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tdog

Thermals

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Would you mind putting that into the Safety & Training forum to start a conversation there maybe? Because the more people that see it, the better.

Wendy W.



Per Wendy's (wmw999) request - here is a cut and paste of a post I made yesterday about a canopy colapse on landing... I am not an expert, take it for what it is worth.

I did not see anyone yet talk of thermals. I am much more experienced in paragliding than skydiving – and in paragliding we really respect the thermals as they are what we need to fly – but at the same time can cause all sorts of havoc close to the ground. The following MAY HAVE NOTHING to do with this incident – but is food for thought.

Thermals are bubbles of rising air. They might extend all the way from the ground to a cloud or they might be just a bubble. I have been told to study a 1970 hippie lava light, as the rising lava in the light is nothing more than a thermal.

If a thermal bubble leaves the ground and rushes up in a column of air, there is a void that must be filled - with the same amount of air going down or sideways outside the thermal as is going up in the thermal. Again, think of the lava light – as the lava rises, the oil fills the void where the lava was. In other words, if you land near a thermal that is bursting, you can be in the middle of a gust of wind that is going down or sideways filling the area under the thermal. I have been in a thermal that went up at 1,400 feet per minute – which is faster than a lot of jump planes. Somewhere there must have been air going down 1,400 feet per minute to fill the void.

If you see a wind indicator (wind sock) quickly change directions, you might have just witnessed a thermal near by. On a quiet day in a field of tall grass you can hear them leave too, just a quick rustle of the grass is all you hear.

A lot of times thermals are the most aggressive close to the ground as they are narrow and get wider as they go up. They can be explosive off of a super heated asphalt driveway or black roof. There are some “surface tension” forces that keep the thermals close to the ground until they break off. If the wind changes a bit, it might be all it takes to make a thermal release.

In paragliding, you know you are about to enter a thermal when you start to feel turbulence or even go down a bit. You actually judge your angle of attack into the thermal by looking at how the wing turns as you enter it. If your wing flies straight but surges back evenly, you entered it straight on. If your wing turns, part of your wing hit the thermal first causing the turn. If your wing surges forward, you probably just left the thermal.

It is very easy on a large paraglidng wing for half of your wing to be in a thermal and the other half not – causing all sorts of fun things – like asymmetric collapses. You could “hear” them in your wing all the time, they sounded like fabric getting loose then springing tight. Big asymmetrics could collapse more than half a canopy.

On very active thermal days, only the advanced would dare to fly paraglidng canopies/wings because you could experience all sort of "asymmetric collapses” or other dynamic unexpected events.

Paragliders are rated by DHV ratings, 1 thru 4 where 1 is the safest to fly, which rate their handling in stalls and collapses. My DHV 1 GIN Bolero glider turns 90-180 degrees in an asymmetric collapse and must spontaneously recover to get the DHV 1 rating. Gliders rated higher might need pilot intervention to recover from a collapse. Turning = loss of altitude = hit the ground hard any way you look at it. Have you ever studied what might happen to your canopy under an asymmetric? How do you fix it?

To avoid thermals close to the ground, I avoided ground treatments that absorb heat, like rock (pea gravel) or cement. In paraglidng – we liked the green soccer fields, but I don’t think DZ have those. ‘-)

Thermals are caused by heated air on the ground being abnormally hotter than the air above. They “break” off of any pointed object, as small as a shrub. We were taught – turn the ground upside down after a rainstorm and anywhere water would drip off is where thermals rise. It is a mistake to think thermals only happen on hot days, because temperature difference, not just warm air, causes thermals. If the atmosphere is cold and the tarmac is hot – expect a greater thermal than normal even if the outside air temperature is freezing.

There are all sorts of mathematical equations used to predict thermals and the strength of thermals, some available on the 1-800-WXBRIEF FAA Flight Service Center pre-flight briefing system, such as the “wave soaring forecast” and the “K index”. The K index measures stability in the atmosphere. You can also speak to a pre-flight briefer who can help interpret the data – but since I don’t speak pilot, I was always intimidated to talk to the humans and only played the recorded messages.

If you are interested, you can study the “lapse rate” which is the phenomenon that as air gets thinner higher you go up in the atmosphere, the air pressure goes down and so does temperature. Physics says pressure and temperature are related due to fact higher pressure causes molecules to be closer to each other. Pure science says that the “dry adiabatic lapse rate” is 5.5 degrees per 1000 feet. This means, if you jump out of a plane 12K above the ground, expect it to be 66 degrees colder at 12K than at the DZ because the air is under less pressure.

But our flying areas do not exist in scientific test tubes – there is instability in the atmosphere. If the actual temperature, lets say 2K up, is more than 11 degrees colder than the ground temperature – you are bound to experience even more aggressive thermals than normal as the atmosphere tries to find balance.

