fly it
might as well be compared to the flapping of butterfly wings for all
the effect it has.

What makes thermals and dust devils start turning is local effects. Surface wind
combined with unequal heating, ridges, valleys, treelines.. etc..

ian
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Old Nov 10, 2012, 09:27 PM
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David Forbes's Avatar
United States, FL, Gainesville
Joined Dec 2005
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Several years ago ther was a post that showed some incredible thermal imaging of an active field. IIRC it was from some mid western university. It showed that at scale we usually play at it was chaos. Wish I could find it.
Dave
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Old Nov 10, 2012, 09:33 PM
ein flugel schplinterizer
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USA, FL, Pensacola
Joined Sep 2004
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Quote:
Originally Posted by GliderJim View Post
I'm pretty sure a thermal could turn in either direction, it just depends on what forces are there to initiate the rotation. Maybe a thermal out in the middle of a field with no other obstacles will follow the north/south hemisphere "rule", but if it kicks up near an obstacle such as a tree line or something, with a little breeze, I could see it kicking off in either direction. If I fill my tub with water and open the drain, I can get the water to spin in either direction by giving it a little swirl with my finger. Once it's started in a direction, it'll hold that until the tub drains. Too much momentum for any other weak forces to change what's happening. Just speculation of course.
I gotta go with this 100%. "Thermal" is a broad definition. We get afternoon wave action at work from sea breezes that have an element of heated air to them. You can park up there sometimes with little or no ground speed and hang till your tired of flying.
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Old Nov 11, 2012, 03:14 AM
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Windsor, Canada, near Detroit
Joined Jul 2007
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proof

as i said, am open to proof. please show me videos of dust devils that turn to the right and after seeing enough of them, i will buy that idea that perhaps thermals, if they turn, do it at random. that proof will be the only way to make certain that dust devils turn at random.

but in 1st place, am beginning to doubt that thermals turn. cumulus don't seem to turn, and there is the chance that thermals develop same way. especially if we consider that thermals feed the clouds that may become cumulus.

the uncertains of meteorology!
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Old Nov 11, 2012, 03:16 AM
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Windsor, Canada, near Detroit
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something like that

Quote:
Originally Posted by David Forbes View Post
Several years ago ther was a post that showed some incredible thermal imaging of an active field. IIRC it was from some mid western university. It showed that at scale we usually play at it was chaos. Wish I could find it.
Dave
great to hear that some1 did that. so there is the chance that it can be done.

we wish that you could find it too!
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Old Nov 11, 2012, 03:37 AM
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terrific work

Quote:
Originally Posted by Daemon View Post
In Austria.. right hand turning
http://www.youtube.com/watch?v=AxRT60-kw78

Yes, thermals turn both ways, and sometimes no way. They are bubbles and
ribbons, ridges and jets and they pull the wind this way and that. There are even
thermals that remain nearly stationary as the wind blows through them because
they're only feeding from the upwind side. I think the biggest thing people overlook is
how much horizontal motion is usually involved to get a thermal going
and to keep it fed and how terrain triggers and guides thermals because of
deflected horizontal movement of the air close to the ground. While warm air wants to rise if
there's cooler air above, the rising air has to be replaced by air from the sides which
may be warm, in which case it feeds and accelerates the thermal upward, while also
accelerating the horizontal winds below. We all know to watch the
flags pointing toward the thermal, but the trick is to understand how the terrain will
influence that wind so you know where to go looking for the next thermal and the next.

I wrote up quite a lengthy post description of slermals (slope + thermal) a few years back
complete with some pretty 3D diagrams I made up to represent my visualization of thermals.
Check it out.
http://www.rcgroups.com/forums/showt...64#post4966743

ian
thank you sir. amazing study. but it is so large that i just saved for further study. i overlooked it when my next message overlapped with yours. in my case, i fly at the flats of the central region of the great lakes, where there are no hills around, so the slope effect doesn't show and i have to deal with thermals only, although this makes things easier for me.
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Old Nov 11, 2012, 09:23 AM
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Michigan, USA
Joined Jul 2006
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If you check out the video on this page...
http://www.radiocarbonart.com/secret...ermal-soaring/
...at about the 30 second mark are a couple shots of thermals carrying dust. They are not spinning violently like most dust devils you see. Very gentle and slow with no discernible rotation. The full length video has many more shots like those where you can watch the thermal for a long time.
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Old Nov 11, 2012, 12:23 PM
1984 is no longer fiction
War is Peace's Avatar
United States
Joined Mar 2012
426 Posts
I do know thermals happen at night;

