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Old Sep 18, 2011, 05:14 PM
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United States, MA, Waltham
Joined Dec 2001
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You're right that it can be a "matter of field" or at least location. It also depends on what you fly.

Today I watched a 10 minute flight with a Supra, most of which was below 100 meters! Lowest I've ever caught a thermal was at about 8 feet (2.5 meters), but that, of course, was a freak situation, given that it was a 43 oz. (1.2kg) 2 meter model. I've also seen a 2 meter model with a 15(!) oz. per square foot wing loading climb out from around 20 feet. If I've got the units right, that's about 46 gm/dm. We have a couple of guys in our club who climb out with their Bubble Dancers from hand tosses.

Around here, most days we have less than 20km/h (12mph or so) wind.

However, it's usually true that the thermals are stronger at higher altitudes. At least up to a point.
Quote:
Originally Posted by marc.pujol View Post
Finding a thermal is like finding mushrooms. It is a matter of field.

SNIP
Also think that there is a minimum altitude to catch the thermal. With few wind and with good accumulator, of course thermals can be catch from 20m altitude. But this is quiet accasional. Most of the time, thermal are small, wind is more than 20 km/h and then, you can only catch thermal starting at 100m to 200m...

SNIP

Marc
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Old Sep 19, 2011, 08:53 AM
Marc PUJOL
Joined Feb 2010
115 Posts
It's clear that the air is different from one region to an other. In western Europe (GB,B,NL,F,Gr...) it seams that we do not have such so good condition you have.

Yesterday, it was, once again a windy day in Paris area (>20km/h) and thermals was as usual for september very small (1m/s max over 100m) They pass at quiet high speed. It was only possible to take thems for 5 min and then speed up to try to return home. Easy gliders was floating without going upfront... A windy day!

I observed some storch that was on the way to spain for 10 minuts. It was difficult also for them to progress agains the wind without flapping...
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Old Sep 20, 2011, 12:37 PM
planepainter
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Mt. Juliet, TN
Joined Sep 2008
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Last Sunday I again went out to the field with my Aspire E. Again the wind was out of the south and again there seemed to be areas of noticeable sink. This time I ranged around the sink areas hoping to find the connected lift but had little success. It was easier to do with electric as the motor allowed me to stay aloft longer and range further...

The Aspire signals lift well and loves thermals so I guess one would have to chalk it up to inexperience on my part to find the elusive lift that day.


But even the worst day soaring is better than the best day fishing....


PP
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Old Sep 20, 2011, 01:40 PM
Duane
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No. VA
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The worst sink is often upwind of the thermal, so try searching downwind of the sink. Sometimes, it's just sink though.
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Old Sep 21, 2011, 07:54 AM
Marc PUJOL
Joined Feb 2010
115 Posts
Sinks are more large downwing. So if you go up wind, you will need to cross a larger area of sink.
In front of a thermal (upwind of a thermal), the sink is usually smaller in size (the wind compress the sinking zone).
So if you do not know where is the lift, take the chance to go down wind or enventually cross wind.

That's what you mean isn't it?

Marc
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Old Sep 21, 2011, 08:37 AM
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United States, OR, Eugene
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It could be that conditions in your area recently have been caused by a temperature inversion. With warmer air aloft thermals cannot penetrate the warmer air above. On days with a stronger or consistent lapse rate(decreasing temperature with altitude) thermals will organize and rise unimpeded until they form fair weather cumulus or given enough moisture and atmospheric conditions potentially they will develop into full thunderstorms.

I fly jets for a living and since I also fly sailplanes for fun I pay attention to conditions while departing or arriving at low altitude. On a typical day, particularly mid day through late afternoon, it's bumpy below the haze layer due to both thermal activity and sometimes mechanical turbulence(caused by wind passing over obstacles, buildings, trees, hills etc...). On some days it is much more pronounced, particularly on days with what we call fair weather cumulus clouds. On others it can be very tame, more specifically, when there is a temperature inversion which basically puts a lid on any organized thermal activity.
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Old Sep 21, 2011, 09:18 AM
Egads! It's a GIRL!
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United States, CA, San Jose
Joined Jul 2011
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Tell me more about inversions, please. Is there a way to see them? Is it like smoggy days, where the haze is right above my head?

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Old Sep 21, 2011, 09:55 AM
Duane
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Some good info and a few pictures here.
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Old Sep 21, 2011, 11:20 AM
Egads! It's a GIRL!
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Thanks! (why didn't I think of that?)

Those pictures remind me of "Spare the Air" days. So if it's super smoggy, IRS probably no good flying.
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Old Sep 21, 2011, 04:12 PM
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I remember camping once on a damp day when there was an inversion. Campfire smoke just stopped at a particular altitude, very low. You could walk up the hill and go right through it.

I have also seen, on a very calm evening, a layer of mist maybe a couple of feet thick, from perhaps 3 feet agl to 5 feet AGL. A real trip to walk into it and duck to look under, stand up to look over.

A thermocline is an inversion in water, if memory serves. I seem to recall one that would freeze your feet if you treaded water when swimming. The transition was so sharp I thought I could see the transition, presumably from refraction which made it look almost like the surface of the water.
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Old Sep 21, 2011, 04:35 PM
Egads! It's a GIRL!
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United States, CA, San Jose
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Cool.

Cool!

COOL!!
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Old Sep 21, 2011, 05:53 PM
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United States, OR, Eugene
Joined Aug 2011
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Nice Wikipedia link Wazmo.

I think today was a good example of a situation where there may have been an inversion layer higher up here in my area.

My first launch today was around noon with calm winds and sunny skies. As soon as I disconnected from the high start I was in good lift so I trimmed for a nice slow circle and rode the lift up for a few minutes until the airplane just seemed to hang and bobble around like it was around a thermal but I just couldn't seem to find the center of the thermal so I trimmed for cruise flight to find more lift. I didn't have any trouble finding more lift but again around the same altitude the airplane just seemed to stop climbing. I repeated the same senario for the next thirty minutes until I got a bit bored and decided to practice landings.

Since the air was dry and clean there was no real visual evidence that there was a temperature inversion aloft but it sure seems to be the case. I have seen many days while flying where there are evenly distributed clouds much like cumulus clouds except they were flat on top because they couldn't penetrate the warmer air above and they weren't stratus clouds because they weren't formed in a continuous smooth layer. I suspect if there was enough moisture in the air today I would have seen clouds much like the ones I just described.
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Old Sep 22, 2011, 08:33 PM
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Quote:
Originally Posted by marc.pujol View Post
I

Yesterday, it was, once again a windy day in Paris area (>20km/h) and thermals was as usual for september very small (1m/s max over 100m) They pass at quiet high speed. It was only possible to take thems for 5 min and then speed up to try to return home. ..
I am interested how you determined the thermal '1m/s max over 100m' lift rate.
Telemetry, or observation??
G2
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Old Sep 24, 2011, 02:53 PM
Marc PUJOL
Joined Feb 2010
115 Posts
Quote:
Originally Posted by Gemini2 View Post
I am interested how you determined the thermal '1m/s max over 100m' lift rate.
Telemetry, or observation??
G2
I use Xerivision telemetry. I register it or I activate the head up display that provide me the info in "real time".

I have then now quiet some curves and experience on how it go up.

It's very interesting how accurate is the eye. 1M/s appears to be quiet high. The glider seems to jump in the sky. and 0.1m/s is easilly triggered by the eyes without much experience.

Make the experience. It's quiet amusing.

Marc
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