Oh, thermals cause clouds – the reason why paragliders fly “cloud streets” of thermals across country. It is possible to experience “cloud suck” also, where the thermals are so strong you get trapped in a cloud and must use advanced techniques to lose altitude.

Note – I am not an expert at this. Someone with more experience is invited to correct me. But my point is, aggressive thermals can cause turbulence close to the ground, which can very easily cause landings to be rough.

EDITED TO ADD PHOTO.

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That's quite a picture!
I'd heard of, but never seen, these partial collapses. There have been fatalities that seem to mirror that kind of thing in the recent past, too.

Maybe there's a whole lot we could learn.

Wendy W.
There is nothing more dangerous than breaking a basic safety rule and getting away with it. It removes fear of the consequences and builds false confidence. (tbrown)

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FWIW I've started to take paragliding classes (busted knee put the kybosh on it) and I'm impressed by the complexity and depth of knowledge about the canopy. Uses of different line groups for different control configurations like "rabits ears" etc and all sorts of stuff that just isn't there and often has no place in skydiving. Even the attention to wind details when kiting isn't something you're generally aware of until you try this.

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Intresting reading.

You quote -5.5 per thousand, is that F? I was told -3C per 1000. No mention of temperature inversions (where it is warmer at altitude than at ground level). We often check with our pilots to find out what the temperature is but this is mainly to see how many extra sweaters we need to wear.

Most DZ have a grass landing area greater than a football field, but do have hazzards (runway, hanger etc.) which can cause termals and/or wind turbulance and are normally considered when picking the landing area.

you say if the bubble of air rises at 1,400 ft per min there must be air somewhere going down at the same speed. I'm no expert but wouldn't the effect be similar to a bus driving along an open road where the front deflects the airwhich rushes past the sides of the bus then curls round the back creating a small pocket (slipstream effect). The air disturbance is fairly localised and reduces rapidly (someone standing on the near kerb could almost be sucked onto the road whereas on the far kerb they would only feel a slight breeze [normal width 2 lane road).
I am not belittling your post, just suggesting that the effects of the bubble are more localised then your post implies.
There is also 'wind shear' to consider but this tend to occur at a height that does not cause concerns coming into land (scary when it gets you whatever the altitude).
In my experience, turbulance is a greater risk the thermals on landing but then I do jump in the UK, on a good day the 'hard lads' jump in tee shirts.

Thank for the info.


Get out, Land on a green bit. If you get the pull somewhere in between it would help.

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You quote -5.5 per thousand, is that F? I was told -3C per 1000.



I looked it up to make sure I remembered correctly. Here is a link. (5.5 F per 1000 FEET or 9.8C per 1 KM)

http://www.campusprogram.com/reference/en/wikipedia/d/dr/dry_adiabatic_lapse_rate.html

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you say if the bubble of air rises at 1,400 ft per min there must be air somewhere going down at the same speed. I'm no expert but wouldn't the effect be similar to a bus driving along an open road where the front deflects the airwhich rushes past the sides of the bus then curls round the back creating a small pocket (slipstream effect). The air disturbance is fairly localised and reduces rapidly (someone standing on the near kerb could almost be sucked onto the road whereas on the far kerb they would only feel a slight breeze [normal width 2 lane road).
I am not belittling your post, just suggesting that the effects of the bubble are more localised then your post implies.



I know nothing of fluid dynamics, but I am guessing that the types of slipstreams can be different depending on many factors.

I think the bus analogy works great for a thermal a few hundred feet off the ground since the bus and thermal are both moving in open space. But, it does not illustrate a thermal close to the ground in my opinion. Imagine a bus parked against a wall (the ground) and taking off with incredible acceleration. For the first moment, nearest the wall, the bus leaves a negative pressure pocket where the bus was parked. Air HAS to fill this void, so you HAVE to get a sideways or downward movement. If the thermal had 200,000 cubic feet of displacement, 200,000 cubic feet of air has to fill the void. How it gets there is anyone's guess - but I don't want to be flying when it happens. ;-)

I am thinking you might have illustrated a good point – thermals might have more intense wind effects close to the ground where they originate. Anyone have a real large smoke machine so we can fill a DZ with smoke and watch the thermals?

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So -3C per 1000 ft would be about right & your figure was in F. Thanks for the confirmation.

I now understand the theory of the trermals close to the ground (more than I did), There would be a rush of air into the void if the bubble breaks it's surface tention in the explosive manner you mention. Didn't pick up that at first. Food for thought isn't it?

Still it would pay not to get in the way of these thermals, (at least you would see the bus coming) at altitude you have a chance to recover but low to the ground would be an even more scary moment.

I knew about thermals from tarmac, roads, buildings etc. but not about the 'surface tention' theory.

Hope this thread helps others to be more aware of the dangers.


Get out, Land on a green bit. If you get the pull somewhere in between it would help.

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