TRUE STORY: During the 1978 Fall Soaring Festival I decided to fly at night. My Bird of Time was covered in transparent yellow monokote and I had weight reducing holes in all the ribs. I took a long clear plastic tube and filled it with the light emitting goop from a cyalume light stick.

I then slid the tube into the wing and launched my glider. The entire wing glowed making it very easy to see the glider. It was about 10:30 at night and I got a 15 minute flight, playing with a very weak thermal. (never went up very much, just mostly reduced sink)

Over the years I have learned thermals are like women: from a distance they seem similar, but once you get involved you realize they are all different.

Some left me wanting more
Some were boring
Some teased me relentlessly
Some were so frightening I had to leave
Some went away with no warning
And every once in a while one made my fantasies come true.
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Old Nov 11, 2012, 03:47 PM
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Windsor, Canada, near Detroit
Joined Jul 2007
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thermals: the making

at the beginning of this thread i posted a link to an article where it stated that thermals are like horizontal donuts that climb, with the wind going up at the center and down at the edges.
we all believed that article (it was back in the 60's), but recently i found another approach. see:
http://www.xcmag.com/2007/07/thermal...rt-1-thermals/
where he brings an explanation that comes very close to how cumulus nimbus grow, including the same shape, so am beginning to lean towards this idea.
so this way thermals don't turn left or right, just wind goes up inside and down outside-and this matches with the other theory in that sense.
and the idea of trimming the planes to glide to the left from those days could be to balance the torque of the motor, be rubber bands or engine-and somehow it became customary and we did gliders that way too. just a guess.
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Old Nov 13, 2012, 02:40 AM
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Angleton, Texas
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Old Nov 13, 2012, 08:48 AM
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United States, MT
Joined Mar 2008
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TTK, that has already been posted in this thread, post # 13.
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Old Nov 15, 2012, 12:05 AM
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Windsor, Canada, near Detroit
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thermals up

from my memories i recall my planes suddenly tilting, slowing down, closing their turn, raising their nose and almost stalling but not, just climbing: they were into a thermal! but when they were released, there was sort of stalling and away they went. but not sinking, just gliding. occasionally they catched a downdraft, but that was not usual.

so, are really downdrafts at the outside of the conduit that goes up?
if we consider that thermals may develop same way as cumulus do, as they are the start of the cumulus, if we look at a cumulus nimbus forming, do you see any thing going down at the outside? i don't. i just see the whole thing growing up.
it may be downdrafts nearby, but not necessarily so.
that's what i recall.
let's watch more closely next time we get into a thermal and bring our comments.
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Old Nov 15, 2012, 05:20 AM
greg
ciurpita's Avatar
somerset, nj
Joined Feb 2005
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Quote:
Originally Posted by phil alvirez View Post
so, are really downdrafts at the outside of the conduit that goes up?
from my full-scale experience being in a glider with a vario, the vario usually indicates strong sink surrounding lift.

if air goes up, what fills the vacuum it leave behind? what happens to air in the space above the lift?

greg
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Old Nov 15, 2012, 07:15 AM
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Windsor, Canada, near Detroit
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now we know

Quote:
Originally Posted by ciurpita View Post
from my full-scale experience being in a glider with a vario, the vario usually indicates strong sink surrounding lift.

if air goes up, what fills the vacuum it leave behind? what happens to air in the space above the lift?

greg
thank you very much for your input.
with this data, now we know for sure what's happening at the edge of a thermal.
and the conclusion just makes sense.
thanks again.
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Old Nov 15, 2012, 09:15 AM
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Canada, ON, Cambridge
Joined Apr 2010
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Im not convinced there is a flow of downward air the entire length of the column. Its more about turbulent air around the perimeter , Some of it down air.

Its no different that calm water circling around the edge of a raging river

Varios measure barometric pressure
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