Thermals 101: A Paraglider’s PerspectiveBy tdog on 2004-11-08
Photos: Example of asymmetric collapse
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 paragliding 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 paragliding 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 paragliding – 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.
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